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DBI(3)		      User Contributed Perl Documentation		DBI(3)

NAME
       DBI - Database independent interface for	Perl

SYNOPSIS
	 use DBI;

	 @driver_names = DBI->available_drivers;
	 %drivers      = DBI->installed_drivers;
	 @data_sources = DBI->data_sources($driver_name, \%attr);

	 $dbh =	DBI->connect($data_source, $username, $auth, \%attr);

	 $rv  =	$dbh->do($statement);
	 $rv  =	$dbh->do($statement, \%attr);
	 $rv  =	$dbh->do($statement, \%attr, @bind_values);

	 $ary_ref  = $dbh->selectall_arrayref($statement);
	 $hash_ref = $dbh->selectall_hashref($statement, $key_field);

	 $ary_ref  = $dbh->selectcol_arrayref($statement);
	 $ary_ref  = $dbh->selectcol_arrayref($statement, \%attr);

	 @row_ary  = $dbh->selectrow_array($statement);
	 $ary_ref  = $dbh->selectrow_arrayref($statement);
	 $hash_ref = $dbh->selectrow_hashref($statement);

	 $sth =	$dbh->prepare($statement);
	 $sth =	$dbh->prepare_cached($statement);

	 $rc = $sth->bind_param($p_num,	$bind_value);
	 $rc = $sth->bind_param($p_num,	$bind_value, $bind_type);
	 $rc = $sth->bind_param($p_num,	$bind_value, \%attr);

	 $rv = $sth->execute;
	 $rv = $sth->execute(@bind_values);
	 $rv = $sth->execute_array(\%attr, ...);

	 $rc = $sth->bind_col($col_num,	\$col_variable);
	 $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);

	 @row_ary  = $sth->fetchrow_array;
	 $ary_ref  = $sth->fetchrow_arrayref;
	 $hash_ref = $sth->fetchrow_hashref;

	 $ary_ref  = $sth->fetchall_arrayref;
	 $ary_ref  = $sth->fetchall_arrayref( $slice, $max_rows	);

	 $hash_ref = $sth->fetchall_hashref( $key_field	);

	 $rv  =	$sth->rows;

	 $rc  =	$dbh->begin_work;
	 $rc  =	$dbh->commit;
	 $rc  =	$dbh->rollback;

	 $quoted_string	= $dbh->quote($string);

	 $rc  =	$h->err;
	 $str =	$h->errstr;
	 $rv  =	$h->state;

	 $rc  =	$dbh->disconnect;

       The synopsis above only lists the major methods and parameters.

   GETTING HELP
       General

       Before asking any questions, reread this	document, consult the archives
       and read	the DBI	FAQ. The archives are listed at	the end	of this
       document	and on the DBI home page <http://dbi.perl.org/support/>

       You might also like to read the Advanced	DBI Tutorial at
       <http://www.slideshare.net/Tim.Bunce/dbi-advanced-tutorial-2007>

       To help you make	the best use of	the dbi-users mailing list, and	any
       other lists or forums you may use, I recommend that you read "Getting
       Answers"	by Mike	Ash: <http://mikeash.com/getting_answers.html>.

       Mailing Lists

       If you have questions about DBI,	or DBD driver modules, you can get
       help from the dbi-users@perl.org	mailing	list. This is the best way to
       get help. You don't have	to subscribe to	the list in order to post,
       though I'd recommend it.	You can	get help on subscribing	and using the
       list by emailing	dbi-users-help@perl.org.

       Please note that	Tim Bunce does not maintain the	mailing	lists or the
       web pages (generous volunteers do that).	 So please don't send mail
       directly	to him;	he just	doesn't	have the time to answer	questions
       personally. The dbi-users mailing list has lots of experienced people
       who should be able to help you if you need it. If you do	email Tim he
       is very likely to just forward it to the	mailing	list.

       IRC

       DBI IRC Channel:	#dbi on	irc.perl.org (<irc://irc.perl.org/#dbi>)

       Online

       StackOverflow has a DBI tag
       <http://stackoverflow.com/questions/tagged/dbi> with over 800
       questions.

       The DBI home page at <http://dbi.perl.org/> and the DBI FAQ at
       <http://faq.dbi-support.com/> may be worth a visit.  They include links
       to other	resources, but are rather out-dated.

       Reporting a Bug

       If you think you've found a bug then please read	"How to	Report Bugs
       Effectively" by Simon Tatham:
       <http://www.chiark.greenend.org.uk/~sgtatham/bugs.html>.

       If you think you've found a memory leak then read "Memory Leaks".

       Your problem is most likely related to the specific DBD driver module
       you're using. If	that's the case	then click on the 'Bugs' link on the
       <http://metacpan.org> page for your driver. Only	submit a bug report
       against the DBI itself if you're	sure that your issue isn't related to
       the driver you're using.

   NOTES
       This is the DBI specification that corresponds to DBI version 1.642
       (see DBI::Changes for details).

       The DBI is evolving at a	steady pace, so	it's good to check that	you
       have the	latest copy.

       The significant user-visible changes in each release are	documented in
       the DBI::Changes	module so you can read them by executing "perldoc
       DBI::Changes".

       Some DBI	changes	require	changes	in the drivers,	but the	drivers	can
       take some time to catch up. Newer versions of the DBI have added
       features	that may not yet be supported by the drivers you use.  Talk to
       the authors of your drivers if you need a new feature that is not yet
       supported.

       Features	added after DBI	1.21 (February 2002) are marked	in the text
       with the	version	number of the DBI release they first appeared in.

       Extensions to the DBI API often use the "DBIx::*" namespace.  See
       "Naming Conventions and Name Space". DBI	extension modules can be found
       at <https://metacpan.org/search?q=DBIx>.	 And all modules related to
       the DBI can be found at <https://metacpan.org/search?q=DBI>.

DESCRIPTION
       The DBI is a database access module for the Perl	programming language.
       It defines a set	of methods, variables, and conventions that provide a
       consistent database interface, independent of the actual	database being
       used.

       It is important to remember that	the DBI	is just	an interface.  The DBI
       is a layer of "glue" between an application and one or more database
       driver modules.	It is the driver modules which do most of the real
       work. The DBI provides a	standard interface and framework for the
       drivers to operate within.

       This document often uses	terms like references, objects,	methods.  If
       you're not familiar with	those terms then it would be a good idea to
       read at least the following perl	manuals	first: perlreftut, perldsc,
       perllol,	and perlboot.

   Architecture	of a DBI Application
		    |<-	Scope of DBI ->|
			 .-.   .--------------.	  .-------------.
	 .-------.	 | |---| XYZ Driver   |---| XYZ	Engine	|
	 | Perl	 |	 | |   `--------------'	  `-------------'
	 | script|  |A|	 |D|   .--------------.	  .-------------.
	 | using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
	 | DBI	 |  |I|	 |I|   `--------------'	  `-------------'
	 | API	 |	 | |...
	 |methods|	 | |...	Other drivers
	 `-------'	 | |...
			 `-'

       The API,	or Application Programming Interface, defines the call
       interface and variables for Perl	scripts	to use.	The API	is implemented
       by the Perl DBI extension.

       The DBI "dispatches" the	method calls to	the appropriate	driver for
       actual execution.  The DBI is also responsible for the dynamic loading
       of drivers, error checking and handling,	providing default
       implementations for methods, and	many other non-database	specific
       duties.

       Each driver contains implementations of the DBI methods using the
       private interface functions of the corresponding	database engine.  Only
       authors of sophisticated/multi-database applications or generic library
       functions need be concerned with	drivers.

   Notation and	Conventions
       The following conventions are used in this document:

	 $dbh	 Database handle object
	 $sth	 Statement handle object
	 $drh	 Driver	handle object (rarely seen or used in applications)
	 $h	 Any of	the handle types above ($dbh, $sth, or $drh)
	 $rc	 General Return	Code  (boolean:	true=ok, false=error)
	 $rv	 General Return	Value (typically an integer)
	 @ary	 List of values	returned from the database, typically a	row of data
	 $rows	 Number	of rows	processed (if available, else -1)
	 $fh	 A filehandle
	 undef	 NULL values are represented by	undefined values in Perl
	 \%attr	 Reference to a	hash of	attribute values passed	to methods

       Note that Perl will automatically destroy database and statement	handle
       objects if all references to them are deleted.

   Outline Usage
       To use DBI, first you need to load the DBI module:

	 use DBI;
	 use strict;

       (The "use strict;" isn't	required but is	strongly recommended.)

       Then you	need to	"connect" to your data source and get a	handle for
       that connection:

	 $dbh =	DBI->connect($dsn, $user, $password,
			     { RaiseError => 1,	AutoCommit => 0	});

       Since connecting	can be expensive, you generally	just connect at	the
       start of	your program and disconnect at the end.

       Explicitly defining the required	"AutoCommit" behaviour is strongly
       recommended and may become mandatory in a later version.	 This
       determines whether changes are automatically committed to the database
       when executed, or need to be explicitly committed later.

       The DBI allows an application to	"prepare" statements for later
       execution.  A prepared statement	is identified by a statement handle
       held in a Perl variable.	 We'll call the	Perl variable $sth in our
       examples.

       The typical method call sequence	for a "SELECT" statement is:

	 prepare,
	   execute, fetch, fetch, ...
	   execute, fetch, fetch, ...
	   execute, fetch, fetch, ...

       for example:

	 $sth =	$dbh->prepare("SELECT foo, bar FROM table WHERE	baz=?");

	 $sth->execute(	$baz );

	 while ( @row =	$sth->fetchrow_array ) {
	   print "@row\n";
	 }

       For queries that	are not	executed many times at once, it	is often
       cleaner to use the higher level select wrappers:

	 $row_hashref =	$dbh->selectrow_hashref("SELECT	foo, bar FROM table WHERE baz=?", undef, $baz);

	 $arrayref_of_row_hashrefs = $dbh->selectall_arrayref(
	   "SELECT foo,	bar FROM table WHERE baz BETWEEN ? AND ?",
	   { Slice => {} }, $baz_min, $baz_max);

       The typical method call sequence	for a non-"SELECT" statement is:

	 prepare,
	   execute,
	   execute,
	   execute.

       for example:

	 $sth =	$dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");

	 while(<CSV>) {
	   chomp;
	   my ($foo,$bar,$baz) = split /,/;
	       $sth->execute( $foo, $bar, $baz );
	 }

       The "do()" method is a wrapper of prepare and execute that can be
       simpler for non repeated	non-"SELECT" statements	(or with drivers that
       don't support placeholders):

	 $rows_affected	= $dbh->do("UPDATE your_table SET foo =	foo + 1");

	 $rows_affected	= $dbh->do("DELETE FROM	table WHERE foo	= ?", undef, $foo);

       To commit your changes to the database (when "AutoCommit" is off):

	 $dbh->commit;	# or call $dbh->rollback; to undo changes

       Finally,	when you have finished working with the	data source, you
       should "disconnect" from	it:

	 $dbh->disconnect;

   General Interface Rules & Caveats
       The DBI does not	have a concept of a "current session". Every session
       has a handle object (i.e., a $dbh) returned from	the "connect" method.
       That handle object is used to invoke database related methods.

       Most data is returned to	the Perl script	as strings. (Null values are
       returned	as "undef".)  This allows arbitrary precision numeric data to
       be handled without loss of accuracy.  Beware that Perl may not preserve
       the same	accuracy when the string is used as a number.

       Dates and times are returned as character strings in the	current
       default format of the corresponding database engine.  Time zone effects
       are database/driver dependent.

       Perl supports binary data in Perl strings, and the DBI will pass	binary
       data to and from	the driver without change. It is up to the driver
       implementors to decide how they wish to handle such binary data.

       Perl supports two kinds of strings: Unicode (utf8 internally) and non-
       Unicode (defaults to iso-8859-1 if forced to assume an encoding).
       Drivers should accept both kinds	of strings and,	if required, convert
       them to the character set of the	database being used. Similarly,	when
       fetching	from the database character data that isn't iso-8859-1 the
       driver should convert it	into utf8.

       Multiple	SQL statements may not be combined in a	single statement
       handle ($sth), although some databases and drivers do support this
       (notably	Sybase and SQL Server).

       Non-sequential record reads are not supported in	this version of	the
       DBI.  In	other words, records can only be fetched in the	order that the
       database	returned them, and once	fetched	they are forgotten.

       Positioned updates and deletes are not directly supported by the	DBI.
       See the description of the "CursorName" attribute for an	alternative.

       Individual driver implementors are free to provide any private
       functions and/or	handle attributes that they feel are useful.  Private
       driver functions	can be invoked using the DBI "func()" method.  Private
       driver attributes are accessed just like	standard attributes.

       Many methods have an optional "\%attr" parameter	which can be used to
       pass information	to the driver implementing the method. Except where
       specifically documented,	the "\%attr" parameter can only	be used	to
       pass driver specific hints. In general, you can ignore "\%attr"
       parameters or pass it as	"undef".

   Naming Conventions and Name Space
       The DBI package and all packages	below it ("DBI::*") are	reserved for
       use by the DBI. Extensions and related modules use the "DBIx::"
       namespace (see <http://www.perl.com/CPAN/modules/by-module/DBIx/>).
       Package names beginning with "DBD::" are	reserved for use by DBI
       database	drivers.  All environment variables used by the	DBI or by
       individual DBDs begin with ""DBI_"" or ""DBD_"".

       The letter case used for	attribute names	is significant and plays an
       important part in the portability of DBI	scripts.  The case of the
       attribute name is used to signify who defined the meaning of that name
       and its values.

	 Case of name  Has a meaning defined by
	 ------------  ------------------------
	 UPPER_CASE    Standards, e.g.,	 X/Open, ISO SQL92 etc (portable)
	 MixedCase     DBI API (portable), underscores are not used.
	 lower_case    Driver or database engine specific (non-portable)

       It is of	the utmost importance that Driver developers only use
       lowercase attribute names when defining private attributes. Private
       attribute names must be prefixed	with the driver	name or	suitable
       abbreviation (e.g., ""ora_"" for	Oracle,	""ing_"" for Ingres, etc).

   SQL - A Query Language
       Most DBI	drivers	require	applications to	use a dialect of SQL
       (Structured Query Language) to interact with the	database engine.  The
       "Standards Reference Information" section provides links	to useful
       information about SQL.

       The DBI itself does not mandate or require any particular language to
       be used;	it is language independent. In ODBC terms, the DBI is in
       "pass-thru" mode, although individual drivers might not be. The only
       requirement is that queries and other statements	must be	expressed as a
       single string of	characters passed as the first argument	to the
       "prepare" or "do" methods.

       For an interesting diversion on the real	history	of RDBMS and SQL, from
       the people who made it happen, see:

	 http://www.mcjones.org/System_R/SQL_Reunion_95/sqlr95.html

       Follow the "Full	Contents" then "Intergalactic dataspeak" links for the
       SQL history.

   Placeholders	and Bind Values
       Some drivers support placeholders and bind values.  Placeholders, also
       called parameter	markers, are used to indicate values in	a database
       statement that will be supplied later, before the prepared statement is
       executed.  For example, an application might use	the following to
       insert a	row of data into the SALES table:

	 INSERT	INTO sales (product_code, qty, price) VALUES (?, ?, ?)

       or the following, to select the description for a product:

	 SELECT	description FROM products WHERE	product_code = ?

       The "?" characters are the placeholders.	 The association of actual
       values with placeholders	is known as binding, and the values are
       referred	to as bind values.  Note that the "?" is not enclosed in
       quotation marks,	even when the placeholder represents a string.

       Some drivers also allow placeholders like ":"name and ":"N (e.g., ":1",
       ":2", and so on)	in addition to "?", but	their use is not portable.

       If the ":"N form	of placeholder is supported by the driver you're
       using, then you should be able to use either "bind_param" or "execute"
       to bind values. Check your driver documentation.

       Some drivers allow you to prevent the recognition of a placeholder by
       placing a single	backslash character ("\") immediately before it. The
       driver will remove the backslash	character and ignore the placeholder,
       passing it unchanged to the backend. If the driver supports this	then
       "get_info"(9000)	will return true.

       With most drivers, placeholders can't be	used for any element of	a
       statement that would prevent the	database server	from validating	the
       statement and creating a	query execution	plan for it. For example:

	 "SELECT name, age FROM	?"	   # wrong (will probably fail)
	 "SELECT name, ?   FROM	people"	   # wrong (but	may not	'fail')

       Also, placeholders can only represent single scalar values.  For
       example,	the following statement	won't work as expected for more	than
       one value:

	 "SELECT name, age FROM	people WHERE name IN (?)"    # wrong
	 "SELECT name, age FROM	people WHERE name IN (?,?)"  # two names

       When using placeholders with the	SQL "LIKE" qualifier, you must
       remember	that the placeholder substitutes for the whole string.	So you
       should use ""...	LIKE ? ..."" and include any wildcard characters in
       the value that you bind to the placeholder.

       NULL Values

       Undefined values, or "undef", are used to indicate NULL values.	You
       can insert and update columns with a NULL value as you would a non-NULL
       value.  These examples insert and update	the column "age" with a	NULL
       value:

	 $sth =	$dbh->prepare(qq{
	   INSERT INTO people (fullname, age) VALUES (?, ?)
	 });
	 $sth->execute("Joe Bloggs", undef);

	 $sth =	$dbh->prepare(qq{
	   UPDATE people SET age = ? WHERE fullname = ?
	 });
	 $sth->execute(undef, "Joe Bloggs");

       However,	care must be taken when	trying to use NULL values in a "WHERE"
       clause.	Consider:

	 SELECT	fullname FROM people WHERE age = ?

       Binding an "undef" (NULL) to the	placeholder will not select rows which
       have a NULL "age"!  At least for	database engines that conform to the
       SQL standard.  Refer to the SQL manual for your database	engine or any
       SQL book	for the	reasons	for this.  To explicitly select	NULLs you have
       to say ""WHERE age IS NULL"".

       A common	issue is to have a code	fragment handle	a value	that could be
       either "defined"	or "undef" (non-NULL or	NULL) at runtime.  A simple
       technique is to prepare the appropriate statement as needed, and
       substitute the placeholder for non-NULL cases:

	 $sql_clause = defined $age? "age = ?" : "age IS NULL";
	 $sth =	$dbh->prepare(qq{
	   SELECT fullname FROM	people WHERE $sql_clause
	 });
	 $sth->execute(defined $age ? $age : ());

       The following technique illustrates qualifying a	"WHERE"	clause with
       several columns,	whose associated values	("defined" or "undef") are in
       a hash %h:

	 for my	$col ("age", "phone", "email") {
	   if (defined $h{$col}) {
	     push @sql_qual, "$col = ?";
	     push @sql_bind, $h{$col};
	   }
	   else	{
	     push @sql_qual, "$col IS NULL";
	   }
	 }
	 $sql_clause = join(" AND ", @sql_qual);
	 $sth =	$dbh->prepare(qq{
	     SELECT fullname FROM people WHERE $sql_clause
	 });
	 $sth->execute(@sql_bind);

       The techniques above call prepare for the SQL statement with each call
       to execute.  Because calls to prepare() can be expensive, performance
       can suffer when an application iterates many times over statements like
       the above.

       A better	solution is a single "WHERE" clause that supports both NULL
       and non-NULL comparisons.  Its SQL statement would need to be prepared
       only once for all cases,	thus improving performance.  Several examples
       of "WHERE" clauses that support this are	presented below.  But each
       example lacks portability, robustness, or simplicity.  Whether an
       example is supported on your database engine depends on what SQL
       extensions it provides, and where it supports the "?"  placeholder in a
       statement.

	 0)  age = ?
	 1)  NVL(age, xx) = NVL(?, xx)
	 2)  ISNULL(age, xx) = ISNULL(?, xx)
	 3)  DECODE(age, ?, 1, 0) = 1
	 4)  age = ? OR	(age IS	NULL AND ? IS NULL)
	 5)  age = ? OR	(age IS	NULL AND SP_ISNULL(?) =	1)
	 6)  age = ? OR	(age IS	NULL AND ? = 1)

       Statements formed with the above	"WHERE"	clauses	require	execute
       statements as follows.  The arguments are required, whether their
       values are "defined" or "undef".

	 0,1,2,3)  $sth->execute($age);
	 4,5)	   $sth->execute($age, $age);
	 6)	   $sth->execute($age, defined($age) ? 0 : 1);

       Example 0 should	not work (as mentioned earlier), but may work on a few
       database	engines	anyway (e.g. Sybase).  Example 0 is part of examples
       4, 5, and 6, so if example 0 works, these other examples	may work, even
       if the engine does not properly support the right hand side of the "OR"
       expression.

       Examples	1 and 2	are not	robust:	they require that you provide a	valid
       column value xx (e.g. '~') which	is not present in any row.  That means
       you must	have some notion of what data won't be stored in the column,
       and expect clients to adhere to that.

       Example 5 requires that you provide a stored procedure (SP_ISNULL in
       this example) that acts as a function: it checks	whether	a value	is
       null, and returns 1 if it is, or	0 if not.

       Example 6, the least simple, is probably	the most portable, i.e., it
       should work with	most, if not all, database engines.

       Here is a table that indicates which examples above are known to	work
       on various database engines:

			  -----Examples------
			  0  1	2  3  4	 5  6
			  -  -	-  -  -	 -  -
	 Oracle	9	  N  Y	N  Y  Y	 ?  Y
	 Informix IDS 9	  N  N	N  Y  N	 Y  Y
	 MS SQL		  N  N	Y  N  Y	 ?  Y
	 Sybase		  Y  N	N  N  N	 N  Y
	 AnyData,DBM,CSV  Y  N	N  N  Y	 Y* Y
	 SQLite	3.3	  N  N	N  N  Y	 N  N
	 MSAccess	  N  N	N  N  Y	 N  Y

       * Works only because Example 0 works.

       DBI provides a sample perl script that will test	the examples above on
       your database engine and	tell you which ones work.  It is located in
       the ex/ subdirectory of the DBI source distribution, or here:
       <https://github.com/perl5-dbi/dbi/blob/master/ex/perl_dbi_nulls_test.pl>
       Please use the script to	help us	fill-in	and maintain this table.

       Performance

       Without using placeholders, the insert statement	shown previously would
       have to contain the literal values to be	inserted and would have	to be
       re-prepared and re-executed for each row. With placeholders, the	insert
       statement only needs to be prepared once. The bind values for each row
       can be given to the "execute" method each time it's called. By avoiding
       the need	to re-prepare the statement for	each row, the application
       typically runs many times faster. Here's	an example:

	 my $sth = $dbh->prepare(q{
	   INSERT INTO sales (product_code, qty, price)	VALUES (?, ?, ?)
	 }) or die $dbh->errstr;
	 while (<>) {
	     chomp;
	     my	($product_code,	$qty, $price) =	split /,/;
	     $sth->execute($product_code, $qty,	$price)	or die $dbh->errstr;
	 }
	 $dbh->commit or die $dbh->errstr;

       See "execute" and "bind_param" for more details.

       The "q{...}" style quoting used in this example avoids clashing with
       quotes that may be used in the SQL statement. Use the double-quote like
       "qq{...}" operator if you want to interpolate variables into the
       string.	See "Quote and Quote-like Operators" in	perlop for more
       details.

       See also	the "bind_columns" method, which is used to associate Perl
       variables with the output columns of a "SELECT" statement.

THE DBI	PACKAGE	AND CLASS
       In this section,	we cover the DBI class methods,	utility	functions, and
       the dynamic attributes associated with generic DBI handles.

   DBI Constants
       Constants representing the values of the	SQL standard types can be
       imported	individually by	name, or all together by importing the special
       ":sql_types" tag.

       The names and values of all the defined SQL standard types can be
       produced	like this:

	 foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
	   printf "%s=%d\n", $_, &{"DBI::$_"};
	 }

       These constants are defined by SQL/CLI, ODBC or both.  "SQL_BIGINT" has
       conflicting codes in SQL/CLI and	ODBC, DBI uses the ODBC	one.

       See the "type_info", "type_info_all", and "bind_param" methods for
       possible	uses.

       Note that just because the DBI defines a	named constant for a given
       data type doesn't mean that drivers will	support	that data type.

   DBI Class Methods
       The following methods are provided by the DBI class:

       "parse_dsn"

	 ($scheme, $driver, $attr_string, $attr_hash, $driver_dsn) = DBI->parse_dsn($dsn)
	     or	die "Can't parse DBI DSN '$dsn'";

       Breaks apart a DBI Data Source Name (DSN) and returns the individual
       parts. If $dsn doesn't contain a	valid DSN then parse_dsn() returns an
       empty list.

       $scheme is the first part of the	DSN and	is currently always 'dbi'.
       $driver is the driver name, possibly defaulted to $ENV{DBI_DRIVER}, and
       may be undefined.  $attr_string is the contents of the optional
       attribute string, which may be undefined.  If $attr_string is not empty
       then $attr_hash is a reference to a hash	containing the parsed
       attribute names and values.  $driver_dsn	is the last part of the	DBI
       DSN string. For example:

	 ($scheme, $driver, $attr_string, $attr_hash, $driver_dsn)
	     = DBI->parse_dsn("dbi:MyDriver(RaiseError=>1):db=test;port=42");
	 $scheme      =	'dbi';
	 $driver      =	'MyDriver';
	 $attr_string =	'RaiseError=>1';
	 $attr_hash   =	{ 'RaiseError' => '1' };
	 $driver_dsn  =	'db=test;port=42';

       The parse_dsn() method was added	in DBI 1.43.

       "connect"

	 $dbh =	DBI->connect($data_source, $username, $password)
		   or die $DBI::errstr;
	 $dbh =	DBI->connect($data_source, $username, $password, \%attr)
		   or die $DBI::errstr;

       Establishes a database connection, or session, to the requested
       $data_source.  Returns a	database handle	object if the connection
       succeeds. Use "$dbh->disconnect"	to terminate the connection.

       If the connect fails (see below), it returns "undef" and	sets both
       $DBI::err and $DBI::errstr. (It does not	explicitly set $!.) You	should
       generally test the return status	of "connect" and "print	$DBI::errstr"
       if it has failed.

       Multiple	simultaneous connections to multiple databases through
       multiple	drivers	can be made via	the DBI. Simply	make one "connect"
       call for	each database and keep a copy of each returned database
       handle.

       The $data_source	value must begin with ""dbi:"driver_name":"".  The
       driver_name specifies the driver	that will be used to make the
       connection. (Letter case	is significant.)

       As a convenience, if the	$data_source parameter is undefined or empty,
       the DBI will substitute the value of the	environment variable
       "DBI_DSN".  If just the driver_name part	is empty (i.e.,	the
       $data_source prefix is ""dbi::""), the environment variable
       "DBI_DRIVER" is used. If	neither	variable is set, then "connect"	dies.

       Examples	of $data_source	values are:

	 dbi:DriverName:database_name
	 dbi:DriverName:database_name@hostname:port
	 dbi:DriverName:database=database_name;host=hostname;port=port

       There is	no standard for	the text following the driver name. Each
       driver is free to use whatever syntax it	wants. The only	requirement
       the DBI makes is	that all the information is supplied in	a single
       string.	You must consult the documentation for the drivers you are
       using for a description of the syntax they require.

       It is recommended that drivers support the ODBC style, shown in the
       last example above. It is also recommended that they support the	three
       common names '"host"', '"port"',	and '"database"' (plus '"db"' as an
       alias for "database"). This simplifies automatic	construction of	basic
       DSNs: "dbi:$driver:database=$db;host=$host;port=$port".	Drivers	should
       aim to 'do something reasonable'	when given a DSN in this form, but if
       any part	is meaningless for that	driver (such as	'port' for Informix)
       it should generate an error if that part	is not empty.

       If the environment variable "DBI_AUTOPROXY" is defined (and the driver
       in $data_source is not ""Proxy"") then the connect request will
       automatically be	changed	to:

	 $ENV{DBI_AUTOPROXY};dsn=$data_source

       "DBI_AUTOPROXY" is typically set	as
       ""dbi:Proxy:hostname=...;port=..."".  If	$ENV{DBI_AUTOPROXY} doesn't
       begin with '"dbi:"' then	"dbi:Proxy:" will be prepended to it first.
       See the DBD::Proxy documentation	for more details.

       If $username or $password are undefined (rather than just empty), then
       the DBI will substitute the values of the "DBI_USER" and	"DBI_PASS"
       environment variables, respectively.  The DBI will warn if the
       environment variables are not defined.  However,	the everyday use of
       these environment variables is not recommended for security reasons.
       The mechanism is	primarily intended to simplify testing.	 See below for
       alternative way to specify the username and password.

       "DBI->connect" automatically installs the driver	if it has not been
       installed yet. Driver installation either returns a valid driver
       handle, or it dies with an error	message	that includes the string
       ""install_driver"" and the underlying problem. So "DBI->connect"	will
       die on a	driver installation failure and	will only return "undef" on a
       connect failure,	in which case $DBI::errstr will	hold the error
       message.	 Use "eval" if you need	to catch the ""install_driver""	error.

       The $data_source	argument (with the ""dbi:...:""	prefix removed)	and
       the $username and $password arguments are then passed to	the driver for
       processing. The DBI does	not define any interpretation for the contents
       of these	fields.	 The driver is free to interpret the $data_source,
       $username, and $password	fields in any way, and supply whatever
       defaults	are appropriate	for the	engine being accessed.	(Oracle, for
       example,	uses the ORACLE_SID and	TWO_TASK environment variables if no
       $data_source is specified.)

       The "AutoCommit"	and "PrintError" attributes for	each connection
       default to "on".	(See "AutoCommit" and "PrintError" for more
       information.)  However, it is strongly recommended that you explicitly
       define "AutoCommit" rather than rely on the default. The	"PrintWarn"
       attribute defaults to true.  The	"RaiseWarn" attribute defaults to
       false.

       The "\%attr" parameter can be used to alter the default settings	of
       "PrintError", "RaiseError", "AutoCommit", and other attributes. For
       example:

	 $dbh =	DBI->connect($data_source, $user, $pass, {
	       PrintError => 0,
	       AutoCommit => 0
	 });

       The username and	password can also be specified using the attributes
       "Username" and "Password", in which case	they take precedence over the
       $username and $password parameters.

       You can also define connection attribute	values within the $data_source
       parameter. For example:

	 dbi:DriverName(PrintWarn=>0,PrintError=>0,Taint=>1):...

       Individual attributes values specified in this way take precedence over
       any conflicting values specified	via the	"\%attr" parameter to
       "connect".

       The "dbi_connect_method"	attribute can be used to specify which driver
       method should be	called to establish the	connection. The	only useful
       values are 'connect', 'connect_cached', or some specialized case	like
       'Apache::DBI::connect' (which is	automatically the default when running
       within Apache).

       Where possible, each session ($dbh) is independent from the
       transactions in other sessions. This is useful when you need to hold
       cursors open across transactions--for example, if you use one session
       for your	long lifespan cursors (typically read-only) and	another	for
       your short update transactions.

       For compatibility with old DBI scripts, the driver can be specified by
       passing its name	as the fourth argument to "connect" (instead of
       "\%attr"):

	 $dbh =	DBI->connect($data_source, $user, $pass, $driver);

       In this "old-style" form	of "connect", the $data_source should not
       start with ""dbi:driver_name:"".	(If it does, the embedded driver_name
       will be ignored). Also note that	in this	older form of "connect", the
       "$dbh->{AutoCommit}" attribute is undefined, the	"$dbh->{PrintError}"
       attribute is off, and the old "DBI_DBNAME" environment variable is
       checked if "DBI_DSN" is not defined. Beware that	this "old-style"
       "connect" will soon be withdrawn	in a future version of DBI.

       "connect_cached"

	 $dbh =	DBI->connect_cached($data_source, $username, $password)
		   or die $DBI::errstr;
	 $dbh =	DBI->connect_cached($data_source, $username, $password,	\%attr)
		   or die $DBI::errstr;

       "connect_cached"	is like	"connect", except that the database handle
       returned	is also	stored in a hash associated with the given parameters.
       If another call is made to "connect_cached" with	the same parameter
       values, then the	corresponding cached $dbh will be returned if it is
       still valid.  The cached	database handle	is replaced with a new
       connection if it	has been disconnected or if the	"ping" method fails.

       Note that the behaviour of this method differs in several respects from
       the behaviour of	persistent connections implemented by Apache::DBI.
       However,	if Apache::DBI is loaded then "connect_cached" will use	it.

       Caching connections can be useful in some applications, but it can also
       cause problems, such as too many	connections, and so should be used
       with care. In particular, avoid changing	the attributes of a database
       handle created via connect_cached() because it will affect other	code
       that may	be using the same handle. When connect_cached()	returns	a
       handle the attributes will be reset to their initial values.  This can
       cause problems, especially with the "AutoCommit"	attribute.

       Also, to	ensure that the	attributes passed are always the same, avoid
       passing references inline. For example, the "Callbacks" attribute is
       specified as a hash reference. Be sure to declare it external to	the
       call to connect_cached(), such that the hash reference is not re-
       created on every	call. A	package-level lexical works well:

	 package MyDBH;
	 my $cb	= {
	     'connect_cached.reused' =>	sub { delete $_[4]->{AutoCommit} },
	 };

	 sub dbh {
	     DBI->connect_cached( $dsn,	$username, $auth, { Callbacks => $cb });
	 }

       Where multiple separate parts of	a program are using connect_cached()
       to connect to the same database with the	same (initial) attributes it
       is a good idea to add a private attribute to the	connect_cached() call
       to effectively limit the	scope of the caching. For example:

	 DBI->connect_cached(..., { private_foo_cachekey => "Bar", ... });

       Handles returned	from that connect_cached() call	will only be returned
       by other	connect_cached() call elsewhere	in the code if those other
       calls also pass in the same attribute values, including the private
       one.  (I've used	"private_foo_cachekey" here as an example, you can use
       any attribute name with a "private_" prefix.)

       Taking that one step further, you can limit a particular
       connect_cached()	call to	return handles unique to that one place	in the
       code by setting the private attribute to	a unique value for that	place:

	 DBI->connect_cached(..., { private_foo_cachekey => __FILE__.__LINE__, ... });

       By using	a private attribute you	still get connection caching for the
       individual calls	to connect_cached() but, by making separate database
       connections for separate	parts of the code, the database	handles	are
       isolated	from any attribute changes made	to other handles.

       The cache can be	accessed (and cleared) via the "CachedKids" attribute:

	 my $CachedKids_hashref	= $dbh->{Driver}->{CachedKids};
	 %$CachedKids_hashref =	() if $CachedKids_hashref;

       "available_drivers"

	 @ary =	DBI->available_drivers;
	 @ary =	DBI->available_drivers($quiet);

       Returns a list of all available drivers by searching for	"DBD::*"
       modules through the directories in @INC.	By default, a warning is given
       if some drivers are hidden by others of the same	name in	earlier
       directories. Passing a true value for $quiet will inhibit the warning.

       "installed_drivers"

	 %drivers = DBI->installed_drivers();

       Returns a list of driver	name and driver	handle pairs for all drivers
       'installed' (loaded) into the current process.  The driver name does
       not include the 'DBD::' prefix.

       To get a	list of	all drivers available in your perl installation	you
       can use "available_drivers".

       Added in	DBI 1.49.

       "installed_versions"

	 DBI->installed_versions;
	 @ary  = DBI->installed_versions;
	 $hash = DBI->installed_versions;

       Calls available_drivers() and attempts to load each of them in turn
       using install_driver().	For each load that succeeds the	driver name
       and version number are added to a hash. When running under
       DBI::PurePerl drivers which appear not be pure-perl are ignored.

       When called in array context the	list of	successfully loaded drivers is
       returned	(without the 'DBD::' prefix).

       When called in scalar context an	extra entry for	the "DBI" is added
       (and "DBI::PurePerl" if appropriate) and	a reference to the hash	is
       returned.

       When called in a	void context the installed_versions() method will
       print out a formatted list of the hash contents,	one per	line, along
       with some other information about the DBI version and OS.

       Due to the potentially high memory cost and unknown risks of loading in
       an unknown number of drivers that just happen to	be installed on	the
       system, this method is not recommended for general use.	Use
       available_drivers() instead.

       The installed_versions()	method is primarily intended as	a quick	way to
       see from	the command line what's	installed. For example:

	 perl -MDBI -e 'DBI->installed_versions'

       The installed_versions()	method was added in DBI	1.38.

       "data_sources"

	 @ary =	DBI->data_sources($driver);
	 @ary =	DBI->data_sources($driver, \%attr);

       Returns a list of data sources (databases) available via	the named
       driver.	If $driver is empty or "undef",	then the value of the
       "DBI_DRIVER" environment	variable is used.

       The driver will be loaded if it hasn't been already. Note that if the
       driver loading fails then data_sources()	dies with an error message
       that includes the string	""install_driver"" and the underlying problem.

       Data sources are	returned in a form suitable for	passing	to the
       "connect" method	(that is, they will include the	""dbi:$driver:""
       prefix).

       Note that many drivers have no way of knowing what data sources might
       be available for	it. These drivers return an empty or incomplete	list
       or may require driver-specific attributes.

       There is	also a data_sources() method defined for database handles.

       "trace"

	 DBI->trace($trace_setting)
	 DBI->trace($trace_setting, $trace_filename)
	 DBI->trace($trace_setting, $trace_filehandle)
	 $trace_setting	= DBI->trace;

       The "DBI->trace"	method sets the	global default trace settings and
       returns the previous trace settings. It can also	be used	to change
       where the trace output is sent.

       There's a similar method, "$h->trace", which sets the trace settings
       for the specific	handle it's called on.

       See the "TRACING" section for full details about	the DBI's powerful
       tracing facilities.

       "visit_handles"

	 DBI->visit_handles( $coderef );
	 DBI->visit_handles( $coderef, $info );

       Where $coderef is a reference to	a subroutine and $info is an arbitrary
       value which, if undefined, defaults to a	reference to an	empty hash.
       Returns $info.

       For each	installed driver handle, if any, $coderef is invoked as:

	 $coderef->($driver_handle, $info);

       If the execution	of $coderef returns a true value then
       "visit_child_handles" is	called on that child handle and	passed the
       returned	value as $info.

       For example:

	 my $info = $dbh->{Driver}->visit_child_handles(sub {
	     my	($h, $info) = @_;
	     ++$info->{	$h->{Type} }; #	count types of handles (dr/db/st)
	     return $info; # visit kids
	 });

       See also	"visit_child_handles".

   DBI Utility Functions
       In addition to the DBI methods listed in	the previous section, the DBI
       package also provides several utility functions.

       These can be imported into your code by listing them in the "use"
       statement. For example:

	 use DBI qw(neat data_diff);

       Alternatively, all these	utility	functions (except hash)	can be
       imported	using the ":utils" import tag. For example:

	 use DBI qw(:utils);

       "data_string_desc"

	 $description =	data_string_desc($string);

       Returns an informal description of the string. For example:

	 UTF8 off, ASCII, 42 characters	42 bytes
	 UTF8 off, non-ASCII, 42 characters 42 bytes
	 UTF8 on, non-ASCII, 4 characters 6 bytes
	 UTF8 on but INVALID encoding, non-ASCII, 4 characters 6 bytes
	 UTF8 off, undef

       The initial "UTF8" on/off refers	to Perl's internal SvUTF8 flag.	 If
       $string has the SvUTF8 flag set but the sequence	of bytes it contains
       are not a valid UTF-8 encoding then data_string_desc() will report
       "UTF8 on	but INVALID encoding".

       The "ASCII" vs "non-ASCII" portion shows	"ASCII"	if all the characters
       in the string are ASCII (have code points <= 127).

       The data_string_desc() function was added in DBI	1.46.

       "data_string_diff"

	 $diff = data_string_diff($a, $b);

       Returns an informal description of the first character difference
       between the strings. If both $a and $b contain the same sequence	of
       characters then data_string_diff() returns an empty string.  For
       example:

	Params a & b	 Result
	------------	 ------
	'aaa', 'aaa'	 ''
	'aaa', 'abc'	 'Strings differ at index 2: a[2]=a, b[2]=b'
	'aaa', undef	 'String b is undef, string a has 3 characters'
	'aaa', 'aa'	 'String b truncated after 2 characters'

       Unicode characters are reported in "\x{XXXX}" format. Unicode code
       points in the range U+0800 to U+08FF are	unassigned and most likely to
       occur due to double-encoding. Characters	in this	range are reported as
       "\x{08XX}='C'" where "C"	is the corresponding latin-1 character.

       The data_string_diff() function only considers logical characters and
       not the underlying encoding. See	"data_diff" for	an alternative.

       The data_string_diff() function was added in DBI	1.46.

       "data_diff"

	 $diff = data_diff($a, $b);
	 $diff = data_diff($a, $b, $logical);

       Returns an informal description of the difference between two strings.
       It calls	"data_string_desc" and "data_string_diff" and returns the
       combined	results	as a multi-line	string.

       For example, "data_diff("abc", "ab\x{263a}")" will return:

	 a: UTF8 off, ASCII, 3 characters 3 bytes
	 b: UTF8 on, non-ASCII,	3 characters 5 bytes
	 Strings differ	at index 2: a[2]=c, b[2]=\x{263A}

       If $a and $b are	identical in both the characters they contain and
       their physical encoding then data_diff()	returns	an empty string.  If
       $logical	is true	then physical encoding differences are ignored (but
       are still reported if there is a	difference in the characters).

       The data_diff() function	was added in DBI 1.46.

       "neat"

	 $str =	neat($value);
	 $str =	neat($value, $maxlen);

       Return a	string containing a neat (and tidy) representation of the
       supplied	value.

       Strings will be quoted, although	internal quotes	will not be escaped.
       Values known to be numeric will be unquoted. Undefined (NULL) values
       will be shown as	"undef"	(without quotes).

       If the string is	flagged	internally as utf8 then	double quotes will be
       used, otherwise single quotes are used and unprintable characters will
       be replaced by dot (.).

       For result strings longer than $maxlen the result string	will be
       truncated to "$maxlen-4"	and ""...'"" will be appended.	If $maxlen is
       0 or "undef", it	defaults to $DBI::neat_maxlen which, in	turn, defaults
       to 400.

       This function is	designed to format values for human consumption.  It
       is used internally by the DBI for "trace" output. It should typically
       not be used for formatting values for database use.  (See also
       "quote".)

       "neat_list"

	 $str =	neat_list(\@listref, $maxlen, $field_sep);

       Calls "neat" on each element of the list	and returns a string
       containing the results joined with $field_sep. $field_sep defaults to
       ", ".

       "looks_like_number"

	 @bool = looks_like_number(@array);

       Returns true for	each element that looks	like a number.	Returns	false
       for each	element	that does not look like	a number.  Returns "undef" for
       each element that is undefined or empty.

       "hash"

	 $hash_value = DBI::hash($buffer, $type);

       Return a	32-bit integer 'hash' value corresponding to the contents of
       $buffer.	 The $type parameter selects which kind	of hash	algorithm
       should be used.

       For the technically curious, type 0 (which is the default if $type
       isn't specified)	is based on the	Perl 5.1 hash except that the value is
       forced to be negative (for obscure historical reasons).	Type 1 is the
       better "Fowler /	Noll / Vo" (FNV) hash. See
       <http://www.isthe.com/chongo/tech/comp/fnv/> for	more information.
       Both types are implemented in C and are very fast.

       This function doesn't have much to do with databases, except that it
       can sometimes be	handy to store such values in a	database.  It also
       doesn't have much to do with perl hashes, like %foo.

       "sql_type_cast"

	 $sts =	DBI::sql_type_cast($sv,	$sql_type, $flags);

       sql_type_cast attempts to cast $sv to the SQL type (see DBI Constants)
       specified in $sql_type. At present only the SQL types "SQL_INTEGER",
       "SQL_DOUBLE" and	"SQL_NUMERIC" are supported.

       For "SQL_INTEGER" the effect is similar to using	the value in an
       expression that requires	an integer. It gives the perl scalar an
       'integer	aspect'.  (Technically the value gains an IV, or possibly a UV
       or NV if	the value is too large for an IV.)

       For "SQL_DOUBLE"	the effect is similar to using the value in an
       expression that requires	a general numeric value. It gives the perl
       scalar a	'numeric aspect'.  (Technically	the value gains	an NV.)

       "SQL_NUMERIC" is	similar	to "SQL_INTEGER" or "SQL_DOUBLE" but more
       general and more	cautious.  It will look	at the string first and	if it
       looks like an integer (that will	fit in an IV or	UV) it will act	like
       "SQL_INTEGER", if it looks like a floating point	value it will act like
       "SQL_DOUBLE", if	it looks like neither then it will do nothing -	and
       thereby avoid the warnings that would be	generated by "SQL_INTEGER" and
       "SQL_DOUBLE" when given non-numeric data.

       $flags may be:

       "DBIstcf_DISCARD_STRING"
	   If this flag	is specified then when the driver successfully casts
	   the bound perl scalar to a non-string type then the string portion
	   of the scalar will be discarded.

       "DBIstcf_STRICT"
	   If $sv cannot be cast to the	requested $sql_type then by default it
	   is left untouched and no error is generated.	If you specify
	   "DBIstcf_STRICT" and	the cast fails,	this will generate an error.

       The returned $sts value is:

	 -2 sql_type is	not handled
	 -1 sv is undef	so unchanged
	  0 sv could not be cast cleanly and DBIstcf_STRICT was	used
	  1 sv could not be cast and DBIstcf_STRICT was	not used
	  2 sv was cast	successfully

       This method is exported by the :utils tag and was introduced in DBI
       1.611.

   DBI Dynamic Attributes
       Dynamic attributes are always associated	with the last handle used
       (that handle is represented by $h in the	descriptions below).

       Where an	attribute is equivalent	to a method call, then refer to	the
       method call for all related documentation.

       Warning:	these attributes are provided as a convenience but they	do
       have limitations. Specifically, they have a short lifespan: because
       they are	associated with	the last handle	used, they should only be used
       immediately after calling the method that "sets"	them.  If in any
       doubt, use the corresponding method call.

       $DBI::err

       Equivalent to "$h->err".

       $DBI::errstr

       Equivalent to "$h->errstr".

       $DBI::state

       Equivalent to "$h->state".

       $DBI::rows

       Equivalent to "$h->rows". Please	refer to the documentation for the
       "rows" method.

       $DBI::lasth

       Returns the DBI object handle used for the most recent DBI method call.
       If the last DBI method call was a DESTROY then $DBI::lasth will return
       the handle of the parent	of the destroyed handle, if there is one.

METHODS	COMMON TO ALL HANDLES
       The following methods can be used by all	types of DBI handles.

       "err"

	 $rv = $h->err;

       Returns the native database engine error	code from the last driver
       method called. The code is typically an integer but you should not
       assume that.

       The DBI resets $h->err to undef before almost all DBI method calls, so
       the value only has a short lifespan. Also, for most drivers, the
       statement handles share the same	error variable as the parent database
       handle, so calling a method on one handle may reset the error on	the
       related handles.

       (Methods	which don't reset err before being called include err()	and
       errstr(), obviously, state(), rows(), func(), trace(), trace_msg(),
       ping(), and the tied hash attribute FETCH() and STORE() methods.)

       If you need to test for specific	error conditions and have your program
       be portable to different	database engines, then you'll need to
       determine what the corresponding	error codes are	for all	those engines
       and test	for all	of them.

       The DBI uses the	value of $DBI::stderr as the "err" value for internal
       errors.	Drivers	should also do likewise.  The default value for
       $DBI::stderr is 2000000000.

       A driver	may return 0 from err()	to indicate a warning condition	after
       a method	call. Similarly, a driver may return an	empty string to
       indicate	a 'success with	information' condition.	In both	these cases
       the value is false but not undef. The errstr() and state() methods may
       be used to retrieve extra information in	these cases.

       See "set_err" for more information.

       "errstr"

	 $str =	$h->errstr;

       Returns the native database engine error	message	from the last DBI
       method called. This has the same	lifespan issues	as the "err" method
       described above.

       The returned string may contain multiple	messages separated by newline
       characters.

       The errstr() method should not be used to test for errors, use err()
       for that, because drivers may return 'success with information' or
       warning messages	via errstr() for methods that have not 'failed'.

       See "set_err" for more information.

       "state"

	 $str =	$h->state;

       Returns a state code in the standard SQLSTATE five character format.
       Note that the specific success code 00000 is translated to any empty
       string (false). If the driver does not support SQLSTATE (and most
       don't), then state() will return	"S1000"	(General Error)	for all
       errors.

       The driver is free to return any	value via "state", e.g., warning
       codes, even if it has not declared an error by returning	a true value
       via the "err" method described above.

       The state() method should not be	used to	test for errors, use err() for
       that, because drivers may return	a 'success with	information' or
       warning state code via state() for methods that have not	'failed'.

       "set_err"

	 $rv = $h->set_err($err, $errstr);
	 $rv = $h->set_err($err, $errstr, $state);
	 $rv = $h->set_err($err, $errstr, $state, $method);
	 $rv = $h->set_err($err, $errstr, $state, $method, $rv);

       Set the "err", "errstr",	and "state" values for the handle.  This
       method is typically only	used by	DBI drivers and	DBI subclasses.

       If the "HandleSetErr" attribute holds a reference to a subroutine it is
       called first. The subroutine can	alter the $err,	$errstr, $state, and
       $method values. See "HandleSetErr" for full details.  If	the subroutine
       returns a true value then the handle "err", "errstr", and "state"
       values are not altered and set_err() returns an empty list (it normally
       returns $rv which defaults to undef, see	below).

       Setting "err" to	a true value indicates an error	and will trigger the
       normal DBI error	handling mechanisms, such as "RaiseError" and
       "HandleError", if they are enabled, when	execution returns from the DBI
       back to the application.

       Setting "err" to	"" indicates an	'information' state, and setting it to
       "0" indicates a 'warning' state.	Setting	"err" to "undef" also sets
       "errstr"	to undef, and "state" to "", irrespective of the values	of the
       $errstr and $state parameters.

       The $method parameter provides an alternate method name for the
       "RaiseError"/"PrintError"/"RaiseWarn"/"PrintWarn" error string instead
       of the fairly unhelpful '"set_err"'.

       The "set_err" method normally returns undef.  The $rv parameter
       provides	an alternate return value.

       Some special rules apply	if the "err" or	"errstr" values	for the	handle
       are already set...

       If "errstr" is true then: "" [err was %s	now %s]"" is appended if $err
       is true and "err" is already true and the new err value differs from
       the original one. Similarly "" [state was %s now	%s]"" is appended if
       $state is true and "state" is already true and the new state value
       differs from the	original one. Finally ""\n"" and the new $errstr are
       appended	if $errstr differs from	the existing errstr value. Obviously
       the %s's	above are replaced by the corresponding	values.

       The handle "err"	value is set to	$err if: $err is true; or handle "err"
       value is	undef; or $err is defined and the length is greater than the
       handle "err" length. The	effect is that an 'information'	state only
       overrides undef;	a 'warning' overrides undef or 'information', and an
       'error' state overrides anything.

       The handle "state" value	is set to $state if $state is true and the
       handle "err" value was set (by the rules	above).

       Support for warning and information states was added in DBI 1.41.

       "trace"

	 $h->trace($trace_settings);
	 $h->trace($trace_settings, $trace_filename);
	 $trace_settings = $h->trace;

       The trace() method is used to alter the trace settings for a handle
       (and any	future children	of that	handle).  It can also be used to
       change where the	trace output is	sent.

       There's a similar method, "DBI->trace", which sets the global default
       trace settings.

       See the "TRACING" section for full details about	the DBI's powerful
       tracing facilities.

       "trace_msg"

	 $h->trace_msg($message_text);
	 $h->trace_msg($message_text, $min_level);

       Writes $message_text to the trace file if the trace level is greater
       than or equal to	$min_level (which defaults to 1).  Can also be called
       as "DBI->trace_msg($msg)".

       See "TRACING" for more details.

       "func"

	 $h->func(@func_arguments, $func_name) or die ...;

       The "func" method can be	used to	call private non-standard and non-
       portable	methods	implemented by the driver. Note	that the function name
       is given	as the last argument.

       It's also important to note that	the func() method does not clear a
       previous	error ($DBI::err etc.) and it does not trigger automatic error
       detection (RaiseError etc.) so you must check the return	status and/or
       $h->err to detect errors.

       (This method is not directly related to calling stored procedures.
       Calling stored procedures is currently not defined by the DBI.  Some
       drivers,	such as	DBD::Oracle, support it	in non-portable	ways.  See
       driver documentation for	more details.)

       See also	install_method() in DBI::DBD for how you can avoid needing to
       use func() and gain direct access to driver-private methods.

       "can"

	 $is_implemented = $h->can($method_name);

       Returns true if $method_name is implemented by the driver or a default
       method is provided by the DBI's driver base class.  It returns false
       where a driver hasn't implemented a method and the default method is
       provided	by the DBI's driver base class is just an empty	stub.

       "parse_trace_flags"

	 $trace_settings_integer = $h->parse_trace_flags($trace_settings);

       Parses a	string containing trace	settings and returns the corresponding
       integer value used internally by	the DBI	and drivers.

       The $trace_settings argument is a string	containing a trace level
       between 0 and 15	and/or trace flag names	separated by vertical bar
       (""|"") or comma	("","")	characters. For	example: "SQL|3|foo".

       It uses the parse_trace_flag() method, described	below, to process the
       individual trace	flag names.

       The parse_trace_flags() method was added	in DBI 1.42.

       "parse_trace_flag"

	 $bit_flag = $h->parse_trace_flag($trace_flag_name);

       Returns the bit flag corresponding to the trace flag name in
       $trace_flag_name.  Drivers are expected to override this	method and
       check if	$trace_flag_name is a driver specific trace flags and, if not,
       then call the DBI's default parse_trace_flag().

       The parse_trace_flag() method was added in DBI 1.42.

       "private_attribute_info"

	 $hash_ref = $h->private_attribute_info();

       Returns a reference to a	hash whose keys	are the	names of driver-
       private handle attributes available for the kind	of handle (driver,
       database, statement) that the method was	called on.

       For example, the	return value when called with a	DBD::Sybase $dbh could
       look like this:

	 {
	     syb_dynamic_supported => undef,
	     syb_oc_version => undef,
	     syb_server_version	=> undef,
	     syb_server_version_string => undef,
	 }

       and when	called with a DBD::Sybase $sth they could look like this:

	 {
	     syb_types => undef,
	     syb_proc_status =>	undef,
	     syb_result_type =>	undef,
	 }

       The values should be undef. Meanings may	be assigned to particular
       values in future.

       "swap_inner_handle"

	 $rc = $h1->swap_inner_handle( $h2 );
	 $rc = $h1->swap_inner_handle( $h2, $allow_reparent );

       Brain transplants for handles. You don't	need to	know about this	unless
       you want	to become a handle surgeon.

       A DBI handle is a reference to a	tied hash. A tied hash has an inner
       hash that actually holds	the contents.  The swap_inner_handle() method
       swaps the inner hashes between two handles. The $h1 and $h2 handles
       still point to the same tied hashes, but	what those hashes are tied to
       has been	swapped.  In effect $h1	becomes	$h2 and	vice-versa. This is
       powerful	stuff, expect problems.	Use with care.

       As a small safety measure, the two handles, $h1 and $h2,	have to	share
       the same	parent unless $allow_reparent is true.

       The swap_inner_handle() method was added	in DBI 1.44.

       Here's a	quick kind of 'diagram'	as a worked example to help think
       about what's happening:

	   Original state:
		   dbh1o -> dbh1i
		   sthAo -> sthAi(dbh1i)
		   dbh2o -> dbh2i

	   swap_inner_handle dbh1o with	dbh2o:
		   dbh2o -> dbh1i
		   sthAo -> sthAi(dbh1i)
		   dbh1o -> dbh2i

	   create new sth from dbh1o:
		   dbh2o -> dbh1i
		   sthAo -> sthAi(dbh1i)
		   dbh1o -> dbh2i
		   sthBo -> sthBi(dbh2i)

	   swap_inner_handle sthAo with	sthBo:
		   dbh2o -> dbh1i
		   sthBo -> sthAi(dbh1i)
		   dbh1o -> dbh2i
		   sthAo -> sthBi(dbh2i)

       "visit_child_handles"

	 $h->visit_child_handles( $coderef );
	 $h->visit_child_handles( $coderef, $info );

       Where $coderef is a reference to	a subroutine and $info is an arbitrary
       value which, if undefined, defaults to a	reference to an	empty hash.
       Returns $info.

       For each	child handle of	$h, if any, $coderef is	invoked	as:

	 $coderef->($child_handle, $info);

       If the execution	of $coderef returns a true value then
       "visit_child_handles" is	called on that child handle and	passed the
       returned	value as $info.

       For example:

	 # count database connections with names (DSN) matching	a pattern
	 my $connections = 0;
	 $dbh->{Driver}->visit_child_handles(sub {
	     my	($h, $info) = @_;
	     ++$connections if $h->{Name} =~ /foo/;
	     return 0; # don't visit kids
	 })

       See also	"visit_handles".

ATTRIBUTES COMMON TO ALL HANDLES
       These attributes	are common to all types	of DBI handles.

       Some attributes are inherited by	child handles. That is,	the value of
       an inherited attribute in a newly created statement handle is the same
       as the value in the parent database handle. Changes to attributes in
       the new statement handle	do not affect the parent database handle and
       changes to the database handle do not affect existing statement
       handles,	only future ones.

       Attempting to set or get	the value of an	unknown	attribute generates a
       warning,	except for private driver specific attributes (which all have
       names starting with a lowercase letter).

       Example:

	 $h->{AttributeName} = ...;    # set/write
	 ... = $h->{AttributeName};    # get/read

       "Warn"

       Type: boolean, inherited

       The "Warn" attribute enables useful warnings for	certain	bad practices.
       It is enabled by	default	and should only	be disabled in rare
       circumstances.  Since warnings are generated using the Perl "warn"
       function, they can be intercepted using the Perl	$SIG{__WARN__} hook.

       The "Warn" attribute is not related to the "PrintWarn" attribute.

       "Active"

       Type: boolean, read-only

       The "Active" attribute is true if the handle object is "active".	This
       is rarely used in applications. The exact meaning of active is somewhat
       vague at	the moment. For	a database handle it typically means that the
       handle is connected to a	database ("$dbh->disconnect" sets "Active"
       off).  For a statement handle it	typically means	that the handle	is a
       "SELECT"	that may have more data	to fetch. (Fetching all	the data or
       calling "$sth->finish" sets "Active" off.)

       "Executed"

       Type: boolean

       The "Executed" attribute	is true	if the handle object has been
       "executed".  Currently only the $dbh do() method	and the	$sth
       execute(), execute_array(), and execute_for_fetch() methods set the
       "Executed" attribute.

       When it's set on	a handle it is also set	on the parent handle at	the
       same time. So calling execute() on a $sth also sets the "Executed"
       attribute on the	parent $dbh.

       The "Executed" attribute	for a database handle is cleared by the
       commit()	and rollback() methods (even if	they fail). The	"Executed"
       attribute of a statement	handle is not cleared by the DBI under any
       circumstances and so acts as a permanent	record of whether the
       statement handle	was ever used.

       The "Executed" attribute	was added in DBI 1.41.

       "Kids"

       Type: integer, read-only

       For a driver handle, "Kids" is the number of currently existing
       database	handles	that were created from that driver handle.  For	a
       database	handle,	"Kids" is the number of	currently existing statement
       handles that were created from that database handle.  For a statement
       handle, the value is zero.

       "ActiveKids"

       Type: integer, read-only

       Like "Kids", but	only counting those that are "Active" (as above).

       "CachedKids"

       Type: hash ref

       For a database handle, "CachedKids" returns a reference to the cache
       (hash) of statement handles created by the "prepare_cached" method.
       For a driver handle, returns a reference	to the cache (hash) of
       database	handles	created	by the "connect_cached"	method.

       "Type"

       Type: scalar, read-only

       The "Type" attribute identifies the type	of a DBI handle.  Returns "dr"
       for driver handles, "db"	for database handles and "st" for statement
       handles.

       "ChildHandles"

       Type: array ref

       The ChildHandles	attribute contains a reference to an array of all the
       handles created by this handle which are	still accessible.  The
       contents	of the array are weak-refs and will become undef when the
       handle goes out of scope. (They're cleared out occasionally.)

       "ChildHandles" returns undef if your perl version does not support weak
       references (check the Scalar::Util module).  The	referenced array
       returned	should be treated as read-only.

       For example, to enumerate all driver handles, database handles and
       statement handles:

	   sub show_child_handles {
	       my ($h, $level) = @_;
	       printf "%sh %s %s\n", $h->{Type}, "\t" x	$level,	$h;
	       show_child_handles($_, $level + 1)
		   for (grep { defined } @{$h->{ChildHandles}});
	   }

	   my %drivers = DBI->installed_drivers();
	   show_child_handles($_, 0) for (values %drivers);

       "CompatMode"

       Type: boolean, inherited

       The "CompatMode"	attribute is used by emulation layers (such as
       Oraperl)	to enable compatible behaviour in the underlying driver	(e.g.,
       DBD::Oracle) for	this handle. Not normally set by application code.

       It also has the effect of disabling the 'quick FETCH' of	attribute
       values from the handles attribute cache.	So all attribute values	are
       handled by the drivers own FETCH	method.	This makes them	slightly
       slower but is useful for	special-purpose	drivers	like DBD::Multiplex.

       "InactiveDestroy"

       Type: boolean

       The default value, false, means a handle	will be	fully destroyed	as
       normal when the last reference to it is removed,	just as	you'd expect.

       If set true then	the handle will	be treated by the DESTROY as if	it was
       no longer Active, and so	the database engine related effects of
       DESTROYing a handle will	be skipped.  Think of the name as meaning
       'treat the handle as not-Active in the DESTROY method'.

       For a database handle, this attribute does not disable an explicit call
       to the disconnect method, only the implicit call	from DESTROY that
       happens if the handle is	still marked as	"Active".

       This attribute is specifically designed for use in Unix applications
       that "fork" child processes.  For some drivers, when the	child process
       exits the destruction of	inherited handles cause	the corresponding
       handles in the parent process to	cease working.

       Either the parent or the	child process, but not both, should set
       "InactiveDestroy" true on all their shared handles. Alternatively, and
       preferably, the "AutoInactiveDestroy" can be set	in the parent on
       connect.

       To help tracing applications using fork the process id is shown in the
       trace log whenever a DBI	or handle trace() method is called.  The
       process id also shown for every method call if the DBI trace level (not
       handle trace level) is set high enough to show the trace	from the DBI's
       method dispatcher, e.g. >= 9.

       "AutoInactiveDestroy"

       Type: boolean, inherited

       The "InactiveDestroy" attribute,	described above, needs to be
       explicitly set in the child process after a fork(), on every active
       database	and statement handle.  This is a problem if the	code that
       performs	the fork() is not under	your control, perhaps in a third-party
       module.	Use "AutoInactiveDestroy" to get around	this situation.

       If set true, the	DESTROY	method will check the process id of the	handle
       and, if different from the current process id, it will set the
       InactiveDestroy attribute.  It is strongly recommended that
       "AutoInactiveDestroy" is	enabled	on all new code	(it's only not enabled
       by default to avoid backwards compatibility problems).

       This is the example it's	designed to deal with:

	   my $dbh = DBI->connect(...);
	   some_code_that_forks(); # Perhaps without your knowledge
	   # Child process dies, destroying the	inherited dbh
	   $dbh->do(...); # Breaks because parent $dbh is now broken

       The "AutoInactiveDestroy" attribute was added in	DBI 1.614.

       "PrintWarn"

       Type: boolean, inherited

       The "PrintWarn" attribute controls the printing of warnings recorded by
       the driver.  When set to	a true value (the default) the DBI will	check
       method calls to see if a	warning	condition has been set.	If so, the DBI
       will effectively	do a "warn("$class $method warning: $DBI::errstr")"
       where $class is the driver class	and $method is the name	of the method
       which failed. E.g.,

	 DBD::Oracle::db execute warning: ... warning text here	...

       If desired, the warnings	can be caught and processed using a
       $SIG{__WARN__} handler or modules like CGI::Carp	and CGI::ErrorWrap.

       See also	"set_err" for how warnings are recorded	and "HandleSetErr" for
       how to influence	it.

       Fetching	the full details of warnings can require an extra round-trip
       to the database server for some drivers.	In which case the driver may
       opt to only fetch the full details of warnings if the "PrintWarn"
       attribute is true. If "PrintWarn" is false then these drivers should
       still indicate the fact that there were warnings	by setting the warning
       string to, for example: "3 warnings".

       "PrintError"

       Type: boolean, inherited

       The "PrintError"	attribute can be used to force errors to generate
       warnings	(using "warn") in addition to returning	error codes in the
       normal way.  When set "on", any method which results in an error
       occurring will cause the	DBI to effectively do a	"warn("$class $method
       failed: $DBI::errstr")" where $class is the driver class	and $method is
       the name	of the method which failed. E.g.,

	 DBD::Oracle::db prepare failed: ... error text	here ...

       By default, "DBI->connect" sets "PrintError" "on".

       If desired, the warnings	can be caught and processed using a
       $SIG{__WARN__} handler or modules like CGI::Carp	and CGI::ErrorWrap.

       "RaiseWarn"

       Type: boolean, inherited

       The "RaiseWarn" attribute can be	used to	force warnings to raise
       exceptions rather then simply printing them. It is "off"	by default.
       When set	"on", any method which sets warning condition will cause the
       DBI to effectively do a "die("$class $method warning: $DBI::errstr")",
       where $class is the driver class	and $method is the name	of the method
       that sets warning condition. E.g.,

	 DBD::Oracle::db execute warning: ... warning text here	...

       If you turn "RaiseWarn" on then you'd normally turn "PrintWarn" off.
       If "PrintWarn" is also on, then the "PrintWarn" is done first
       (naturally).

       This attribute was added	in DBI 1.643.

       "RaiseError"

       Type: boolean, inherited

       The "RaiseError"	attribute can be used to force errors to raise
       exceptions rather than simply return error codes	in the normal way. It
       is "off"	by default.  When set "on", any	method which results in	an
       error will cause	the DBI	to effectively do a "die("$class $method
       failed: $DBI::errstr")",	where $class is	the driver class and $method
       is the name of the method that failed. E.g.,

	 DBD::Oracle::db prepare failed: ... error text	here ...

       If you turn "RaiseError"	on then	you'd normally turn "PrintError" off.
       If "PrintError" is also on, then	the "PrintError" is done first
       (naturally).

       Typically "RaiseError" is used in conjunction with "eval", or a module
       like Try::Tiny or TryCatch, to catch the	exception that's been thrown
       and handle it.  For example:

	 use Try::Tiny;

	 try {
	   ...
	   $sth->execute();
	   ...
	 } catch {
	   # $sth->err and $DBI::err will be true if error was from DBI
	   warn	$_; # print the	error (which Try::Tiny puts into $_)
	   ... # do whatever you need to deal with the error
	 };

       In the catch block the $DBI::lasth variable can be useful for diagnosis
       and reporting if	you can't be sure which	handle triggered the error.
       For example, $DBI::lasth->{Type}	and $DBI::lasth->{Statement}.

       See also	"Transactions".

       If you want to temporarily turn "RaiseError" off	(inside	a library
       function	that is	likely to fail,	for example), the recommended way is
       like this:

	 {
	   local $h->{RaiseError};  # localize and turn	off for	this block
	   ...
	 }

       The original value will automatically and reliably be restored by Perl,
       regardless of how the block is exited.  The same	logic applies to other
       attributes, including "PrintError".

       "HandleError"

       Type: code ref, inherited

       The "HandleError" attribute can be used to provide your own alternative
       behaviour in case of errors. If set to a	reference to a subroutine then
       that subroutine is called when an error is detected (at the same	point
       that "RaiseError" and "PrintError" are handled).	It is called also when
       "RaiseWarn" is enabled and a warning is detected.

       The subroutine is called	with three parameters: the error message
       string that "RaiseError", "RaiseWarn" or	"PrintError" would use,	the
       DBI handle being	used, and the first value being	returned by the	method
       that failed (typically undef).

       If the subroutine returns a false value then the	"RaiseError",
       "RaiseWarn" and/or "PrintError" attributes are checked and acted	upon
       as normal.

       For example, to "die" with a full stack trace for any error:

	 use Carp;
	 $h->{HandleError} = sub { confess(shift) };

       Or to turn errors into exceptions:

	 use Exception;	# or your own favourite	exception module
	 $h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };

       It is possible to 'stack' multiple HandleError handlers by using
       closures:

	 sub your_subroutine {
	   my $previous_handler	= $h->{HandleError};
	   $h->{HandleError} = sub {
	     return 1 if $previous_handler and &$previous_handler(@_);
	     ... your code here	...
	   };
	 }

       Using a "my" inside a subroutine	to store the previous "HandleError"
       value is	important.  See	perlsub	and perlref for	more information about
       closures.

       It is possible for "HandleError"	to alter the error message that	will
       be used by "RaiseError",	"RaiseWarn" and	"PrintError" if	it returns
       false.  It can do that by altering the value of $_[0]. This example
       appends a stack trace to	all errors and,	unlike the previous example
       using Carp::confess, this will work "PrintError"	as well	as
       "RaiseError":

	 $h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]);	0; };

       It is also possible for "HandleError" to	hide an	error, to a limited
       degree, by using	"set_err" to reset $DBI::err and $DBI::errstr, and
       altering	the return value of the	failed method. For example:

	 $h->{HandleError} = sub {
	   return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
	   return 0 unless $_[1]->err == 1234; # the error to 'hide'
	   $h->set_err(undef,undef);   # turn off the error
	   $_[2] = [ ... ];    # supply	alternative return value
	   return 1;
	 };

       This only works for methods which return	a single value and is hard to
       make reliable (avoiding infinite	loops, for example) and	so isn't
       recommended for general use!  If	you find a good	use for	it then	please
       let me know.

       "HandleSetErr"

       Type: code ref, inherited

       The "HandleSetErr" attribute can	be used	to intercept the setting of
       handle "err", "errstr", and "state" values.  If set to a	reference to a
       subroutine then that subroutine is called whenever set_err() is called,
       typically by the	driver or a subclass.

       The subroutine is called	with five arguments, the first five that were
       passed to set_err(): the	handle,	the "err", "errstr", and "state"
       values being set, and the method	name. These can	be altered by changing
       the values in the @_ array. The return value affects set_err()
       behaviour, see "set_err"	for details.

       It is possible to 'stack' multiple HandleSetErr handlers	by using
       closures. See "HandleError" for an example.

       The "HandleSetErr" and "HandleError" subroutines	differ in subtle but
       significant ways. HandleError is	only invoked at	the point where	the
       DBI is about to return to the application with "err" set	true.  It's
       not invoked by the failure of a method that's been called by another
       DBI method.  HandleSetErr, on the other hand, is	called whenever
       set_err() is called with	a defined "err"	value, even if false.  So it's
       not just	for errors, despite the	name, but also warn and	info states.
       The set_err() method, and thus HandleSetErr, may	be called multiple
       times within a method and is usually invoked from deep within driver
       code.

       In theory a driver can use the return value from	HandleSetErr via
       set_err() to decide whether to continue or not. If set_err() returns an
       empty list, indicating that the HandleSetErr code has 'handled' the
       'error',	the driver could then continue instead of failing (if that's a
       reasonable thing	to do).	 This isn't excepted to	be common and any such
       cases should be clearly marked in the driver documentation and
       discussed on the	dbi-dev	mailing	list.

       The "HandleSetErr" attribute was	added in DBI 1.41.

       "ErrCount"

       Type: unsigned integer

       The "ErrCount" attribute	is incremented whenever	the set_err() method
       records an error. It isn't incremented by warnings or information
       states. It is not reset by the DBI at any time.

       The "ErrCount" attribute	was added in DBI 1.41. Older drivers may not
       have been updated to use	set_err() to record errors and so this
       attribute may not be incremented	when using them.

       "ShowErrorStatement"

       Type: boolean, inherited

       The "ShowErrorStatement"	attribute can be used to cause the relevant
       Statement text to be appended to	the error messages generated by	the
       "RaiseError", "PrintError", "RaiseWarn" and "PrintWarn" attributes.
       Only applies to errors on statement handles plus	the prepare(), do(),
       and the various "select*()" database handle methods.  (The exact	format
       of the appended text is subject to change.)

       If "$h->{ParamValues}" returns a	hash reference of parameter
       (placeholder) values then those are formatted and appended to the end
       of the Statement	text in	the error message.

       "TraceLevel"

       Type: integer, inherited

       The "TraceLevel"	attribute can be used as an alternative	to the "trace"
       method to set the DBI trace level and trace flags for a specific
       handle.	See "TRACING" for more details.

       The "TraceLevel"	attribute is especially	useful combined	with "local"
       to alter	the trace settings for just a single block of code.

       "FetchHashKeyName"

       Type: string, inherited

       The "FetchHashKeyName" attribute	is used	to specify whether the
       fetchrow_hashref() method should	perform	case conversion	on the field
       names used for the hash keys. For historical reasons it defaults	to
       '"NAME"'	but it is recommended to set it	to '"NAME_lc"' (convert	to
       lower case) or '"NAME_uc"' (convert to upper case) according to your
       preference.  It can only	be set for driver and database handles.	 For
       statement handles the value is frozen when prepare() is called.

       "ChopBlanks"

       Type: boolean, inherited

       The "ChopBlanks"	attribute can be used to control the trimming of
       trailing	space characters from fixed width character (CHAR) fields. No
       other field types are affected, even where field	values have trailing
       spaces.

       The default is false (although it is possible that the default may
       change).	 Applications that need	specific behaviour should set the
       attribute as needed.

       Drivers are not required	to support this	attribute, but any driver
       which does not support it must arrange to return	"undef"	as the
       attribute value.

       "LongReadLen"

       Type: unsigned integer, inherited

       The "LongReadLen" attribute may be used to control the maximum length
       of 'long' type fields (LONG, BLOB, CLOB,	MEMO, etc.) which the driver
       will read from the database automatically when it fetches each row of
       data.

       The "LongReadLen" attribute only	relates	to fetching and	reading	long
       values; it is not involved in inserting or updating them.

       A value of 0 means not to automatically fetch any long data.  Drivers
       may return undef	or an empty string for long fields when	"LongReadLen"
       is 0.

       The default is typically	0 (zero) or 80 bytes but may vary between
       drivers.	 Applications fetching long fields should set this value to
       slightly	larger than the	longest	long field value to be fetched.

       Some databases return some long types encoded as	pairs of hex digits.
       For these types,	"LongReadLen" relates to the underlying	data length
       and not the doubled-up length of	the encoded string.

       Changing	the value of "LongReadLen" for a statement handle after	it has
       been "prepare"'d	will typically have no effect, so it's common to set
       "LongReadLen" on	the $dbh before	calling	"prepare".

       For most	drivers	the value used here has	a direct effect	on the memory
       used by the statement handle while it's active, so don't	be too
       generous. If you	can't be sure what value to use	you could execute an
       extra select statement to determine the longest value.  For example:

	 $dbh->{LongReadLen} = $dbh->selectrow_array(qq{
	     SELECT MAX(OCTET_LENGTH(long_column_name))
	     FROM table	WHERE ...
	 });
	 $sth =	$dbh->prepare(qq{
	     SELECT long_column_name, ... FROM table WHERE ...
	 });

       You may need to take extra care if the table can	be modified between
       the first select	and the	second being executed. You may also need to
       use a different function	if OCTET_LENGTH() does not work	for long types
       in your database. For example, for Sybase use DATALENGTH() and for
       Oracle use LENGTHB().

       See also	"LongTruncOk" for information on truncation of long types.

       "LongTruncOk"

       Type: boolean, inherited

       The "LongTruncOk" attribute may be used to control the effect of
       fetching	a long field value which has been truncated (typically because
       it's longer than	the value of the "LongReadLen" attribute).

       By default, "LongTruncOk" is false and so fetching a long value that
       needs to	be truncated will cause	the fetch to fail.  (Applications
       should always be	sure to	check for errors after a fetch loop in case an
       error, such as a	divide by zero or long field truncation, caused	the
       fetch to	terminate prematurely.)

       If a fetch fails	due to a long field truncation when "LongTruncOk" is
       false, many drivers will	allow you to continue fetching further rows.

       See also	"LongReadLen".

       "TaintIn"

       Type: boolean, inherited

       If the "TaintIn"	attribute is set to a true value and Perl is running
       in taint	mode (e.g., started with the "-T" option), then	all the
       arguments to most DBI method calls are checked for being	tainted. This
       may change.

       The attribute defaults to off, even if Perl is in taint mode.  See
       perlsec for more	about taint mode.  If Perl is not running in taint
       mode, this attribute has	no effect.

       When fetching data that you trust you can turn off the TaintIn
       attribute, for that statement handle, for the duration of the fetch
       loop.

       The "TaintIn" attribute was added in DBI	1.31.

       "TaintOut"

       Type: boolean, inherited

       If the "TaintOut" attribute is set to a true value and Perl is running
       in taint	mode (e.g., started with the "-T" option), then	most data
       fetched from the	database is considered tainted.	This may change.

       The attribute defaults to off, even if Perl is in taint mode.  See
       perlsec for more	about taint mode.  If Perl is not running in taint
       mode, this attribute has	no effect.

       When fetching data that you trust you can turn off the TaintOut
       attribute, for that statement handle, for the duration of the fetch
       loop.

       Currently only fetched data is tainted. It is possible that the results
       of other	DBI method calls, and the value	of fetched attributes, may
       also be tainted in future versions. That	change may well	break your
       applications unless you take great care now. If you use DBI Taint mode,
       please report your experience and any suggestions for changes.

       The "TaintOut" attribute	was added in DBI 1.31.

       "Taint"

       Type: boolean, inherited

       The "Taint" attribute is	a shortcut for "TaintIn" and "TaintOut"	(it is
       also present for	backwards compatibility).

       Setting this attribute sets both	"TaintIn" and "TaintOut", and
       retrieving it returns a true value if and only if "TaintIn" and
       "TaintOut" are both set to true values.

       "Profile"

       Type: inherited

       The "Profile" attribute enables the collection and reporting of method
       call timing statistics.	See the	DBI::Profile module documentation for
       much more detail.

       The "Profile" attribute was added in DBI	1.24.

       "ReadOnly"

       Type: boolean, inherited

       An application can set the "ReadOnly" attribute of a handle to a	true
       value to	indicate that it will not be attempting	to make	any changes
       using that handle or any	children of it.

       Note that the exact definition of 'read only' is	rather fuzzy.  For
       more details see	the documentation for the driver you're	using.

       If the driver can make the handle truly read-only then it should
       (unless doing so	would have unpleasant side effect, like	changing the
       consistency level from per-statement to per-session).  Otherwise	the
       attribute is simply advisory.

       A driver	can set	the "ReadOnly" attribute itself	to indicate that the
       data it is connected to cannot be changed for some reason.

       If the driver cannot ensure the "ReadOnly" attribute is adhered to it
       will record a warning.  In this case reading the	"ReadOnly" attribute
       back after it is	set true will return true even if the underlying
       driver cannot ensure this (so any application knows the application
       declared	itself ReadOnly).

       Library modules and proxy drivers can use the attribute to influence
       their behavior.	For example, the DBD::Gofer driver considers the
       "ReadOnly" attribute when making	a decision about whether to retry an
       operation that failed.

       The attribute should be set to 1	or 0 (or undef). Other values are
       reserved.

       "Callbacks"

       Type: hash ref

       The DBI callback	mechanism lets you intercept, and optionally replace,
       any method call on a DBI	handle.	At the extreme,	it lets	you become a
       puppet master, deceiving	the application	in any way you want.

       The "Callbacks" attribute is a hash reference where the keys are	DBI
       method names and	the values are code references.	For each key naming a
       method, the DBI will execute the	associated code	reference before
       executing the method.

       The arguments to	the code reference will	be the same as to the method,
       including the invocant (a database handle or statement handle). For
       example,	say that to callback to	some code on a call to "prepare()":

	 $dbh->{Callbacks} = {
	     prepare =>	sub {
		 my ($dbh, $query, $attrs) = @_;
		 print "Preparing q{$query}\n"
	     },
	 };

       The callback would then be executed when	you called the "prepare()"
       method:

	 $dbh->prepare('SELECT 1');

       And the output of course	would be:

	 Preparing q{SELECT 1}

       Because callbacks are executed before the methods they're associated
       with, you can modify the	arguments before they're passed	on to the
       method call. For	example, to make sure that all calls to	"prepare()"
       are immediately prepared	by DBD::Pg, add	a callback that	makes sure
       that the	"pg_prepare_now" attribute is always set:

	 my $dbh = DBI->connect($dsn, $username, $auth,	{
	     Callbacks => {
		 prepare => sub	{
		     $_[2] ||= {};
		     $_[2]->{pg_prepare_now} = 1;
		     return; # must return nothing
		 },
	     }
	 });

       Note that we are	editing	the contents of	@_ directly. In	this case
       we've created the attributes hash if it's not passed to the "prepare"
       call.

       You can also prevent the	associated method from ever executing. While a
       callback	executes, $_ holds the method name. (This allows multiple
       callbacks to share the same code	reference and still know what method
       was called.)  To	prevent	the method from	executing, simply "undef $_".
       For example, if you wanted to disable calls to "ping()",	you could do
       this:

	 $dbh->{Callbacks} = {
	     ping => sub {
		 # tell	dispatch to not	call the method:
		 undef $_;
		 # return this value instead:
		 return	"42 bells";
	     }
	 };

       As with other attributes, Callbacks can be specified on a handle	or via
       the attributes to "connect()". Callbacks	can also be applied to a
       statement methods on a statement	handle.	For example:

	 $sth->{Callbacks} = {
	     execute =>	sub {
		 print "Executing ", shift->{Statement}, "\n";
	     }
	 };

       The "Callbacks" attribute of a database handle isn't copied to any
       statement handles it creates. So	setting	callbacks for a	statement
       handle requires you to set the "Callbacks" attribute on the statement
       handle yourself,	as in the example above, or use	the special
       "ChildCallbacks"	key described below.

       Special Keys in Callbacks Attribute

       In addition to DBI handle method	names, the "Callbacks" hash reference
       supports	four additional	keys.

       The first is the	"ChildCallbacks" key. When a statement handle is
       created from a database handle the "ChildCallbacks" key of the database
       handle's	"Callbacks" attribute, if any, becomes the new "Callbacks"
       attribute of the	statement handle.  This	allows you to define callbacks
       for all statement handles created from a	database handle. For example,
       if you wanted to	count how many times "execute" was called in your
       application, you	could write:

	 my $exec_count	= 0;
	 my $dbh = DBI->connect( $dsn, $username, $auth, {
	     Callbacks => {
		 ChildCallbacks	=> {
		     execute =>	sub { $exec_count++; return; }
		 }
	     }
	 });

	 END {
	     print "The	execute	method was called $exec_count times\n";
	 }

       The other three special keys are	"connect_cached.new",
       "connect_cached.connected", and "connect_cached.reused".	These keys
       define callbacks	that are called	when "connect_cached()"	is called, but
       allow different behaviors depending on whether a	new handle is created
       or a handle is returned.	The callback is	invoked	with these arguments:
       "$dbh, $dsn, $user, $auth, $attr".

       For example, some applications uses "connect_cached()" to connect with
       "AutoCommit" enabled and	then disable "AutoCommit" temporarily for
       transactions. If	"connect_cached()" is called during a transaction,
       perhaps in a utility method, then it might select the same cached
       handle and then force "AutoCommit" on, forcing a	commit of the
       transaction. See	the "connect_cached" documentation for one way to deal
       with that. Here we'll describe an alternative approach using a
       callback.

       Because the "connect_cached.new"	and "connect_cached.reused" callbacks
       are invoked before "connect_cached()" has applied the connect
       attributes, you can use them to edit the	attributes that	will be
       applied.	To prevent a cached handle from	having its transactions
       committed before	it's returned, you can eliminate the "AutoCommit"
       attribute in a "connect_cached.reused" callback,	like so:

	 my $cb	= {
	     'connect_cached.reused' =>	sub { delete $_[4]->{AutoCommit} },
	 };

	 sub dbh {
	     my	$self =	shift;
	     DBI->connect_cached( $dsn,	$username, $auth, {
		 PrintError => 0,
		 RaiseError => 1,
		 AutoCommit => 1,
		 Callbacks  => $cb,
	     });
	 }

       The upshot is that new database handles are created with	"AutoCommit"
       enabled,	while cached database handles are left in whatever transaction
       state they happened to be in when retrieved from	the cache.

       Note that we've also used a lexical for the callbacks hash reference.
       This is because "connect_cached()" returns a new	database handle	if any
       of the attributes passed	to is have changed. If we used an inline hash
       reference, "connect_cached()" would return a new	database handle	every
       time. Which would rather	defeat the purpose.

       A more common application for callbacks is setting connection state
       only when a new connection is made (by connect()	or connect_cached()).
       Adding a	callback to the	connected method (when using "connect")	or via
       "connect_cached.connected" (when	useing connect_cached()>) makes	this
       easy.  The connected() method is	a no-op	by default (unless you
       subclass	the DBI	and change it).	The DBI	calls it to indicate that a
       new connection has been made and	the connection attributes have all
       been set. You can give it a bit of added	functionality by applying a
       callback	to it. For example, to make sure that MySQL understands	your
       application's ANSI-compliant SQL, set it	up like	so:

	 my $dbh = DBI->connect($dsn, $username, $auth,	{
	     Callbacks => {
		 connected => sub {
		     shift->do(q{
			 SET SESSION sql_mode='ansi,strict_trans_tables,no_auto_value_on_zero';
		     });
		     return;
		 },
	     }
	 });

       If you're using "connect_cached()", use the "connect_cached.connected"
       callback, instead. This is because "connected()"	is called for both new
       and reused database handles, but	you want to execute a callback only
       the when	a new database handle is returned. For example,	to set the
       time zone on connection to a PostgreSQL database, try this:

	 my $cb	= {
	     'connect_cached.connected'	=> sub {
		 shift->do('SET	timezone = UTC');
	     }
	 };

	 sub dbh {
	     my	$self =	shift;
	     DBI->connect_cached( $dsn,	$username, $auth, { Callbacks => $cb });
	 }

       One significant limitation with callbacks is that there can only	be one
       per method per handle. This means it's easy for one use of callbacks to
       interfere with, or typically simply overwrite, another use of
       callbacks. For this reason modules using	callbacks should document the
       fact clearly so application authors can tell if use of callbacks	by the
       module will clash with use of callbacks by the application.

       You might be able to work around	this issue by taking a copy of the
       original	callback and calling it	within your own. For example:

	 my $prev_cb = $h->{Callbacks}{method_name};
	 $h->{Callbacks}{method_name} =	sub {
	   if ($prev_cb) {
	       my @result = $prev_cb->(@_);
	       return @result if not $_; # $prev_cb vetoed call
	   }
	   ... your callback logic here	...
	 };

       "private_your_module_name_*"

       The DBI provides	a way to store extra information in a DBI handle as
       "private" attributes. The DBI will allow	you to store and retrieve any
       attribute which has a name starting with	""private_"".

       It is strongly recommended that you use just one	private	attribute
       (e.g., use a hash ref) and give it a long and unambiguous name that
       includes	the module or application name that the	attribute relates to
       (e.g., ""private_YourFullModuleName_thingy"").

       Because of the way the Perl tie mechanism works you cannot reliably use
       the "||=" operator directly to initialise the attribute,	like this:

	 my $foo = $dbh->{private_yourmodname_foo} ||= { ... };	# WRONG

       you should use a	two step approach like this:

	 my $foo = $dbh->{private_yourmodname_foo};
	 $foo ||= $dbh->{private_yourmodname_foo} = { ... };

       This attribute is primarily of interest to people sub-classing DBI, or
       for applications	to piggy-back extra information	onto DBI handles.

DBI DATABASE HANDLE OBJECTS
       This section covers the methods and attributes associated with database
       handles.

   Database Handle Methods
       The following methods are specified for DBI database handles:

       "clone"

	 $new_dbh = $dbh->clone(\%attr);

       The "clone" method duplicates the $dbh connection by connecting with
       the same	parameters ($dsn, $user, $password) as originally used.

       The attributes for the cloned connect are the same as those used	for
       the original connect, with any other attributes in "\%attr" merged over
       them.  Effectively the same as doing:

	 %attributes_used = ( %original_attributes, %attr );

       If \%attr is not	given then it defaults to a hash containing all	the
       attributes in the attribute cache of $dbh excluding any non-code
       references, plus	the main boolean attributes (RaiseError, PrintError,
       AutoCommit, etc.). This behaviour is unreliable and so use of clone
       without an argument is deprecated and may cause a warning in a future
       release.

       The clone method	can be used even if the	database handle	is
       disconnected.

       The "clone" method was added in DBI 1.33.

       "data_sources"

	 @ary =	$dbh->data_sources();
	 @ary =	$dbh->data_sources(\%attr);

       Returns a list of data sources (databases) available via	the $dbh
       driver's	data_sources() method, plus any	extra data sources that	the
       driver can discover via the connected $dbh. Typically the extra data
       sources are other databases managed by the same server process that the
       $dbh is connected to.

       Data sources are	returned in a form suitable for	passing	to the
       "connect" method	(that is, they will include the	""dbi:$driver:""
       prefix).

       The data_sources() method, for a	$dbh, was added	in DBI 1.38.

       "do"

	 $rows = $dbh->do($statement)		or die $dbh->errstr;
	 $rows = $dbh->do($statement, \%attr)	or die $dbh->errstr;
	 $rows = $dbh->do($statement, \%attr, @bind_values) or die ...

       Prepare and execute a single statement. Returns the number of rows
       affected	or "undef" on error. A return value of "-1" means the number
       of rows is not known, not applicable, or	not available.

       This method is typically	most useful for	non-"SELECT" statements	that
       either cannot be	prepared in advance (due to a limitation of the
       driver) or do not need to be executed repeatedly. It should not be used
       for "SELECT" statements because it does not return a statement handle
       (so you can't fetch any data).

       The default "do"	method is logically similar to:

	 sub do	{
	     my($dbh, $statement, $attr, @bind_values) = @_;
	     my	$sth = $dbh->prepare($statement, $attr)	or return undef;
	     $sth->execute(@bind_values) or return undef;
	     my	$rows =	$sth->rows;
	     ($rows == 0) ? "0E0" : $rows; # always return true	if no error
	 }

       For example:

	 my $rows_deleted = $dbh->do(q{
	     DELETE FROM table
	     WHERE status = ?
	 }, undef, 'DONE') or die $dbh->errstr;

       Using placeholders and @bind_values with	the "do" method	can be useful
       because it avoids the need to correctly quote any variables in the
       $statement. But if you'll be executing the statement many times then
       it's more efficient to "prepare"	it once	and call "execute" many	times
       instead.

       The "q{...}" style quoting used in this example avoids clashing with
       quotes that may be used in the SQL statement. Use the double-quote-like
       "qq{...}" operator if you want to interpolate variables into the
       string.	See "Quote and Quote-like Operators" in	perlop for more
       details.

       Note drivers are	free to	avoid the overhead of creating an DBI
       statement handle	for do(), especially if	there are no parameters. In
       this case error handlers, if invoked during do(), will be passed	the
       database	handle.

       "last_insert_id"

	 $rv = $dbh->last_insert_id();
	 $rv = $dbh->last_insert_id($catalog, $schema, $table, $field);
	 $rv = $dbh->last_insert_id($catalog, $schema, $table, $field, \%attr);

       Returns a value 'identifying' the row just inserted, if possible.
       Typically this would be a value assigned	by the database	server to a
       column with an auto_increment or	serial type.  Returns undef if the
       driver does not support the method or can't determine the value.

       The $catalog, $schema, $table, and $field parameters may	be required
       for some	drivers	(see below).  If you don't know	the parameter values
       and your	driver does not	need them, then	use "undef" for	each.

       There are several caveats to be aware of	with this method if you	want
       to use it for portable applications:

       * For some drivers the value may	only available immediately after the
       insert statement	has executed (e.g., mysql, Informix).

       * For some drivers the $catalog,	$schema, $table, and $field parameters
       are required, for others	they are ignored (e.g.,	mysql).

       * Drivers may return an indeterminate value if no insert	has been
       performed yet.

       * For some drivers the value may	only be	available if placeholders have
       not been	used (e.g., Sybase, MS SQL). In	this case the value returned
       would be	from the last non-placeholder insert statement.

       * Some drivers may need driver-specific hints about how to get the
       value. For example, being told the name of the database 'sequence'
       object that holds the value. Any	such hints are passed as driver-
       specific	attributes in the \%attr parameter.

       * If the	underlying database offers nothing better, then	some drivers
       may attempt to implement	this method by executing ""select max($field)
       from $table"". Drivers using any	approach like this should issue	a
       warning if "AutoCommit" is true because it is generally unsafe -
       another process may have	modified the table between your	insert and the
       select. For situations where you	know it	is safe, such as when you have
       locked the table, you can silence the warning by	passing	"Warn" => 0 in
       \%attr.

       * If no insert has been performed yet, or the last insert failed, then
       the value is implementation defined.

       Given all the caveats above, it's clear that this method	must be	used
       with care.

       The "last_insert_id" method was added in	DBI 1.38.

       "selectrow_array"

	 @row_ary = $dbh->selectrow_array($statement);
	 @row_ary = $dbh->selectrow_array($statement, \%attr);
	 @row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and "fetchrow_array"
       into a single call. If called in	a list context,	it returns the first
       row of data from	the statement.	The $statement parameter can be	a
       previously prepared statement handle, in	which case the "prepare" is
       skipped.

       If any method fails, and	"RaiseError" is	not set, "selectrow_array"
       will return an empty list.

       If called in a scalar context for a statement handle that has more than
       one column, it is undefined whether the driver will return the value of
       the first column	or the last. So	don't do that.	Also, in a scalar
       context,	an "undef" is returned if there	are no more rows or if an
       error occurred. That "undef" can't be distinguished from	an "undef"
       returned	because	the first field	value was NULL.	 For these reasons you
       should exercise some caution if you use "selectrow_array" in a scalar
       context,	or just	don't do that.

       "selectrow_arrayref"

	 $ary_ref = $dbh->selectrow_arrayref($statement);
	 $ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
	 $ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchrow_arrayref" into	a single call. It returns the first row	of
       data from the statement.	 The $statement	parameter can be a previously
       prepared	statement handle, in which case	the "prepare" is skipped.

       If any method fails, and	"RaiseError" is	not set, "selectrow_arrayref"
       will return undef.

       "selectrow_hashref"

	 $hash_ref = $dbh->selectrow_hashref($statement);
	 $hash_ref = $dbh->selectrow_hashref($statement, \%attr);
	 $hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchrow_hashref" into a single	call. It returns the first row of data
       from the	statement.  The	$statement parameter can be a previously
       prepared	statement handle, in which case	the "prepare" is skipped.

       If any method fails, and	"RaiseError" is	not set, "selectrow_hashref"
       will return undef.

       "selectall_arrayref"

	 $ary_ref = $dbh->selectall_arrayref($statement);
	 $ary_ref = $dbh->selectall_arrayref($statement, \%attr);
	 $ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchall_arrayref" into	a single call. It returns a reference to an
       array containing	a reference to an array	(or hash, see below) for each
       row of data fetched.

       The $statement parameter	can be a previously prepared statement handle,
       in which	case the "prepare" is skipped. This is recommended if the
       statement is going to be	executed many times.

       If "RaiseError" is not set and any method except	"fetchall_arrayref"
       fails then "selectall_arrayref" will return "undef"; if
       "fetchall_arrayref" fails then it will return with whatever data	has
       been fetched thus far. You should check "$dbh->err" afterwards (or use
       the "RaiseError"	attribute) to discover if the data is complete or was
       truncated due to	an error.

       The "fetchall_arrayref" method called by	"selectall_arrayref" supports
       a $max_rows parameter. You can specify a	value for $max_rows by
       including a '"MaxRows"' attribute in \%attr. In which case finish() is
       called for you after fetchall_arrayref()	returns.

       The "fetchall_arrayref" method called by	"selectall_arrayref" also
       supports	a $slice parameter. You	can specify a value for	$slice by
       including a '"Slice"' or	'"Columns"' attribute in \%attr. The only
       difference between the two is that if "Slice" is	not defined and
       "Columns" is an array ref, then the array is assumed to contain column
       index values (which count from 1), rather than perl array index values.
       In which	case the array is copied and each value	decremented before
       passing to "/fetchall_arrayref".

       You may often want to fetch an array of rows where each row is stored
       as a hash. That can be done simply using:

	 my $emps = $dbh->selectall_arrayref(
	     "SELECT ename FROM	emp ORDER BY ename",
	     { Slice =>	{} }
	 );
	 foreach my $emp ( @$emps ) {
	     print "Employee: $emp->{ename}\n";
	 }

       Or, to fetch into an array instead of an	array ref:

	 @result = @{ $dbh->selectall_arrayref($sql, { Slice =>	{} }) };

       See "fetchall_arrayref" method for more details.

       "selectall_array"

	 @ary =	$dbh->selectall_array($statement);
	 @ary =	$dbh->selectall_array($statement, \%attr);
	 @ary =	$dbh->selectall_array($statement, \%attr, @bind_values);

       This is a convenience wrapper around selectall_arrayref that returns
       the rows	directly as a list, rather than	a reference to an array	of
       rows.

       Note that if "RaiseError" is not	set then you can't tell	the difference
       between returning no rows and an	error. Using RaiseError	is best
       practice.

       The "selectall_array" method was	added in DBI 1.635.

       "selectall_hashref"

	 $hash_ref = $dbh->selectall_hashref($statement, $key_field);
	 $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
	 $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);

       This utility method combines "prepare", "execute" and
       "fetchall_hashref" into a single	call. It returns a reference to	a hash
       containing one entry, at	most, for each row, as returned	by
       fetchall_hashref().

       The $statement parameter	can be a previously prepared statement handle,
       in which	case the "prepare" is skipped.	This is	recommended if the
       statement is going to be	executed many times.

       The $key_field parameter	defines	which column, or columns, are used as
       keys in the returned hash. It can either	be the name of a single	field,
       or a reference to an array containing multiple field names. Using
       multiple	names yields a tree of nested hashes.

       If a row	has the	same key as an earlier row then	it replaces the
       earlier row.

       If any method except "fetchall_hashref" fails, and "RaiseError" is not
       set, "selectall_hashref"	will return "undef".  If "fetchall_hashref"
       fails and "RaiseError" is not set, then it will return with whatever
       data it has fetched thus	far. $DBI::err should be checked to catch
       that.

       See fetchall_hashref() for more details.

       "selectcol_arrayref"

	 $ary_ref = $dbh->selectcol_arrayref($statement);
	 $ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
	 $ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);

       This utility method combines "prepare", "execute", and fetching one
       column from all the rows, into a	single call. It	returns	a reference to
       an array	containing the values of the first column from each row.

       The $statement parameter	can be a previously prepared statement handle,
       in which	case the "prepare" is skipped. This is recommended if the
       statement is going to be	executed many times.

       If any method except "fetch" fails, and "RaiseError" is not set,
       "selectcol_arrayref" will return	"undef".  If "fetch" fails and
       "RaiseError" is not set,	then it	will return with whatever data it has
       fetched thus far. $DBI::err should be checked to	catch that.

       The "selectcol_arrayref"	method defaults	to pushing a single column
       value (the first) from each row into the	result array. However, it can
       also push another column, or even multiple columns per row, into	the
       result array. This behaviour can	be specified via a '"Columns"'
       attribute which must be a ref to	an array containing the	column number
       or numbers to use. For example:

	 # get array of	id and name pairs:
	 my $ary_ref = $dbh->selectcol_arrayref("select	id, name from table", {	Columns=>[1,2] });
	 my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name

       You can specify a maximum number	of rows	to fetch by including a
       '"MaxRows"' attribute in	\%attr.

       "prepare"

	 $sth =	$dbh->prepare($statement)	   or die $dbh->errstr;
	 $sth =	$dbh->prepare($statement, \%attr)  or die $dbh->errstr;

       Prepares	a statement for	later execution	by the database	engine and
       returns a reference to a	statement handle object.

       The returned statement handle can be used to get	attributes of the
       statement and invoke the	"execute" method. See "Statement Handle
       Methods".

       Drivers for engines without the concept of preparing a statement	will
       typically just store the	statement in the returned handle and process
       it when "$sth->execute" is called. Such drivers are unlikely to give
       much useful information about the statement, such as
       "$sth->{NUM_OF_FIELDS}",	until after "$sth->execute" has	been called.
       Portable	applications should take this into account.

       In general, DBI drivers do not parse the	contents of the	statement
       (other than simply counting any Placeholders).  The statement is	passed
       directly	to the database	engine,	sometimes known	as pass-thru mode.
       This has	advantages and disadvantages. On the plus side,	you can	access
       all the functionality of	the engine being used. On the downside,	you're
       limited if you're using a simple	engine,	and you	need to	take extra
       care if writing applications intended to	be portable between engines.

       Portable	applications should not	assume that a new statement can	be
       prepared	and/or executed	while still fetching results from a previous
       statement.

       Some command-line SQL tools use statement terminators, like a
       semicolon, to indicate the end of a statement. Such terminators should
       not normally be used with the DBI.

       "prepare_cached"

	 $sth =	$dbh->prepare_cached($statement)
	 $sth =	$dbh->prepare_cached($statement, \%attr)
	 $sth =	$dbh->prepare_cached($statement, \%attr, $if_active)

       Like "prepare" except that the statement	handle returned	will be	stored
       in a hash associated with the $dbh. If another call is made to
       "prepare_cached"	with the same $statement and %attr parameter values,
       then the	corresponding cached $sth will be returned without contacting
       the database server. Be sure to understand the cautions and caveats
       noted below.

       The $if_active parameter	lets you adjust	the behaviour if an already
       cached statement	handle is still	Active.	 There are several
       alternatives:

       0: A warning will be generated, and finish() will be called on the
       statement handle	before it is returned.	This is	the default behaviour
       if $if_active is	not passed.
       1: finish() will	be called on the statement handle, but the warning is
       suppressed.
       2: Disables any checking.
       3: The existing active statement	handle will be removed from the	cache
       and a new statement handle prepared and cached in its place. This is
       the safest option because it doesn't affect the state of	the old
       handle, it just removes it from the cache. [Added in DBI	1.40]

       Here are	some examples of "prepare_cached":

	 sub insert_hash {
	   my ($table, $field_values) =	@_;
	   # sort to keep field	order, and thus	sql, stable for	prepare_cached
	   my @fields =	sort keys %$field_values;
	   my @values =	@{$field_values}{@fields};
	   my $sql = sprintf "insert into %s (%s) values (%s)",
	       $table, join(",", @fields), join(",", ("?")x@fields);
	   my $sth = $dbh->prepare_cached($sql);
	   return $sth->execute(@values);
	 }

	 sub search_hash {
	   my ($table, $field_values) =	@_;
	   # sort to keep field	order, and thus	sql, stable for	prepare_cached
	   my @fields =	sort keys %$field_values;
	   my @values =	@{$field_values}{@fields};
	   my $qualifier = "";
	   $qualifier =	"where ".join("	and ", map { "$_=?" } @fields) if @fields;
	   $sth	= $dbh->prepare_cached("SELECT * FROM $table $qualifier");
	   return $dbh->selectall_arrayref($sth, {}, @values);
	 }

       Caveat emptor: This caching can be useful in some applications, but it
       can also	cause problems and should be used with care. Here is a
       contrived case where caching would cause	a significant problem:

	 my $sth = $dbh->prepare_cached('SELECT	* FROM foo WHERE bar=?');
	 $sth->execute(...);
	 while (my $data = $sth->fetchrow_hashref) {

	   # later, in some other code called within the loop...
	   my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
	   $sth2->execute(...);
	   while (my $data2 = $sth2->fetchrow_arrayref)	{
	     do_stuff(...);
	   }
	 }

       In this example,	since both handles are preparing the exact same
       statement, $sth2	will not be its	own statement handle, but a duplicate
       of $sth returned	from the cache.	The results will certainly not be what
       you expect.  Typically the inner	fetch loop will	work normally,
       fetching	all the	records	and terminating	when there are no more,	but
       now that	$sth is	the same as $sth2 the outer fetch loop will also
       terminate.

       You'll know if you run into this	problem	because	prepare_cached() will
       generate	a warning by default (when $if_active is false).

       The cache used by prepare_cached() is keyed by both the statement and
       any attributes so you can also avoid this issue by doing	something
       like:

	 $sth =	$dbh->prepare_cached("...", { dbi_dummy	=> __FILE__.__LINE__ });

       which will ensure that prepare_cached only returns statements cached by
       that line of code in that source	file.

       Also, to	ensure the attributes passed are always	the same, avoid
       passing references inline. For example, the Slice attribute is
       specified as a reference. Be sure to declare it external	to the call to
       prepare_cached(), such that a new hash reference	is not created on
       every call. See "connect_cached"	for more details and examples.

       If you'd	like the cache to managed intelligently, you can tie the
       hashref returned	by "CachedKids"	to an appropriate caching module, such
       as Tie::Cache::LRU:

	 my $cache;
	 tie %$cache, 'Tie::Cache::LRU', 500;
	 $dbh->{CachedKids} = $cache;

       "commit"

	 $rc  =	$dbh->commit	 or die	$dbh->errstr;

       Commit (make permanent) the most	recent series of database changes if
       the database supports transactions and AutoCommit is off.

       If "AutoCommit" is on, then calling "commit" will issue a "commit
       ineffective with	AutoCommit" warning.

       See also	"Transactions" in the "FURTHER INFORMATION" section below.

       "rollback"

	 $rc  =	$dbh->rollback	 or die	$dbh->errstr;

       Rollback	(undo) the most	recent series of uncommitted database changes
       if the database supports	transactions and AutoCommit is off.

       If "AutoCommit" is on, then calling "rollback" will issue a "rollback
       ineffective with	AutoCommit" warning.

       See also	"Transactions" in the "FURTHER INFORMATION" section below.

       "begin_work"

	 $rc  =	$dbh->begin_work   or die $dbh->errstr;

       Enable transactions (by turning "AutoCommit" off) until the next	call
       to "commit" or "rollback". After	the next "commit" or "rollback",
       "AutoCommit" will automatically be turned on again.

       If "AutoCommit" is already off when "begin_work"	is called then it does
       nothing except return an	error. If the driver does not support
       transactions then when "begin_work" attempts to set "AutoCommit"	off
       the driver will trigger a fatal error.

       See also	"Transactions" in the "FURTHER INFORMATION" section below.

       "disconnect"

	 $rc = $dbh->disconnect	 or warn $dbh->errstr;

       Disconnects the database	from the database handle. "disconnect" is
       typically only used before exiting the program. The handle is of	little
       use after disconnecting.

       The transaction behaviour of the	"disconnect" method is,	sadly,
       undefined.  Some	database systems (such as Oracle and Ingres) will
       automatically commit any	outstanding changes, but others	(such as
       Informix) will rollback any outstanding changes.	 Applications not
       using "AutoCommit" should explicitly call "commit" or "rollback"	before
       calling "disconnect".

       The database is automatically disconnected by the "DESTROY" method if
       still connected when there are no longer	any references to the handle.
       The "DESTROY" method for	each driver should implicitly call "rollback"
       to undo any uncommitted changes.	This is	vital behaviour	to ensure that
       incomplete transactions don't get committed simply because Perl calls
       "DESTROY" on every object before	exiting. Also, do not rely on the
       order of	object destruction during "global destruction",	as it is
       undefined.

       Generally, if you want your changes to be committed or rolled back when
       you disconnect, then you	should explicitly call "commit"	or "rollback"
       before disconnecting.

       If you disconnect from a	database while you still have active statement
       handles (e.g., SELECT statement handles that may	have more data to
       fetch), you will	get a warning. The warning may indicate	that a fetch
       loop terminated early, perhaps due to an	uncaught error.	 To avoid the
       warning call the	"finish" method	on the active handles.

       "ping"

	 $rc = $dbh->ping;

       Attempts	to determine, in a reasonably efficient	way, if	the database
       server is still running and the connection to it	is still working.
       Individual drivers should implement this	function in the	most suitable
       manner for their	database engine.

       The current default implementation always returns true without actually
       doing anything. Actually, it returns ""0	but true"" which is true but
       zero. That way you can tell if the return value is genuine or just the
       default.	Drivers	should override	this method with one that does the
       right thing for their type of database.

       Few applications	would have direct use for this method. See the
       specialized Apache::DBI module for one example usage.

       "get_info"

	 $value	= $dbh->get_info( $info_type );

       Returns information about the implementation, i.e. driver and data
       source capabilities, restrictions etc. It returns "undef" for unknown
       or unimplemented	information types. For example:

	 $database_version  = $dbh->get_info(  18 ); # SQL_DBMS_VER
	 $max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT

       See "Standards Reference	Information" for more detailed information
       about the information types and their meanings and possible return
       values.

       The DBI::Const::GetInfoType module exports a %GetInfoType hash that can
       be used to map info type	names to numbers. For example:

	 $database_version = $dbh->get_info( $GetInfoType{SQL_DBMS_VER}	);

       The names are a merging of the ANSI and ODBC standards (which differ in
       some cases). See	DBI::Const::GetInfoType	for more details.

       Because some DBI	methods	make use of get_info(),	drivers	are strongly
       encouraged to support at	least the following very minimal set of
       information types to ensure the DBI itself works	properly:

	Type  Name			  Example A	Example	B
	----  --------------------------  ------------	----------------
	  17  SQL_DBMS_NAME		  'ACCESS'	'Oracle'
	  18  SQL_DBMS_VER		  '03.50.0000'	'08.01.0721 ...'
	  29  SQL_IDENTIFIER_QUOTE_CHAR	  '`'		'"'
	  41  SQL_CATALOG_NAME_SEPARATOR  '.'		'@'
	 114  SQL_CATALOG_LOCATION	  1		2

       Values from 9000	to 9999	for get_info are officially reserved for use
       by Perl DBI.  Values in that range which	have been assigned a meaning
       are defined here:

       9000: true if a backslash character ("\") before	placeholder-like text
       (e.g. "?", ":foo") will prevent it being	treated	as a placeholder by
       the driver.  The	backslash will be removed before the text is passed to
       the backend.

       "table_info"

	 $sth =	$dbh->table_info( $catalog, $schema, $table, $type );
	 $sth =	$dbh->table_info( $catalog, $schema, $table, $type, \%attr );

	 # then	$sth->fetchall_arrayref	or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about tables	and views that exist in	the database.

       The arguments $catalog, $schema and $table may accept search patterns
       according to the	database/driver, for example: $table = '%FOO%';
       Remember	that the underscore character ('"_"') is a search pattern that
       means match any character, so 'FOO_%' is	the same as 'FOO%' and
       'FOO_BAR%' will match names like	'FOO1BAR'.

       The value of $type is a comma-separated list of one or more types of
       tables to be returned in	the result set.	Each value may optionally be
       quoted, e.g.:

	 $type = "TABLE";
	 $type = "'TABLE','VIEW'";

       In addition the following special cases may also	be supported by	some
       drivers:

       o   If the value	of $catalog is '%' and $schema and $table name are
	   empty strings, the result set contains a list of catalog names.
	   For example:

	     $sth = $dbh->table_info('%', '', '');

       o   If the value	of $schema is '%' and $catalog and $table are empty
	   strings, the	result set contains a list of schema names.

       o   If the value	of $type is '%'	and $catalog, $schema, and $table are
	   all empty strings, the result set contains a	list of	table types.

       If your driver doesn't support one or more of the selection filter
       parameters then you may get back	more than you asked for	and can	do the
       filtering yourself.

       This method can be expensive, and can return a large amount of data.
       (For example, small Oracle installation returns over 2000 rows.)	 So
       it's a good idea	to use the filters to limit the	data as	much as
       possible.

       The statement handle returned has at least the following	fields in the
       order show below. Other fields, after these, may	also be	present.

       TABLE_CAT: Table	catalog	identifier. This field is NULL ("undef") if
       not applicable to the data source, which	is usually the case. This
       field is	empty if not applicable	to the table.

       TABLE_SCHEM: The	name of	the schema containing the TABLE_NAME value.
       This field is NULL ("undef") if not applicable to data source, and
       empty if	not applicable to the table.

       TABLE_NAME: Name	of the table (or view, synonym,	etc).

       TABLE_TYPE: One of the following: "TABLE", "VIEW", "SYSTEM TABLE",
       "GLOBAL TEMPORARY", "LOCAL TEMPORARY", "ALIAS", "SYNONYM" or a type
       identifier that is specific to the data source.

       REMARKS:	A description of the table. May	be NULL	("undef").

       Note that "table_info" might not	return records for all tables.
       Applications can	use any	valid table regardless of whether it's
       returned	by "table_info".

       See also	"tables", "Catalog Methods" and	"Standards Reference
       Information".

       "column_info"

	 $sth =	$dbh->column_info( $catalog, $schema, $table, $column );

	 # then	$sth->fetchall_arrayref	or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about columns in specified tables.

       The arguments $schema, $table and $column may accept search patterns
       according to the	database/driver, for example: $table = '%FOO%';

       Note: The support for the selection criteria is driver specific.	If the
       driver doesn't support one or more of them then you may get back	more
       than you	asked for and can do the filtering yourself.

       Note: If	your driver does not support column_info an undef is returned.
       This is distinct	from asking for	something which	does not exist in a
       driver which supports column_info as a valid statement handle to	an
       empty result-set	will be	returned in this case.

       If the arguments	don't match any	tables then you'll still get a
       statement handle, it'll just return no rows.

       The statement handle returned has at least the following	fields in the
       order shown below. Other	fields,	after these, may also be present.

       TABLE_CAT: The catalog identifier.  This	field is NULL ("undef")	if not
       applicable to the data source, which is often the case.	This field is
       empty if	not applicable to the table.

       TABLE_SCHEM: The	schema identifier.  This field is NULL ("undef") if
       not applicable to the data source, and empty if not applicable to the
       table.

       TABLE_NAME: The table identifier.  Note:	A driver may provide column
       metadata	not only for base tables, but also for derived objects like
       SYNONYMS	etc.

       COLUMN_NAME: The	column identifier.

       DATA_TYPE: The concise data type	code.

       TYPE_NAME: A data source	dependent data type name.

       COLUMN_SIZE: The	column size.  This is the maximum length in characters
       for character data types, the number of digits or bits for numeric data
       types or	the length in the representation of temporal types.  See the
       relevant	specifications for detailed information.

       BUFFER_LENGTH: The length in bytes of transferred data.

       DECIMAL_DIGITS: The total number	of significant digits to the right of
       the decimal point.

       NUM_PREC_RADIX: The radix for numeric precision.	 The value is 10 or 2
       for numeric data	types and NULL ("undef") if not	applicable.

       NULLABLE: Indicates if a	column can accept NULLs.  The following	values
       are defined:

	 SQL_NO_NULLS	       0
	 SQL_NULLABLE	       1
	 SQL_NULLABLE_UNKNOWN  2

       REMARKS:	A description of the column.

       COLUMN_DEF: The default value of	the column, in a format	that can be
       used directly in	an SQL statement.

       Note that this may be an	expression and not simply the text used	for
       the default value in the	original CREATE	TABLE statement. For example,
       given:

	   col1	char(30) default current_user	 -- a 'function'
	   col2	char(30) default 'string'	 -- a string literal

       where "current_user" is the name	of a function, the corresponding
       "COLUMN_DEF" values would be:

	   Database	   col1			    col2
	   --------	   ----			    ----
	   Oracle:	   current_user		    'string'
	   Postgres:	   "current_user"()	    'string'::text
	   MS SQL:	   (user_name())	    ('string')

       SQL_DATA_TYPE: The SQL data type.

       SQL_DATETIME_SUB: The subtype code for datetime and interval data
       types.

       CHAR_OCTET_LENGTH: The maximum length in	bytes of a character or	binary
       data type column.

       ORDINAL_POSITION: The column sequence number (starting with 1).

       IS_NULLABLE: Indicates if the column can	accept NULLs.  Possible	values
       are: 'NO', 'YES'	and ''.

       SQL/CLI defines the following additional	columns:

	 CHAR_SET_CAT
	 CHAR_SET_SCHEM
	 CHAR_SET_NAME
	 COLLATION_CAT
	 COLLATION_SCHEM
	 COLLATION_NAME
	 UDT_CAT
	 UDT_SCHEM
	 UDT_NAME
	 DOMAIN_CAT
	 DOMAIN_SCHEM
	 DOMAIN_NAME
	 SCOPE_CAT
	 SCOPE_SCHEM
	 SCOPE_NAME
	 MAX_CARDINALITY
	 DTD_IDENTIFIER
	 IS_SELF_REF

       Drivers capable of supplying any	of those values	should do so in	the
       corresponding column and	supply undef values for	the others.

       Drivers wishing to provide extra	database/driver	specific information
       should do so in extra columns beyond all	those listed above, and	use
       lowercase field names with the driver-specific prefix (i.e.,
       'ora_...'). Applications	accessing such fields should do	so by name and
       not by column number.

       The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and
       ORDINAL_POSITION.

       Note: There is some overlap with	statement handle attributes (in	perl)
       and SQLDescribeCol (in ODBC). However, SQLColumns provides more
       metadata.

       See also	"Catalog Methods" and "Standards Reference Information".

       "primary_key_info"

	 $sth =	$dbh->primary_key_info(	$catalog, $schema, $table );

	 # then	$sth->fetchall_arrayref	or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about columns that make up the primary key for a table.
       The arguments don't accept search patterns (unlike table_info()).

       The statement handle will return	one row	per column, ordered by
       TABLE_CAT, TABLE_SCHEM, TABLE_NAME, and KEY_SEQ.	 If there is no
       primary key then	the statement handle will fetch	no rows.

       Note: The support for the selection criteria, such as $catalog, is
       driver specific.	 If the	driver doesn't support catalogs	and/or
       schemas,	it may ignore these criteria.

       The statement handle returned has at least the following	fields in the
       order shown below. Other	fields,	after these, may also be present.

       TABLE_CAT: The catalog identifier.  This	field is NULL ("undef")	if not
       applicable to the data source, which is often the case.	This field is
       empty if	not applicable to the table.

       TABLE_SCHEM: The	schema identifier.  This field is NULL ("undef") if
       not applicable to the data source, and empty if not applicable to the
       table.

       TABLE_NAME: The table identifier.

       COLUMN_NAME: The	column identifier.

       KEY_SEQ:	The column sequence number (starting with 1).  Note: This
       field is	named ORDINAL_POSITION in SQL/CLI.

       PK_NAME:	The primary key	constraint identifier.	This field is NULL
       ("undef") if not	applicable to the data source.

       See also	"Catalog Methods" and "Standards Reference Information".

       "primary_key"

	 @key_column_names = $dbh->primary_key(	$catalog, $schema, $table );

       Simple interface	to the primary_key_info() method. Returns a list of
       the column names	that comprise the primary key of the specified table.
       The list	is in primary key column sequence order.  If there is no
       primary key then	an empty list is returned.

       "foreign_key_info"

	 $sth =	$dbh->foreign_key_info(	$pk_catalog, $pk_schema, $pk_table
				      ,	$fk_catalog, $fk_schema, $fk_table );

	 $sth =	$dbh->foreign_key_info(	$pk_catalog, $pk_schema, $pk_table
				      ,	$fk_catalog, $fk_schema, $fk_table
				      ,	\%attr );

	 # then	$sth->fetchall_arrayref	or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       information about foreign keys in and/or	referencing the	specified
       table(s).  The arguments	don't accept search patterns (unlike
       table_info()).

       $pk_catalog, $pk_schema,	$pk_table identify the primary (unique)	key
       table (PKT).

       $fk_catalog, $fk_schema,	$fk_table identify the foreign key table
       (FKT).

       If both PKT and FKT are given, the function returns the foreign key, if
       any, in table FKT that refers to	the primary (unique) key of table PKT.
       (Note: In SQL/CLI, the result is	implementation-defined.)

       If only PKT is given, then the result set contains the primary key of
       that table and all foreign keys that refer to it.

       If only FKT is given, then the result set contains all foreign keys in
       that table and the primary keys to which	they refer.  (Note: In
       SQL/CLI,	the result includes unique keys	too.)

       For example:

	 $sth =	$dbh->foreign_key_info(	undef, $user, 'master');
	 $sth =	$dbh->foreign_key_info(	undef, undef,	undef ,	undef, $user, 'detail');
	 $sth =	$dbh->foreign_key_info(	undef, $user, 'master',	undef, $user, 'detail');

	 # then	$sth->fetchall_arrayref	or $sth->fetchall_hashref etc

       Note: The support for the selection criteria, such as $catalog, is
       driver specific.	 If the	driver doesn't support catalogs	and/or
       schemas,	it may ignore these criteria.

       The statement handle returned has the following fields in the order
       shown below.  Because ODBC never	includes unique	keys, they define
       different columns in the	result set than	SQL/CLI. SQL/CLI column	names
       are shown in parentheses.

       PKTABLE_CAT    (	UK_TABLE_CAT	  ): The primary (unique) key table
       catalog identifier.  This field is NULL ("undef") if not	applicable to
       the data	source,	which is often the case.  This field is	empty if not
       applicable to the table.

       PKTABLE_SCHEM  (	UK_TABLE_SCHEM	  ): The primary (unique) key table
       schema identifier.  This	field is NULL ("undef")	if not applicable to
       the data	source,	and empty if not applicable to the table.

       PKTABLE_NAME   (	UK_TABLE_NAME	  ): The primary (unique) key table
       identifier.

       PKCOLUMN_NAME  (UK_COLUMN_NAME	 ): The	primary	(unique) key column
       identifier.

       FKTABLE_CAT    (	FK_TABLE_CAT	  ): The foreign key table catalog
       identifier.  This field is NULL ("undef") if not	applicable to the data
       source, which is	often the case.	 This field is empty if	not applicable
       to the table.

       FKTABLE_SCHEM  (	FK_TABLE_SCHEM	  ): The foreign key table schema
       identifier.  This field is NULL ("undef") if not	applicable to the data
       source, and empty if not	applicable to the table.

       FKTABLE_NAME   (	FK_TABLE_NAME	  ): The foreign key table identifier.

       FKCOLUMN_NAME  (	FK_COLUMN_NAME	  ): The foreign key column
       identifier.

       KEY_SEQ	      (	ORDINAL_POSITION  ): The column	sequence number
       (starting with 1).

       UPDATE_RULE    (	UPDATE_RULE	  ): The referential action for	the
       UPDATE rule.  The following codes are defined:

	 CASCADE	      0
	 RESTRICT	      1
	 SET NULL	      2
	 NO ACTION	      3
	 SET DEFAULT	      4

       DELETE_RULE    (	DELETE_RULE	  ): The referential action for	the
       DELETE rule.  The codes are the same as for UPDATE_RULE.

       FK_NAME	      (	FK_NAME		  ): The foreign key name.

       PK_NAME	      (	UK_NAME		  ): The primary (unique) key name.

       DEFERRABILITY  (	DEFERABILITY	  ): The deferrability of the foreign
       key constraint.	The following codes are	defined:

	 INITIALLY DEFERRED   5
	 INITIALLY IMMEDIATE  6
	 NOT DEFERRABLE	      7

		      (	UNIQUE_OR_PRIMARY ): This column is necessary if a
       driver includes all candidate (i.e. primary and alternate) keys in the
       result set (as specified	by SQL/CLI).  The value	of this	column is
       UNIQUE if the foreign key references an alternate key and PRIMARY if
       the foreign key references a primary key, or it may be undefined	if the
       driver doesn't have access to the information.

       See also	"Catalog Methods" and "Standards Reference Information".

       "statistics_info"

       Warning:	This method is experimental and	may change.

	 $sth =	$dbh->statistics_info( $catalog, $schema, $table, $unique_only,	$quick );

	 # then	$sth->fetchall_arrayref	or $sth->fetchall_hashref etc

       Returns an active statement handle that can be used to fetch
       statistical information about a table and its indexes.

       The arguments don't accept search patterns (unlike "table_info").

       If the boolean argument $unique_only is true, only UNIQUE indexes will
       be returned in the result set, otherwise	all indexes will be returned.

       If the boolean argument $quick is set, the actual statistical
       information columns (CARDINALITY	and PAGES) will	only be	returned if
       they are	readily	available from the server, and might not be current.
       Some databases may return stale statistics or no	statistics at all with
       this flag set.

       The statement handle will return	at most	one row	per column name	per
       index, plus at most one row for the entire table	itself,	ordered	by
       NON_UNIQUE, TYPE, INDEX_QUALIFIER, INDEX_NAME, and ORDINAL_POSITION.

       Note: The support for the selection criteria, such as $catalog, is
       driver specific.	 If the	driver doesn't support catalogs	and/or
       schemas,	it may ignore these criteria.

       The statement handle returned has at least the following	fields in the
       order shown below. Other	fields,	after these, may also be present.

       TABLE_CAT: The catalog identifier.  This	field is NULL ("undef")	if not
       applicable to the data source, which is often the case.	This field is
       empty if	not applicable to the table.

       TABLE_SCHEM: The	schema identifier.  This field is NULL ("undef") if
       not applicable to the data source, and empty if not applicable to the
       table.

       TABLE_NAME: The table identifier.

       NON_UNIQUE: Unique index	indicator.  Returns 0 for unique indexes, 1
       for non-unique indexes

       INDEX_QUALIFIER:	Index qualifier	identifier.  The identifier that is
       used to qualify the index name when doing a "DROP INDEX"; NULL
       ("undef") is returned if	an index qualifier is not supported by the
       data source.  If	a non-NULL (defined) value is returned in this column,
       it must be used to qualify the index name on a "DROP INDEX" statement;
       otherwise, the TABLE_SCHEM should be used to qualify the	index name.

       INDEX_NAME: The index identifier.

       TYPE: The type of information being returned.  Can be any of the
       following values: 'table', 'btree', 'clustered',	'content', 'hashed',
       or 'other'.

       In the case that	this field is 'table', all fields other	than
       TABLE_CAT, TABLE_SCHEM, TABLE_NAME, TYPE, CARDINALITY, and PAGES	will
       be NULL ("undef").

       ORDINAL_POSITION: Column	sequence number	(starting with 1).

       COLUMN_NAME: The	column identifier.

       ASC_OR_DESC: Column sort	sequence.  "A" for Ascending, "D" for
       Descending, or NULL ("undef") if	not supported for this index.

       CARDINALITY: Cardinality	of the table or	index.	For indexes, this is
       the number of unique values in the index.  For tables, this is the
       number of rows in the table.  If	not supported, the value will be NULL
       ("undef").

       PAGES: Number of	storage	pages used by this table or index.  If not
       supported, the value will be NULL ("undef").

       FILTER_CONDITION: The index filter condition as a string.  If the index
       is not a	filtered index,	or it cannot be	determined whether the index
       is a filtered index, this value is NULL ("undef").  If the index	is a
       filtered	index, but the filter condition	cannot be determined, this
       value is	the empty string ''.  Otherwise	it will	be the literal filter
       condition as a string, such as "SALARY <= 4500".

       See also	"Catalog Methods" and "Standards Reference Information".

       "tables"

	 @names	= $dbh->tables(	$catalog, $schema, $table, $type );
	 @names	= $dbh->tables;	       # deprecated

       Simple interface	to table_info(). Returns a list	of matching table
       names, possibly including a catalog/schema prefix.

       See "table_info"	for a description of the parameters.

       If "$dbh->get_info(29)" returns true (29	is SQL_IDENTIFIER_QUOTE_CHAR)
       then the	table names are	constructed and	quoted by "quote_identifier"
       to ensure they are usable even if they contain whitespace or reserved
       words etc. This means that the table names returned will	include	quote
       characters.

       "type_info_all"

	 $type_info_all	= $dbh->type_info_all;

       Returns a reference to an array which holds information about each data
       type variant supported by the database and driver. The array and	its
       contents	should be treated as read-only.

       The first item is a reference to	an 'index' hash	of "Name ="> "Index"
       pairs.  The items following that	are references to arrays, one per
       supported data type variant. The	leading	index hash defines the names
       and order of the	fields within the arrays that follow it.  For example:

	 $type_info_all	= [
	   {   TYPE_NAME	 => 0,
	       DATA_TYPE	 => 1,
	       COLUMN_SIZE	 => 2,	   # was PRECISION originally
	       LITERAL_PREFIX	 => 3,
	       LITERAL_SUFFIX	 => 4,
	       CREATE_PARAMS	 => 5,
	       NULLABLE		 => 6,
	       CASE_SENSITIVE	 => 7,
	       SEARCHABLE	 => 8,
	       UNSIGNED_ATTRIBUTE=> 9,
	       FIXED_PREC_SCALE	 => 10,	   # was MONEY originally
	       AUTO_UNIQUE_VALUE => 11,	   # was AUTO_INCREMENT	originally
	       LOCAL_TYPE_NAME	 => 12,
	       MINIMUM_SCALE	 => 13,
	       MAXIMUM_SCALE	 => 14,
	       SQL_DATA_TYPE	 => 15,
	       SQL_DATETIME_SUB	 => 16,
	       NUM_PREC_RADIX	 => 17,
	       INTERVAL_PRECISION=> 18,
	   },
	   [ 'VARCHAR',	SQL_VARCHAR,
	       undef, "'","'", undef,0,	1,1,0,0,0,undef,1,255, undef
	   ],
	   [ 'INTEGER',	SQL_INTEGER,
	       undef,  "", "", undef,0,	0,1,0,0,0,undef,0,  0, 10
	   ],
	 ];

       More than one row may have the same value in the	"DATA_TYPE" field if
       there are different ways	to spell the type name and/or there are
       variants	of the type with different attributes (e.g., with and without
       "AUTO_UNIQUE_VALUE" set,	with and without "UNSIGNED_ATTRIBUTE", etc).

       The rows	are ordered by "DATA_TYPE" first and then by how closely each
       type maps to the	corresponding ODBC SQL data type, closest first.

       The meaning of the fields is described in the documentation for the
       "type_info" method.

       An 'index' hash is provided so you don't	need to	rely on	index values
       defined above.  However,	using DBD::ODBC	with some old ODBC drivers may
       return older names, shown as comments in	the example above.  Another
       issue with the index hash is that the lettercase	of the keys is not
       defined.	It is usually uppercase, as show here, but drivers may return
       names with any lettercase.

       Drivers are also	free to	return extra driver-specific columns of
       information - though it's recommended that they start at	column index
       50 to leave room	for expansion of the DBI/ODBC specification.

       The type_info_all() method is not normally used directly.  The
       "type_info" method provides a more usable and useful interface to the
       data.

       "type_info"

	 @type_info = $dbh->type_info($data_type);

       Returns a list of hash references holding information about one or more
       variants	of $data_type. The list	is ordered by "DATA_TYPE" first	and
       then by how closely each	type maps to the corresponding ODBC SQL	data
       type, closest first.  If	called in a scalar context then	only the first
       (best) element is returned.

       If $data_type is	undefined or "SQL_ALL_TYPES", then the list will
       contain hashes for all data type	variants supported by the database and
       driver.

       If $data_type is	an array reference then	"type_info" returns the
       information for the first type in the array that	has any	matches.

       The keys	of the hash follow the same letter case	conventions as the
       rest of the DBI (see "Naming Conventions	and Name Space"). The
       following uppercase items should	always exist, though may be undef:

       TYPE_NAME (string)
	   Data	type name for use in CREATE TABLE statements etc.

       DATA_TYPE (integer)
	   SQL data type number.

       COLUMN_SIZE (integer)
	   For numeric types, this is either the total number of digits	(if
	   the NUM_PREC_RADIX value is 10) or the total	number of bits allowed
	   in the column (if NUM_PREC_RADIX is 2).

	   For string types, this is the maximum size of the string in
	   characters.

	   For date and	interval types,	this is	the maximum number of
	   characters needed to	display	the value.

       LITERAL_PREFIX (string)
	   Characters used to prefix a literal.	A typical prefix is ""'"" for
	   characters, or possibly ""0x"" for binary values passed as
	   hexadecimal.	 NULL ("undef")	is returned for	data types for which
	   this	is not applicable.

       LITERAL_SUFFIX (string)
	   Characters used to suffix a literal.	Typically ""'""	for
	   characters.	NULL ("undef") is returned for data types where	this
	   is not applicable.

       CREATE_PARAMS (string)
	   Parameter names for data type definition. For example,
	   "CREATE_PARAMS" for a "DECIMAL" would be ""precision,scale""	if the
	   DECIMAL type	should be declared as "DECIMAL("precision,scale")"
	   where precision and scale are integer values.  For a	"VARCHAR" it
	   would be ""max length"".  NULL ("undef") is returned	for data types
	   for which this is not applicable.

       NULLABLE	(integer)
	   Indicates whether the data type accepts a NULL value: 0 or an empty
	   string = no,	1 = yes, 2 = unknown.

       CASE_SENSITIVE (boolean)
	   Indicates whether the data type is case sensitive in	collations and
	   comparisons.

       SEARCHABLE (integer)
	   Indicates how the data type can be used in a	WHERE clause, as
	   follows:

	     0 - Cannot	be used	in a WHERE clause
	     1 - Only with a LIKE predicate
	     2 - All comparison	operators except LIKE
	     3 - Can be	used in	a WHERE	clause with any	comparison operator

       UNSIGNED_ATTRIBUTE (boolean)
	   Indicates whether the data type is unsigned.	 NULL ("undef")	is
	   returned for	data types for which this is not applicable.

       FIXED_PREC_SCALE	(boolean)
	   Indicates whether the data type always has the same precision and
	   scale (such as a money type).  NULL ("undef") is returned for data
	   types for which this	is not applicable.

       AUTO_UNIQUE_VALUE (boolean)
	   Indicates whether a column of this data type	is automatically set
	   to a	unique value whenever a	new row	is inserted.  NULL ("undef")
	   is returned for data	types for which	this is	not applicable.

       LOCAL_TYPE_NAME (string)
	   Localized version of	the "TYPE_NAME"	for use	in dialog with users.
	   NULL	("undef") is returned if a localized name is not available (in
	   which case "TYPE_NAME" should be used).

       MINIMUM_SCALE (integer)
	   The minimum scale of	the data type. If a data type has a fixed
	   scale, then "MAXIMUM_SCALE" holds the same value.  NULL ("undef")
	   is returned for data	types for which	this is	not applicable.

       MAXIMUM_SCALE (integer)
	   The maximum scale of	the data type. If a data type has a fixed
	   scale, then "MINIMUM_SCALE" holds the same value.  NULL ("undef")
	   is returned for data	types for which	this is	not applicable.

       SQL_DATA_TYPE (integer)
	   This	column is the same as the "DATA_TYPE" column, except for
	   interval and	datetime data types.  For interval and datetime	data
	   types, the "SQL_DATA_TYPE" field will return	"SQL_INTERVAL" or
	   "SQL_DATETIME", and the "SQL_DATETIME_SUB" field below will return
	   the subcode for the specific	interval or datetime data type.	If
	   this	field is NULL, then the	driver does not	support	or report on
	   interval or datetime	subtypes.

       SQL_DATETIME_SUB	(integer)
	   For interval	or datetime data types,	where the "SQL_DATA_TYPE"
	   field above is "SQL_INTERVAL" or "SQL_DATETIME", this field will
	   hold	the subcode for	the specific interval or datetime data type.
	   Otherwise it	will be	NULL ("undef").

	   Although not	mentioned explicitly in	the standards, it seems	there
	   is a	simple relationship between these values:

	     DATA_TYPE == (10 *	SQL_DATA_TYPE) + SQL_DATETIME_SUB

       NUM_PREC_RADIX (integer)
	   The radix value of the data type. For approximate numeric types,
	   "NUM_PREC_RADIX" contains the value 2 and "COLUMN_SIZE" holds the
	   number of bits. For exact numeric types, "NUM_PREC_RADIX" contains
	   the value 10	and "COLUMN_SIZE" holds	the number of decimal digits.
	   NULL	("undef") is returned either for data types for	which this is
	   not applicable or if	the driver cannot report this information.

       INTERVAL_PRECISION (integer)
	   The interval	leading	precision for interval types. NULL is returned
	   either for data types for which this	is not applicable or if	the
	   driver cannot report	this information.

       For example, to find the	type name for the fields in a select statement
       you can do:

	 @names	= map {	scalar $dbh->type_info($_)->{TYPE_NAME}	} @{ $sth->{TYPE} }

       Since DBI and ODBC drivers vary in how they map their types into	the
       ISO standard types you may need to search for more than one type.
       Here's an example looking for a usable type to store a date:

	 $my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );

       Similarly, to more reliably find	a type to store	small integers,	you
       could use a list	starting with "SQL_SMALLINT", "SQL_INTEGER",
       "SQL_DECIMAL", etc.

       See also	"Standards Reference Information".

       "quote"

	 $sql =	$dbh->quote($value);
	 $sql =	$dbh->quote($value, $data_type);

       Quote a string literal for use as a literal value in an SQL statement,
       by escaping any special characters (such	as quotation marks) contained
       within the string and adding the	required type of outer quotation
       marks.

	 $sql =	sprintf	"SELECT	foo FROM bar WHERE baz = %s",
		       $dbh->quote("Don't");

       For most	database types,	at least those that conform to SQL standards,
       quote would return 'Don''t' (including the outer	quotation marks). For
       others it may return something like 'Don\'t'

       An undefined $value value will be returned as the string	"NULL"
       (without	single quotation marks)	to match how NULLs are represented in
       SQL.

       If $data_type is	supplied, it is	used to	try to determine the required
       quoting behaviour by using the information returned by "type_info".  As
       a special case, the standard numeric types are optimized	to return
       $value without calling "type_info".

       Quote will probably not be able to deal with all	possible input (such
       as binary data or data containing newlines), and	is not related in any
       way with	escaping or quoting shell meta-characters.

       It is valid for the quote() method to return an SQL expression that
       evaluates to the	desired	string.	For example:

	 $quoted = $dbh->quote("one\ntwo\0three")

       may return something like:

	 CONCAT('one', CHAR(12), 'two',	CHAR(0), 'three')

       The quote() method should not be	used with "Placeholders	and Bind
       Values".

       "quote_identifier"

	 $sql =	$dbh->quote_identifier(	$name );
	 $sql =	$dbh->quote_identifier(	$catalog, $schema, $table, \%attr );

       Quote an	identifier (table name etc.) for use in	an SQL statement, by
       escaping	any special characters (such as	double quotation marks)	it
       contains	and adding the required	type of	outer quotation	marks.

       Undefined names are ignored and the remainder are quoted	and then
       joined together,	typically with a dot (".") character. For example:

	 $id = $dbh->quote_identifier( undef, 'Her schema', 'My	table' );

       would, for most database	types, return "Her schema"."My table"
       (including all the double quotation marks).

       If three	names are supplied then	the first is assumed to	be a catalog
       name and	special	rules may be applied based on what "get_info" returns
       for SQL_CATALOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCATION (114).
       For example, for	Oracle:

	 $id = $dbh->quote_identifier( 'link', 'schema', 'table' );

       would return "schema"."table"@"link".

       "take_imp_data"

	 $imp_data = $dbh->take_imp_data;

       Leaves the $dbh in an almost dead, zombie-like, state and returns a
       binary string of	raw implementation data	from the driver	which
       describes the current database connection. Effectively it detaches the
       underlying database API connection data from the	DBI handle.  After
       calling take_imp_data(),	all other methods except "DESTROY" will
       generate	a warning and return undef.

       Why would you want to do	this? You don't, forget	I even mentioned it.
       Unless, that is,	you're implementing something advanced like a multi-
       threaded	connection pool. See DBI::Pool.

       The returned $imp_data can be passed as a "dbi_imp_data"	attribute to a
       later connect() call, even in a separate	thread in the same process,
       where the driver	can use	it to 'adopt' the existing connection that the
       implementation data was taken from.

       Some things to keep in mind...

       * the $imp_data holds the only reference	to the underlying database API
       connection data.	That connection	is still 'live'	and won't be cleaned
       up properly unless the $imp_data	is used	to create a new	$dbh which is
       then allowed to disconnect() normally.

       * using the same	$imp_data to create more than one other	new $dbh at a
       time may	well lead to unpleasant	problems. Don't	do that.

       Any child statement handles are effectively destroyed when
       take_imp_data() is called.

       The "take_imp_data" method was added in DBI 1.36	but wasn't useful till
       1.49.

   Database Handle Attributes
       This section describes attributes specific to database handles.

       Changes to these	database handle	attributes do not affect any other
       existing	or future database handles.

       Attempting to set or get	the value of an	unknown	attribute generates a
       warning,	except for private driver-specific attributes (which all have
       names starting with a lowercase letter).

       Example:

	 $h->{AutoCommit} = ...;       # set/write
	 ... = $h->{AutoCommit};       # get/read

       "AutoCommit"

       Type: boolean

       If true,	then database changes cannot be	rolled-back (undone).  If
       false, then database changes automatically occur	within a
       "transaction", which must either	be committed or	rolled back using the
       "commit"	or "rollback" methods.

       Drivers should always default to	"AutoCommit" mode (an unfortunate
       choice largely forced on	the DBI	by ODBC	and JDBC conventions.)

       Attempting to set "AutoCommit" to an unsupported	value is a fatal
       error.  This is an important feature of the DBI.	Applications that need
       full transaction	behaviour can set "$dbh->{AutoCommit} =	0" (or set
       "AutoCommit" to 0 via "connect")	without	having to check	that the value
       was assigned successfully.

       For the purposes	of this	description, we	can divide databases into
       three categories:

	 Databases which don't support transactions at all.
	 Databases in which a transaction is always active.
	 Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).

       * Databases which don't support transactions at all

       For these databases, attempting to turn "AutoCommit" off	is a fatal
       error.  "commit"	and "rollback" both issue warnings about being
       ineffective while "AutoCommit" is in effect.

       * Databases in which a transaction is always active

       These are typically mainstream commercial relational databases with
       "ANSI standard" transaction behaviour.  If "AutoCommit" is off, then
       changes to the database won't have any lasting effect unless "commit"
       is called (but see also "disconnect"). If "rollback" is called then any
       changes since the last commit are undone.

       If "AutoCommit" is on, then the effect is the same as if	the DBI	called
       "commit"	automatically after every successful database operation. So
       calling "commit"	or "rollback" explicitly while "AutoCommit" is on
       would be	ineffective because the	changes	would have already been
       committed.

       Changing	"AutoCommit" from off to on will trigger a "commit".

       For databases which don't support a specific auto-commit	mode, the
       driver has to commit each statement automatically using an explicit
       "COMMIT"	after it completes successfully	(and roll it back using	an
       explicit	"ROLLBACK" if it fails).  The error information	reported to
       the application will correspond to the statement	which was executed,
       unless it succeeded and the commit or rollback failed.

       * Databases in which a transaction must be explicitly started

       For these databases, the	intention is to	have them act like databases
       in which	a transaction is always	active (as described above).

       To do this, the driver will automatically begin an explicit transaction
       when "AutoCommit" is turned off,	or after a "commit" or "rollback" (or
       when the	application issues the next database operation after one of
       those events).

       In this way, the	application does not have to treat these databases as
       a special case.

       See "commit", "disconnect" and "Transactions" for other important notes
       about transactions.

       "Driver"

       Type: handle

       Holds the handle	of the parent driver. The only recommended use for
       this is to find the name	of the driver using:

	 $dbh->{Driver}->{Name}

       "Name"

       Type: string

       Holds the "name"	of the database. Usually (and recommended to be) the
       same as the ""dbi:DriverName:..."" string used to connect to the
       database, but with the leading ""dbi:DriverName:"" removed.

       "Statement"

       Type: string, read-only

       Returns the statement string passed to the most recent "prepare"	or
       "do" method called in this database handle, even	if that	method failed.
       This is especially useful where "RaiseError" is enabled and the
       exception handler checks	$@ and sees that a 'prepare' method call
       failed.

       "RowCacheSize"

       Type: integer

       A hint to the driver indicating the size	of the local row cache that
       the application would like the driver to	use for	future "SELECT"
       statements.  If a row cache is not implemented, then setting
       "RowCacheSize" is ignored and getting the value returns "undef".

       Some "RowCacheSize" values have special meaning,	as follows:

	 0 - Automatically determine a reasonable cache	size for each C<SELECT>
	 1 - Disable the local row cache
	>1 - Cache this	many rows
	<0 - Cache as many rows	that will fit into this	much memory for	each C<SELECT>.

       Note that large cache sizes may require a very large amount of memory
       (cached rows * maximum size of row). Also, a large cache	will cause a
       longer delay not	only for the first fetch, but also whenever the	cache
       needs refilling.

       See also	the "RowsInCache" statement handle attribute.

       "Username"

       Type: string

       Returns the username used to connect to the database.

DBI STATEMENT HANDLE OBJECTS
       This section lists the methods and attributes associated	with DBI
       statement handles.

   Statement Handle Methods
       The DBI defines the following methods for use on	DBI statement handles:

       "bind_param"

	 $sth->bind_param($p_num, $bind_value)
	 $sth->bind_param($p_num, $bind_value, \%attr)
	 $sth->bind_param($p_num, $bind_value, $bind_type)

       The "bind_param"	method takes a copy of $bind_value and associates it
       (binds it) with a placeholder, identified by $p_num, embedded in	the
       prepared	statement. Placeholders	are indicated with question mark
       character ("?").	For example:

	 $dbh->{RaiseError} = 1;	# save having to check each method call
	 $sth =	$dbh->prepare("SELECT name, age	FROM people WHERE name LIKE ?");
	 $sth->bind_param(1, "John%");	# placeholders are numbered from 1
	 $sth->execute;
	 DBI::dump_results($sth);

       See "Placeholders and Bind Values" for more information.

       Data Types for Placeholders

       The "\%attr" parameter can be used to hint at the data type the
       placeholder should have.	This is	rarely needed. Typically, the driver
       is only interested in knowing if	the placeholder	should be bound	as a
       number or a string.

	 $sth->bind_param(1, $value, { TYPE => SQL_INTEGER });

       As a short-cut for the common case, the data type can be	passed
       directly, in place of the "\%attr" hash reference. This example is
       equivalent to the one above:

	 $sth->bind_param(1, $value, SQL_INTEGER);

       The "TYPE" value	indicates the standard (non-driver-specific) type for
       this parameter. To specify the driver-specific type, the	driver may
       support a driver-specific attribute, such as "{ ora_type	=> 97 }".

       The SQL_INTEGER and other related constants can be imported using

	 use DBI qw(:sql_types);

       See "DBI	Constants" for more information.

       The data	type is	'sticky' in that bind values passed to execute() are
       bound with the data type	specified by earlier bind_param() calls, if
       any.  Portable applications should not rely on being able to change the
       data type after the first "bind_param" call.

       Perl only has string and	number scalar data types. All database types
       that aren't numbers are bound as	strings	and must be in a format	the
       database	will understand	except where the bind_param() TYPE attribute
       specifies a type	that implies a particular format. For example, given:

	 $sth->bind_param(1, $value, SQL_DATETIME);

       the driver should expect	$value to be in	the ODBC standard SQL_DATETIME
       format, which is	'YYYY-MM-DD HH:MM:SS'. Similarly for SQL_DATE,
       SQL_TIME	etc.

       As an alternative to specifying the data	type in	the "bind_param" call,
       you can let the driver pass the value as	the default type ("VARCHAR").
       You can then use	an SQL function	to convert the type within the
       statement.  For example:

	 INSERT	INTO price(code, price)	VALUES (?, CONVERT(MONEY,?))

       The "CONVERT" function used here	is just	an example. The	actual
       function	and syntax will	vary between different databases and is	non-
       portable.

       See also	"Placeholders and Bind Values" for more	information.

       "bind_param_inout"

	 $rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len)  or die $sth->errstr;
	 $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr)	  or ...
	 $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...

       This method acts	like "bind_param", but also enables values to be
       updated by the statement. The statement is typically a call to a	stored
       procedure. The $bind_value must be passed as a reference	to the actual
       value to	be used.

       Note that unlike	"bind_param", the $bind_value variable is not copied
       when "bind_param_inout" is called. Instead, the value in	the variable
       is read at the time "execute" is	called.

       The additional $max_len parameter specifies the minimum amount of
       memory to allocate to $bind_value for the new value. If the value
       returned	from the database is too big to	fit, then the execution	should
       fail. If	unsure what value to use, pick a generous length, i.e.,	a
       length larger than the longest value that would ever be returned.  The
       only cost of using a larger value than needed is	wasted memory.

       Undefined values	or "undef" are used to indicate	null values.  See also
       "Placeholders and Bind Values" for more information.

       "bind_param_array"

	 $rc = $sth->bind_param_array($p_num, $array_ref_or_value)
	 $rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr)
	 $rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type)

       The "bind_param_array" method is	used to	bind an	array of values	to a
       placeholder embedded in the prepared statement which is to be executed
       with "execute_array". For example:

	 $dbh->{RaiseError} = 1;	# save having to check each method call
	 $sth =	$dbh->prepare("INSERT INTO staff (first_name, last_name, dept) VALUES(?, ?, ?)");
	 $sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
	 $sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
	 $sth->bind_param_array(3, "SALES"); # scalar will be reused for each row
	 $sth->execute_array( {	ArrayTupleStatus => \my	@tuple_status }	);

       The %attr ($bind_type) argument is the same as defined for
       "bind_param".  Refer to "bind_param" for	general	details	on using
       placeholders.

       (Note that bind_param_array() can not be	used to	expand a placeholder
       into a list of values for a statement like "SELECT foo WHERE bar	IN
       (?)".  A	placeholder can	only ever represent one	value per execution.)

       Scalar values, including	"undef", may also be bound by
       "bind_param_array". In which case the same value	will be	used for each
       "execute" call. Driver-specific implementations may behave differently,
       e.g., when binding to a stored procedure	call, some databases may
       permit mixing scalars and arrays	as arguments.

       The default implementation provided by DBI (for drivers that have not
       implemented array binding) is to	iteratively call "execute" for each
       parameter tuple provided	in the bound arrays.  Drivers may provide more
       optimized implementations using whatever	bulk operation support the
       database	API provides. The default driver behaviour should match	the
       default DBI behaviour, but always consult your driver documentation as
       there may be driver specific issues to consider.

       Note that the default implementation currently only supports non-data
       returning statements (INSERT, UPDATE, but not SELECT). Also,
       "bind_param_array" and "bind_param" cannot be mixed in the same
       statement execution, and	"bind_param_array" must	be used	with
       "execute_array";	using "bind_param_array" will have no effect for
       "execute".

       The "bind_param_array" method was added in DBI 1.22.

       "execute"

	 $rv = $sth->execute		    or die $sth->errstr;
	 $rv = $sth->execute(@bind_values)  or die $sth->errstr;

       Perform whatever	processing is necessary	to execute the prepared
       statement.  An "undef" is returned if an	error occurs.  A successful
       "execute" always	returns	true regardless	of the number of rows
       affected, even if it's zero (see	below).	It is always important to
       check the return	status of "execute" (and most other DBI	methods) for
       errors if you're	not using "RaiseError".

       For a non-"SELECT" statement, "execute" returns the number of rows
       affected, if known. If no rows were affected, then "execute" returns
       "0E0", which Perl will treat as 0 but will regard as true. Note that it
       is not an error for no rows to be affected by a statement. If the
       number of rows affected is not known, then "execute" returns -1.

       For "SELECT" statements,	execute	simply "starts"	the query within the
       database	engine.	Use one	of the fetch methods to	retrieve the data
       after calling "execute".	 The "execute" method does not return the
       number of rows that will	be returned by the query (because most
       databases can't tell in advance), it simply returns a true value.

       You can tell if the statement was a "SELECT" statement by checking if
       "$sth->{NUM_OF_FIELDS}" is greater than zero after calling "execute".

       If any arguments	are given, then	"execute" will effectively call
       "bind_param" for	each value before executing the	statement.  Values
       bound in	this way are usually treated as	"SQL_VARCHAR" types unless the
       driver can determine the	correct	type (which is rare), or unless
       "bind_param" (or	"bind_param_inout") has	already	been used to specify
       the type.

       Note that passing "execute" an empty array is the same as passing no
       arguments at all, which will execute the	statement with previously
       bound values.  That's probably not what you want.

       If execute() is called on a statement handle that's still active
       ($sth->{Active} is true)	then it	should effectively call	finish() to
       tidy up the previous execution results before starting this new
       execution.

       "execute_array"

	 $tuples = $sth->execute_array(\%attr) or die $sth->errstr;
	 $tuples = $sth->execute_array(\%attr, @bind_values) or	die $sth->errstr;

	 ($tuples, $rows) = $sth->execute_array(\%attr)	or die $sth->errstr;
	 ($tuples, $rows) = $sth->execute_array(\%attr,	@bind_values) or die $sth->errstr;

       Execute the prepared statement once for each parameter tuple (group of
       values) provided	either in the @bind_values, or by prior	calls to
       "bind_param_array", or via a reference passed in	\%attr.

       When called in scalar context the execute_array() method	returns	the
       number of tuples	executed, or "undef" if	an error occurred.  Like
       execute(), a successful execute_array() always returns true regardless
       of the number of	tuples executed, even if it's zero. If there were any
       errors the ArrayTupleStatus array can be	used to	discover which tuples
       failed and with what errors.

       When called in list context the execute_array() method returns two
       scalars;	$tuples	is the same as calling execute_array() in scalar
       context and $rows is the	number of rows affected	for each tuple,	if
       available or -1 if the driver cannot determine this. NOTE, some drivers
       cannot determine	the number of rows affected per	tuple but can provide
       the number of rows affected for the batch.  If you are doing an update
       operation the returned rows affected may	not be what you	expect if, for
       instance, one or	more of	the tuples affected the	same row multiple
       times.  Some drivers may	not yet	support	list context, in which case
       $rows will be undef, or may not be able to provide the number of	rows
       affected	when performing	this batch operation, in which case $rows will
       be -1.

       Bind values for the tuples to be	executed may be	supplied row-wise by
       an "ArrayTupleFetch" attribute, or else column-wise in the @bind_values
       argument, or else column-wise by	prior calls to "bind_param_array".

       Where column-wise binding is used (via the @bind_values argument	or
       calls to	bind_param_array()) the	maximum	number of elements in any one
       of the bound value arrays determines the	number of tuples executed.
       Placeholders with fewer values in their parameter arrays	are treated as
       if padded with undef (NULL) values.

       If a scalar value is bound, instead of an array reference, it is
       treated as a variable length array with all elements having the same
       value. It does not influence the	number of tuples executed, so if all
       bound arrays have zero elements then zero tuples	will be	executed. If
       all bound values	are scalars then one tuple will	be executed, making
       execute_array() act just	like execute().

       The "ArrayTupleFetch" attribute can be used to specify a	reference to a
       subroutine that will be called to provide the bind values for each
       tuple execution.	The subroutine should return an	reference to an	array
       which contains the appropriate number of	bind values, or	return an
       undef if	there is no more data to execute.

       As a convenience, the "ArrayTupleFetch" attribute can also be used to
       specify a statement handle. In which case the fetchrow_arrayref()
       method will be called on	the given statement handle in order to provide
       the bind	values for each	tuple execution.

       The values specified via	bind_param_array() or the @bind_values
       parameter may be	either scalars,	or arrayrefs.  If any @bind_values are
       given, then "execute_array" will	effectively call "bind_param_array"
       for each	value before executing the statement.  Values bound in this
       way are usually treated as "SQL_VARCHAR"	types unless the driver	can
       determine the correct type (which is rare), or unless "bind_param",
       "bind_param_inout", "bind_param_array", or "bind_param_inout_array" has
       already been used to specify the	type.  See "bind_param_array" for
       details.

       The "ArrayTupleStatus" attribute	can be used to specify a reference to
       an array	which will receive the execute status of each executed
       parameter tuple.	Note the "ArrayTupleStatus" attribute was mandatory
       until DBI 1.38.

       For tuples which	are successfully executed, the element at the same
       ordinal position	in the status array is the resulting rowcount (or -1
       if unknown).  If	the execution of a tuple causes	an error, then the
       corresponding status array element will be set to a reference to	an
       array containing	"err", "errstr"	and "state" set	by the failed
       execution.

       If any tuple execution returns an error,	"execute_array"	will return
       "undef".	In that	case, the application should inspect the status	array
       to determine which parameter tuples failed.  Some databases may not
       continue	executing tuples beyond	the first failure. In this case	the
       status array will either	hold fewer elements, or	the elements beyond
       the failure will	be undef.

       If all parameter	tuples are successfully	executed, "execute_array"
       returns the number tuples executed.  If no tuples were executed,	then
       execute_array() returns "0E0", just like	execute() does,	which Perl
       will treat as 0 but will	regard as true.

       For example:

	 $sth =	$dbh->prepare("INSERT INTO staff (first_name, last_name) VALUES	(?, ?)");
	 my $tuples = $sth->execute_array(
	     { ArrayTupleStatus	=> \my @tuple_status },
	     \@first_names,
	     \@last_names,
	 );
	 if ($tuples) {
	     print "Successfully inserted $tuples records\n";
	 }
	 else {
	     for my $tuple (0..@last_names-1) {
		 my $status = $tuple_status[$tuple];
		 $status = [0, "Skipped"] unless defined $status;
		 next unless ref $status;
		 printf	"Failed	to insert (%s, %s): %s\n",
		     $first_names[$tuple], $last_names[$tuple],	$status->[1];
	     }
	 }

       Support for data	returning statements such as SELECT is driver-specific
       and subject to change. At present, the default implementation provided
       by DBI only supports non-data returning statements.

       Transaction semantics when using	array binding are driver and database
       specific.  If "AutoCommit" is on, the default DBI implementation	will
       cause each parameter tuple to be	individually committed (or rolled back
       in the event of an error). If "AutoCommit" is off, the application is
       responsible for explicitly committing the entire	set of bound parameter
       tuples.	Note that different drivers and	databases may have different
       behaviours when some parameter tuples cause failures. In	some cases,
       the driver or database may automatically	rollback the effect of all
       prior parameter tuples that succeeded in	the transaction; other drivers
       or databases may	retain the effect of prior successfully	executed
       parameter tuples. Be sure to check your driver and database for its
       specific	behaviour.

       Note that, in general, performance will usually be better with
       "AutoCommit" turned off,	and using explicit "commit" after each
       "execute_array" call.

       The "execute_array" method was added in DBI 1.22, and ArrayTupleFetch
       was added in 1.36.

       "execute_for_fetch"

	 $tuples = $sth->execute_for_fetch($fetch_tuple_sub);
	 $tuples = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);

	 ($tuples, $rows) = $sth->execute_for_fetch($fetch_tuple_sub);
	 ($tuples, $rows) = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);

       The execute_for_fetch() method is used to perform bulk operations and
       although	it is most often used via the execute_array() method you can
       use it directly.	The main difference between execute_array and
       execute_for_fetch is the	former does column or row-wise binding and the
       latter uses row-wise binding.

       The fetch subroutine, referenced	by $fetch_tuple_sub, is	expected to
       return a	reference to an	array (known as	a 'tuple') or undef.

       The execute_for_fetch() method calls $fetch_tuple_sub, without any
       parameters, until it returns a false value. Each	tuple returned is used
       to provide bind values for an $sth->execute(@$tuple) call.

       In scalar context execute_for_fetch() returns "undef" if	there were any
       errors and the number of	tuples executed	otherwise. Like	execute() and
       execute_array() a zero is returned as "0E0" so execute_for_fetch() is
       only false on error.  If	there were any errors the @tuple_status	array
       can be used to discover which tuples failed and with what errors.

       When called in list context execute_for_fetch() returns two scalars;
       $tuples is the same as calling execute_for_fetch() in scalar context
       and $rows is the	sum of the number of rows affected for each tuple, if
       available or -1 if the driver cannot determine this.  If	you are	doing
       an update operation the returned	rows affected may not be what you
       expect if, for instance,	one or more of the tuples affected the same
       row multiple times.  Some drivers may not yet support list context, in
       which case $rows	will be	undef, or may not be able to provide the
       number of rows affected when performing this batch operation, in	which
       case $rows will be -1.

       If \@tuple_status is passed then	the execute_for_fetch method uses it
       to return status	information. The tuple_status array holds one element
       per tuple. If the corresponding execute() did not fail then the element
       holds the return	value from execute(), which is typically a row count.
       If the execute()	did fail then the element holds	a reference to an
       array containing	($sth->err, $sth->errstr, $sth->state).

       If the driver detects an	error that it knows means no further tuples
       can be executed then it may return, with	an error status, even though
       $fetch_tuple_sub	may still have more tuples to be executed.

       Although	each tuple returned by $fetch_tuple_sub	is effectively used to
       call $sth->execute(@$tuple_array_ref) the exact timing may vary.
       Drivers are free	to accumulate sets of tuples to	pass to	the database
       server in bulk group operations for more	efficient execution.  However,
       the $fetch_tuple_sub is specifically allowed to return the same array
       reference each time (which is what fetchrow_arrayref() usually does).

       For example:

	 my $sel = $dbh1->prepare("select foo, bar from	table1");
	 $sel->execute;

	 my $ins = $dbh2->prepare("insert into table2 (foo, bar) values	(?,?)");
	 my $fetch_tuple_sub = sub { $sel->fetchrow_arrayref };

	 my @tuple_status;
	 $rc = $ins->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
	 my @errors = grep { ref $_ } @tuple_status;

       Similarly, if you already have an array containing the data rows	to be
       processed you'd use a subroutine	to shift off and return	each array ref
       in turn:

	 $ins->execute_for_fetch( sub {	shift @array_of_arrays }, \@tuple_status);

       The "execute_for_fetch" method was added	in DBI 1.38.

       "last_insert_id"

	 $rv = $sth->last_insert_id();
	 $rv = $sth->last_insert_id($catalog, $schema, $table, $field);
	 $rv = $sth->last_insert_id($catalog, $schema, $table, $field, \%attr);

       Returns a value 'identifying' the row inserted by last execution	of the
       statement $sth, if possible.

       For some	drivers	the value may be 'identifying' the row inserted	by the
       last executed statement,	not by $sth.

       See database handle method last_insert_id for all details.

       The "last_insert_id" statement method was added in DBI 1.642.

       "fetchrow_arrayref"

	 $ary_ref = $sth->fetchrow_arrayref;
	 $ary_ref = $sth->fetch;    # alias

       Fetches the next	row of data and	returns	a reference to an array
       holding the field values.  Null fields are returned as "undef" values
       in the array.  This is the fastest way to fetch data, particularly if
       used with "$sth->bind_columns".

       If there	are no more rows or if an error	occurs,	then
       "fetchrow_arrayref" returns an "undef". You should check	"$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the
       "undef" returned	was due	to an error.

       Note that the same array	reference is returned for each fetch, so don't
       store the reference and then use	it after a later fetch.	 Also, the
       elements	of the array are also reused for each row, so take care	if you
       want to take a reference	to an element. See also	"bind_columns".

       "fetchrow_array"

	@ary = $sth->fetchrow_array;

       An alternative to "fetchrow_arrayref". Fetches the next row of data and
       returns it as a list containing the field values.  Null fields are
       returned	as "undef" values in the list.

       If there	are no more rows or if an error	occurs,	then "fetchrow_array"
       returns an empty	list. You should check "$sth->err" afterwards (or use
       the "RaiseError"	attribute) to discover if the empty list returned was
       due to an error.

       If called in a scalar context for a statement handle that has more than
       one column, it is undefined whether the driver will return the value of
       the first column	or the last. So	don't do that.	Also, in a scalar
       context,	an "undef" is returned if there	are no more rows or if an
       error occurred. That "undef" can't be distinguished from	an "undef"
       returned	because	the first field	value was NULL.	 For these reasons you
       should exercise some caution if you use "fetchrow_array"	in a scalar
       context.

       "fetchrow_hashref"

	$hash_ref = $sth->fetchrow_hashref;
	$hash_ref = $sth->fetchrow_hashref($name);

       An alternative to "fetchrow_arrayref". Fetches the next row of data and
       returns it as a reference to a hash containing field name and field
       value pairs.  Null fields are returned as "undef" values	in the hash.

       If there	are no more rows or if an error	occurs,	then
       "fetchrow_hashref" returns an "undef". You should check "$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the
       "undef" returned	was due	to an error.

       The optional $name parameter specifies the name of the statement	handle
       attribute. For historical reasons it defaults to	""NAME"", however
       using either ""NAME_lc""	or ""NAME_uc"" is recommended for portability.

       The keys	of the hash are	the same names returned	by "$sth->{$name}". If
       more than one field has the same	name, there will only be one entry in
       the returned hash for those fields, so statements like ""select foo,
       foo from	bar"" will return only a single	key from "fetchrow_hashref".
       In these	cases use column aliases or "fetchrow_arrayref".  Note that it
       is the database server (and not the DBD implementation) which provides
       the name	for fields containing functions	like "count(*)"	or
       ""max(c_foo)"" and they may clash with existing column names (most
       databases don't care about duplicate column names in a result-set). If
       you want	these to return	as unique names	that are the same across
       databases, use aliases, as in ""select count(*) as cnt""	or ""select
       max(c_foo) mx_foo, ...""	depending on the syntax	your database
       supports.

       Because of the extra work "fetchrow_hashref" and	Perl have to perform,
       it is not as efficient as "fetchrow_arrayref" or	"fetchrow_array".

       By default a reference to a new hash is returned	for each row.  It is
       likely that a future version of the DBI will support an attribute which
       will enable the same hash to be reused for each row. This will give a
       significant performance boost, but it won't be enabled by default
       because of the risk of breaking old code.

       "fetchall_arrayref"

	 $tbl_ary_ref =	$sth->fetchall_arrayref;
	 $tbl_ary_ref =	$sth->fetchall_arrayref( $slice	);
	 $tbl_ary_ref =	$sth->fetchall_arrayref( $slice, $max_rows  );

       The "fetchall_arrayref" method can be used to fetch all the data	to be
       returned	from a prepared	and executed statement handle. It returns a
       reference to an array that contains one reference per row.

       If called on an inactive	statement handle, "fetchall_arrayref" returns
       undef.

       If there	are no rows left to return from	an active statement handle,
       "fetchall_arrayref" returns a reference to an empty array. If an	error
       occurs, "fetchall_arrayref" returns the data fetched thus far, which
       may be none.  You should	check "$sth->err" afterwards (or use the
       "RaiseError" attribute) to discover if the data is complete or was
       truncated due to	an error.

       If $slice is an array reference,	"fetchall_arrayref" uses
       "fetchrow_arrayref" to fetch each row as	an array ref. If the $slice
       array is	not empty then it is used as a slice to	select individual
       columns by perl array index number (starting at 0, unlike column	and
       parameter numbers which start at	1).

       With no parameters, or if $slice	is undefined, "fetchall_arrayref" acts
       as if passed an empty array ref.

       For example, to fetch just the first column of every row:

	 $tbl_ary_ref =	$sth->fetchall_arrayref([0]);

       To fetch	the second to last and last column of every row:

	 $tbl_ary_ref =	$sth->fetchall_arrayref([-2,-1]);

       Those two examples both return a	reference to an	array of array refs.

       If $slice is a hash reference, "fetchall_arrayref" fetches each row as
       a hash reference. If the	$slice hash is empty then the keys in the
       hashes have whatever name lettercase is returned	by default. (See
       "FetchHashKeyName" attribute.) If the $slice hash is not	empty, then it
       is used as a slice to select individual columns by name.	The values of
       the hash	should be set to 1.  The key names of the returned hashes
       match the letter	case of	the names in the parameter hash, regardless of
       the "FetchHashKeyName" attribute.

       For example, to fetch all fields	of every row as	a hash ref:

	 $tbl_ary_ref =	$sth->fetchall_arrayref({});

       To fetch	only the fields	called "foo" and "bar" of every	row as a hash
       ref (with keys named "foo" and "BAR", regardless	of the original
       capitalization):

	 $tbl_ary_ref =	$sth->fetchall_arrayref({ foo=>1, BAR=>1 });

       Those two examples both return a	reference to an	array of hash refs.

       If $slice is a reference	to a hash reference, that hash is used to
       select and rename columns. The keys are 0-based column index numbers
       and the values are the corresponding keys for the returned row hashes.

       For example, to fetch only the first and	second columns of every	row as
       a hash ref (with	keys named "k" and "v" regardless of their original
       names):

	 $tbl_ary_ref =	$sth->fetchall_arrayref( \{ 0 => 'k', 1	=> 'v' } );

       If $max_rows is defined and greater than	or equal to zero then it is
       used to limit the number	of rows	fetched	before returning.
       fetchall_arrayref() can then be called again to fetch more rows.	 This
       is especially useful when you need the better performance of
       fetchall_arrayref() but don't have enough memory	to fetch and return
       all the rows in one go.

       Here's an example (assumes RaiseError is	enabled):

	 my $rows = [];	# cache	for batches of rows
	 while(	my $row	= ( shift(@$rows) || # get row from cache, or reload cache:
			    shift(@{$rows=$sth->fetchall_arrayref(undef,10_000)||[]}) )
	 ) {
	   ...
	 }

       That might be the fastest way to	fetch and process lots of rows using
       the DBI,	but it depends on the relative cost of method calls vs memory
       allocation.

       A standard "while" loop with column binding is often faster because the
       cost of allocating memory for the batch of rows is greater than the
       saving by reducing method calls.	It's possible that the DBI may provide
       a way to	reuse the memory of a previous batch in	future,	which would
       then shift the balance back towards fetchall_arrayref().

       "fetchall_hashref"

	 $hash_ref = $sth->fetchall_hashref($key_field);

       The "fetchall_hashref" method can be used to fetch all the data to be
       returned	from a prepared	and executed statement handle. It returns a
       reference to a hash containing a	key for	each distinct value of the
       $key_field column that was fetched. For each key	the corresponding
       value is	a reference to a hash containing all the selected columns and
       their values, as	returned by "fetchrow_hashref()".

       If there	are no rows to return, "fetchall_hashref" returns a reference
       to an empty hash. If an error occurs, "fetchall_hashref"	returns	the
       data fetched thus far, which may	be none.  You should check "$sth->err"
       afterwards (or use the "RaiseError" attribute) to discover if the data
       is complete or was truncated due	to an error.

       The $key_field parameter	provides the name of the field that holds the
       value to	be used	for the	key for	the returned hash.  For	example:

	 $dbh->{FetchHashKeyName} = 'NAME_lc';
	 $sth =	$dbh->prepare("SELECT FOO, BAR,	ID, NAME, BAZ FROM TABLE");
	 $sth->execute;
	 $hash_ref = $sth->fetchall_hashref('id');
	 print "Name for id 42 is $hash_ref->{42}->{name}\n";

       The $key_field parameter	can also be specified as an integer column
       number (counting	from 1).  If $key_field	doesn't	match any column in
       the statement, as a name	first then as a	number,	then an	error is
       returned.

       For queries returning more than one 'key' column, you can specify
       multiple	column names by	passing	$key_field as a	reference to an	array
       containing one or more key column names (or index numbers).  For
       example:

	 $sth =	$dbh->prepare("SELECT foo, bar,	baz FROM table");
	 $sth->execute;
	 $hash_ref = $sth->fetchall_hashref( [ qw(foo bar) ] );
	 print "For foo	42 and bar 38, baz is $hash_ref->{42}->{38}->{baz}\n";

       The fetchall_hashref() method is	normally used only where the key
       fields values for each row are unique.  If multiple rows	are returned
       with the	same values for	the key	fields then later rows overwrite
       earlier ones.

       "finish"

	 $rc  =	$sth->finish;

       Indicate	that no	more data will be fetched from this statement handle
       before it is either executed again or destroyed.	 You almost certainly
       do not need to call this	method.

       Adding calls to "finish"	after loop that	fetches	all rows is a common
       mistake,	don't do it, it	can mask genuine problems like uncaught	fetch
       errors.

       When all	the data has been fetched from a "SELECT" statement, the
       driver will automatically call "finish" for you.	So you should not call
       it explicitly except when you know that you've not fetched all the data
       from a statement	handle and the handle won't be destroyed soon.

       The most	common example is when you only	want to	fetch just one row,
       but in that case	the "selectrow_*" methods are usually better anyway.

       Consider	a query	like:

	 SELECT	foo FROM table WHERE bar=? ORDER BY baz

       on a very large table. When executed, the database server will have to
       use temporary buffer space to store the sorted rows. If,	after
       executing the handle and	selecting just a few rows, the handle won't be
       re-executed for some time and won't be destroyed, the "finish" method
       can be used to tell the server that the buffer space can	be freed.

       Calling "finish"	resets the "Active" attribute for the statement.  It
       may also	make some statement handle attributes (such as "NAME" and
       "TYPE") unavailable if they have	not already been accessed (and thus
       cached).

       The "finish" method does	not affect the transaction status of the
       database	connection.  It	has nothing to do with transactions. It's
       mostly an internal "housekeeping" method	that is	rarely needed.	See
       also "disconnect" and the "Active" attribute.

       The "finish" method should have been called "discard_pending_rows".

       "rows"

	 $rv = $sth->rows;

       Returns the number of rows affected by the last row affecting command,
       or -1 if	the number of rows is not known	or not available.

       Generally, you can only rely on a row count after a non-"SELECT"
       "execute" (for some specific operations like "UPDATE" and "DELETE"), or
       after fetching all the rows of a	"SELECT" statement.

       For "SELECT" statements,	it is generally	not possible to	know how many
       rows will be returned except by fetching	them all.  Some	drivers	will
       return the number of rows the application has fetched so	far, but
       others may return -1 until all rows have	been fetched.  So use of the
       "rows" method or	$DBI::rows with	"SELECT" statements is not
       recommended.

       One alternative method to get a row count for a "SELECT"	is to execute
       a "SELECT COUNT(*) FROM ..." SQL	statement with the same	"..." as your
       query and then fetch the	row count from that.

       "bind_col"

	 $rc = $sth->bind_col($column_number, \$var_to_bind);
	 $rc = $sth->bind_col($column_number, \$var_to_bind, \%attr );
	 $rc = $sth->bind_col($column_number, \$var_to_bind, $bind_type	);

       Binds a Perl variable and/or some attributes to an output column
       (field) of a "SELECT" statement.	 Column	numbers	count up from 1.  You
       do not need to bind output columns in order to fetch data.  For maximum
       portability between drivers, bind_col() should be called	after
       execute() and not before.  See also "bind_columns" for an example.

       The binding is performed	at a low level using Perl aliasing.  Whenever
       a row is	fetched	from the database $var_to_bind appears to be
       automatically updated simply because it now refers to the same memory
       location	as the corresponding column value.  This makes using bound
       variables very efficient.  Binding a tied variable doesn't work,
       currently.

       The "bind_param"	method performs	a similar, but opposite, function for
       input variables.

       Data Types for Column Binding

       The "\%attr" parameter can be used to hint at the data type formatting
       the column should have. For example, you	can use:

	 $sth->bind_col(1, undef, { TYPE => SQL_DATETIME });

       to specify that you'd like the column (which presumably is some kind of
       datetime	type) to be returned in	the standard format for	SQL_DATETIME,
       which is	'YYYY-MM-DD HH:MM:SS', rather than the native formatting the
       database	would normally use.

       There's no $var_to_bind in that example to emphasize the	point that
       bind_col() works	on the underlying column and not just a	particular
       bound variable.

       As a short-cut for the common case, the data type can be	passed
       directly, in place of the "\%attr" hash reference. This example is
       equivalent to the one above:

	 $sth->bind_col(1, undef, SQL_DATETIME);

       The "TYPE" value	indicates the standard (non-driver-specific) type for
       this parameter. To specify the driver-specific type, the	driver may
       support a driver-specific attribute, such as "{ ora_type	=> 97 }".

       The SQL_DATETIME	and other related constants can	be imported using

	 use DBI qw(:sql_types);

       See "DBI	Constants" for more information.

       Few drivers support specifying a	data type via a	"bind_col" call	(most
       will simply ignore the data type). Fewer	still allow the	data type to
       be altered once set. If you do set a column type	the type should	remain
       sticky through further calls to bind_col	for the	same column if the
       type is not overridden (this is important for instance when you are
       using a slice in	fetchall_arrayref).

       The TYPE	attribute for bind_col() was first specified in	DBI 1.41.

       From DBI	1.611, drivers can use the "TYPE" attribute to attempt to cast
       the bound scalar	to a perl type which more closely matches "TYPE". At
       present DBI supports "SQL_INTEGER", "SQL_DOUBLE"	and "SQL_NUMERIC". See
       "sql_type_cast" for details of how types	are cast.

       Other attributes	for Column Binding

       The "\%attr" parameter may also contain the following attributes:

       "StrictlyTyped"
	   If a	"TYPE" attribute is passed to bind_col,	then the driver	will
	   attempt to change the bound perl scalar to match the	type more
	   closely. If the bound value cannot be cast to the requested "TYPE"
	   then	by default it is left untouched	and no error is	generated. If
	   you specify "StrictlyTyped" as 1 and	the cast fails,	this will
	   generate an error.

	   This	attribute was first added in DBI 1.611.	When 1.611 was
	   released few	drivers	actually supported this	attribute but
	   DBD::Oracle and DBD::ODBC should from versions 1.24.

       "DiscardString"
	   When	the "TYPE" attribute is	passed to "bind_col" and the driver
	   successfully	casts the bound	perl scalar to a non-string type then
	   if "DiscardString" is set to	1, the string portion of the scalar
	   will	be discarded. By default, "DiscardString" is not set.

	   This	attribute was first added in DBI 1.611.	When 1.611 was
	   released few	drivers	actually supported this	attribute but
	   DBD::Oracle and DBD::ODBC should from versions 1.24.

       "bind_columns"

	 $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);

       Calls "bind_col"	for each column	of the "SELECT"	statement.

       The list	of references should have the same number of elements as the
       number of columns in the	"SELECT" statement. If it doesn't then
       "bind_columns" will bind	the elements given, up to the number of
       columns,	and then return	an error.

       For maximum portability between drivers,	bind_columns() should be
       called after execute() and not before.

       For example:

	 $dbh->{RaiseError} = 1; # do this, or check every call	for errors
	 $sth =	$dbh->prepare(q{ SELECT	region,	sales FROM sales_by_region });
	 $sth->execute;
	 my ($region, $sales);

	 # Bind	Perl variables to columns:
	 $rv = $sth->bind_columns(\$region, \$sales);

	 # you can also	use Perl's \(...) syntax (see perlref docs):
	 #     $sth->bind_columns(\($region, $sales));

	 # Column binding is the most efficient	way to fetch data
	 while ($sth->fetch) {
	     print "$region: $sales\n";
	 }

       For compatibility with old scripts, the first parameter will be ignored
       if it is	"undef"	or a hash reference.

       Here's a	more fancy example that	binds columns to the values inside a
       hash (thanks to H.Merijn	Brand):

	 $sth->execute;
	 my %row;
	 $sth->bind_columns( \(	@row{ @{$sth->{NAME_lc}	} } ));
	 while ($sth->fetch) {
	     print "$row{region}: $row{sales}\n";
	 }

       "dump_results"

	 $rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);

       Fetches all the rows from $sth, calls "DBI::neat_list" for each row,
       and prints the results to $fh (defaults to "STDOUT") separated by $lsep
       (default	"\n"). $fsep defaults to ", " and $maxlen defaults to 35.

       This method is designed as a handy utility for prototyping and testing
       queries.	Since it uses "neat_list" to format and	edit the string	for
       reading by humans, it is	not recommended	for data transfer
       applications.

   Statement Handle Attributes
       This section describes attributes specific to statement handles.	Most
       of these	attributes are read-only.

       Changes to these	statement handle attributes do not affect any other
       existing	or future statement handles.

       Attempting to set or get	the value of an	unknown	attribute generates a
       warning,	except for private driver specific attributes (which all have
       names starting with a lowercase letter).

       Example:

	 ... = $h->{NUM_OF_FIELDS};    # get/read

       Some drivers cannot provide valid values	for some or all	of these
       attributes until	after "$sth->execute" has been successfully called.
       Typically the attribute will be "undef" in these	situations.

       Some attributes,	like NAME, are not appropriate to some types of
       statement, like SELECT. Typically the attribute will be "undef" in
       these situations.

       For drivers which support stored	procedures and multiple	result sets
       (see "more_results") these attributes relate to the current result set.

       See also	"finish" to learn more about the effect	it may have on some
       attributes.

       "NUM_OF_FIELDS"

       Type: integer, read-only

       Number of fields	(columns) in the data the prepared statement may
       return.	Statements that	don't return rows of data, like	"DELETE" and
       "CREATE"	set "NUM_OF_FIELDS" to 0 (though it may	be undef in some
       drivers).

       "NUM_OF_PARAMS"

       Type: integer, read-only

       The number of parameters	(placeholders) in the prepared statement.  See
       SUBSTITUTION VARIABLES below for	more details.

       "NAME"

       Type: array-ref,	read-only

       Returns a reference to an array of field	names for each column. The
       names may contain spaces	but should not be truncated or have any
       trailing	space. Note that the names have	the letter case	(upper,	lower
       or mixed) as returned by	the driver being used. Portable	applications
       should use "NAME_lc" or "NAME_uc".

	 print "First column name: $sth->{NAME}->[0]\n";

       Also note that the name returned	for (aggregate)	functions like
       count(*)	or "max(c_foo)"	is determined by the database server and not
       by "DBI"	or the "DBD" backend.

       "NAME_lc"

       Type: array-ref,	read-only

       Like "/NAME" but	always returns lowercase names.

       "NAME_uc"

       Type: array-ref,	read-only

       Like "/NAME" but	always returns uppercase names.

       "NAME_hash"

       Type: hash-ref, read-only

       "NAME_lc_hash"

       Type: hash-ref, read-only

       "NAME_uc_hash"

       Type: hash-ref, read-only

       The "NAME_hash",	"NAME_lc_hash",	and "NAME_uc_hash" attributes return
       column name information as a reference to a hash.

       The keys	of the hash are	the names of the columns.  The letter case of
       the keys	corresponds to the letter case returned	by the "NAME",
       "NAME_lc", and "NAME_uc"	attributes respectively	(as described above).

       The value of each hash entry is the perl	index number of	the
       corresponding column (counting from 0). For example:

	 $sth =	$dbh->prepare("select Id, Name from table");
	 $sth->execute;
	 @row =	$sth->fetchrow_array;
	 print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";

       "TYPE"

       Type: array-ref,	read-only

       Returns a reference to an array of integer values for each column. The
       value indicates the data	type of	the corresponding column.

       The values correspond to	the international standards (ANSI X3.135 and
       ISO/IEC 9075) which, in general terms, means ODBC. Driver-specific
       types that don't	exactly	match standard types should generally return
       the same	values as an ODBC driver supplied by the makers	of the
       database. That might include private type numbers in ranges the vendor
       has officially registered with the ISO working group:

	 ftp://sqlstandards.org/SC32/SQL_Registry/

       Where there's no	vendor-supplied	ODBC driver to be compatible with, the
       DBI driver can use type numbers in the range that is now	officially
       reserved	for use	by the DBI: -9999 to -9000.

       All possible values for "TYPE" should have at least one entry in	the
       output of the "type_info_all" method (see "type_info_all").

       "PRECISION"

       Type: array-ref,	read-only

       Returns a reference to an array of integer values for each column.

       For numeric columns, the	value is the maximum number of digits (without
       considering a sign character or decimal point). Note that the "display
       size" for floating point	types (REAL, FLOAT, DOUBLE) can	be up to 7
       characters greater than the precision (for the sign + decimal point +
       the letter E + a	sign + 2 or 3 digits).

       For any character type column the value is the OCTET_LENGTH, in other
       words the number	of bytes, not characters.

       (More recent standards refer to this as COLUMN_SIZE but we stick	with
       PRECISION for backwards compatibility.)

       "SCALE"

       Type: array-ref,	read-only

       Returns a reference to an array of integer values for each column.
       NULL ("undef") values indicate columns where scale is not applicable.

       "NULLABLE"

       Type: array-ref,	read-only

       Returns a reference to an array indicating the possibility of each
       column returning	a null.	 Possible values are 0 (or an empty string) =
       no, 1 = yes, 2 =	unknown.

	 print "First column may return	NULL\n"	if $sth->{NULLABLE}->[0];

       "CursorName"

       Type: string, read-only

       Returns the name	of the cursor associated with the statement handle, if
       available. If not available or if the database driver does not support
       the "where current of ..." SQL syntax, then it returns "undef".

       "Database"

       Type: dbh, read-only

       Returns the parent $dbh of the statement	handle.

       "Statement"

       Type: string, read-only

       Returns the statement string passed to the "prepare" method.

       "ParamValues"

       Type: hash ref, read-only

       Returns a reference to a	hash containing	the values currently bound to
       placeholders.  The keys of the hash are the 'names' of the
       placeholders, typically integers	starting at 1.	Returns	undef if not
       supported by the	driver.

       See "ShowErrorStatement"	for an example of how this is used.

       * Keys:

       If the driver supports "ParamValues" but	no values have been bound yet
       then the	driver should return a hash with placeholders names in the
       keys but	all the	values undef, but some drivers may return a ref	to an
       empty hash because they can't pre-determine the names.

       It is possible that the keys in the hash	returned by "ParamValues" are
       not exactly the same as those implied by	the prepared statement.	 For
       example,	DBD::Oracle translates '"?"' placeholders into '":pN"' where N
       is a sequence number starting at	1.

       * Values:

       It is possible that the values in the hash returned by "ParamValues"
       are not exactly the same	as those passed	to bind_param()	or execute().
       The driver may have slightly modified values in some way	based on the
       TYPE the	value was bound	with. For example a floating point value bound
       as an SQL_INTEGER type may be returned as an integer.  The values
       returned	by "ParamValues" can be	passed to another bind_param() method
       with the	same TYPE and will be seen by the database as the same value.
       See also	"ParamTypes" below.

       The "ParamValues" attribute was added in	DBI 1.28.

       "ParamTypes"

       Type: hash ref, read-only

       Returns a reference to a	hash containing	the type information currently
       bound to	placeholders.  Returns undef if	not supported by the driver.

       * Keys:

       See "ParamValues" above.

       * Values:

       The hash	values are hashrefs of type information	in the same form as
       that passed to the various bind_param() methods (See "bind_param" for
       the format and values).

       It is possible that the values in the hash returned by "ParamTypes" are
       not exactly the same as those passed to bind_param() or execute().
       Param attributes	specified using	the abbreviated	form, like this:

	   $sth->bind_param(1, SQL_INTEGER);

       are returned in the expanded form, as if	called like this:

	   $sth->bind_param(1, { TYPE => SQL_INTEGER });

       The driver may have modified the	type information in some way based on
       the bound values, other hints provided by the prepare()'d SQL
       statement, or alternate type mappings required by the driver or target
       database	system.	The driver may also add	private	keys (with names
       beginning with the drivers reserved prefix, e.g., odbc_xxx).

       * Example:

       The keys	and values in the returned hash	can be passed to the various
       bind_param() methods to effectively reproduce a previous	param binding.
       For example:

	 # assuming $sth1 is a previously prepared statement handle
	 my $sth2 = $dbh->prepare( $sth1->{Statement} );
	 my $ParamValues = $sth1->{ParamValues}	|| {};
	 my $ParamTypes	 = $sth1->{ParamTypes}	|| {};
	 $sth2->bind_param($_, $ParamValues->{$_}, $ParamTypes->{$_})
	   for keys %{ {%$ParamValues, %$ParamTypes} };
	 $sth2->execute();

       The "ParamTypes"	attribute was added in DBI 1.49. Implementation	is the
       responsibility of individual drivers; the DBI layer default
       implementation simply returns undef.

       "ParamArrays"

       Type: hash ref, read-only

       Returns a reference to a	hash containing	the values currently bound to
       placeholders with "execute_array" or "bind_param_array".	 The keys of
       the hash	are the	'names'	of the placeholders, typically integers
       starting	at 1.  Returns undef if	not supported by the driver or no
       arrays of parameters are	bound.

       Each key	value is an array reference containing a list of the bound
       parameters for that column.

       For example:

	 $sth =	$dbh->prepare("INSERT INTO staff (id, name) values (?,?)");
	 $sth->execute_array({},[1,2], ['fred','dave']);
	 if ($sth->{ParamArrays}) {
	     foreach $param (keys %{$sth->{ParamArrays}}) {
		 printf	"Parameters for	%s : %s\n", $param,
		 join(",", @{$sth->{ParamArrays}->{$param}});
	     }
	 }

       It is possible that the values in the hash returned by "ParamArrays"
       are not exactly the same	as those passed	to "bind_param_array" or
       "execute_array".	 The driver may	have slightly modified values in some
       way based on the	TYPE the value was bound with. For example a floating
       point value bound as an SQL_INTEGER type	may be returned	as an integer.

       It is also possible that	the keys in the	hash returned by "ParamArrays"
       are not exactly the same	as those implied by the	prepared statement.
       For example, DBD::Oracle	translates '"?"'  placeholders into '":pN"'
       where N is a sequence number starting at	1.

       "RowsInCache"

       Type: integer, read-only

       If the driver supports a	local row cache	for "SELECT" statements, then
       this attribute holds the	number of un-fetched rows in the cache.	If the
       driver doesn't, then it returns "undef".	Note that some drivers pre-
       fetch rows on execute, whereas others wait till the first fetch.

       See also	the "RowCacheSize" database handle attribute.

FURTHER	INFORMATION
   Catalog Methods
       An application can retrieve metadata information	from the DBMS by
       issuing appropriate queries on the views	of the Information Schema.
       Unfortunately, "INFORMATION_SCHEMA" views are seldom supported by the
       DBMS.  Special methods (catalog methods)	are available to return	result
       sets for	a small	but important portion of that metadata:

	 column_info
	 foreign_key_info
	 primary_key_info
	 table_info
	 statistics_info

       All catalog methods accept arguments in order to	restrict the result
       sets.  Passing "undef" to an optional argument does not constrain the
       search for that argument.  However, an empty string ('')	is treated as
       a regular search	criteria and will only match an	empty value.

       Note: SQL/CLI and ODBC differ in	the handling of	empty strings. An
       empty string will not restrict the result set in	SQL/CLI.

       Most arguments in the catalog methods accept only ordinary values, e.g.
       the arguments of	"primary_key_info()".  Such arguments are treated as a
       literal string, i.e. the	case is	significant and	quote characters are
       taken literally.

       Some arguments in the catalog methods accept search patterns (strings
       containing '_' and/or '%'), e.g.	the $table argument of
       "column_info()".	 Passing '%' is	equivalent to leaving the argument
       "undef".

       Caveat: The underscore ('_') is valid and often used in SQL
       identifiers.  Passing such a value to a search pattern argument may
       return more rows	than expected!	To include pattern characters as
       literals, they must be preceded by an escape character which can	be
       achieved	with

	 $esc =	$dbh->get_info(	14 );  # SQL_SEARCH_PATTERN_ESCAPE
	 $search_pattern =~ s/([_%])/$esc$1/g;

       The ODBC	and SQL/CLI specifications define a way	to change the default
       behaviour described above: All arguments	(except	list value arguments)
       are treated as identifier if the	"SQL_ATTR_METADATA_ID" attribute is
       set to "SQL_TRUE".  Quoted identifiers are very similar to ordinary
       values, i.e. their body (the string within the quotes) is interpreted
       literally.  Unquoted identifiers	are compared in	UPPERCASE.

       The DBI (currently) does	not support the	"SQL_ATTR_METADATA_ID"
       attribute, i.e. it behaves like an ODBC driver where
       "SQL_ATTR_METADATA_ID" is set to	"SQL_FALSE".

   Transactions
       Transactions are	a fundamental part of any robust database system. They
       protect against errors and database corruption by ensuring that sets of
       related changes to the database take place in atomic (indivisible, all-
       or-nothing) units.

       This section applies to databases that support transactions and where
       "AutoCommit" is off.  See "AutoCommit" for details of using
       "AutoCommit" with various types of databases.

       The recommended way to implement	robust transactions in Perl
       applications is to enable "RaiseError" and catch	the error that's
       'thrown'	as an exception.  For example, using Try::Tiny:

	 use Try::Tiny;
	 $dbh->{AutoCommit} = 0;  # enable transactions, if possible
	 $dbh->{RaiseError} = 1;
	 try {
	     foo(...)	     # do lots of work here
	     bar(...)	     # including inserts
	     baz(...)	     # and updates
	     $dbh->commit;   # commit the changes if we	get this far
	 } catch {
	     warn "Transaction aborted because $_"; # Try::Tiny	copies $@ into $_
	     # now rollback to undo the	incomplete changes
	     # but do it in an eval{} as it may	also fail
	     eval { $dbh->rollback };
	     # add other application on-error-clean-up code here
	 };

       If the "RaiseError" attribute is	not set, then DBI calls	would need to
       be manually checked for errors, typically like this:

	 $h->method(@args) or die $h->errstr;

       With "RaiseError" set, the DBI will automatically "die" if any DBI
       method call on that handle (or a	child handle) fails, so	you don't have
       to test the return value	of each	method call. See "RaiseError" for more
       details.

       A major advantage of the	"eval" approach	is that	the transaction	will
       be properly rolled back if any code (not	just DBI calls)	in the inner
       application dies	for any	reason.	The major advantage of using the
       "$h->{RaiseError}" attribute is that all	DBI calls will be checked
       automatically. Both techniques are strongly recommended.

       After calling "commit" or "rollback" many drivers will not let you
       fetch from a previously active "SELECT" statement handle	that's a child
       of the same database handle. A typical way round	this is	to connect the
       the database twice and use one connection for "SELECT" statements.

       See "AutoCommit"	and "disconnect" for other important information about
       transactions.

   Handling BLOB / LONG	/ Memo Fields
       Many databases support "blob" (binary large objects), "long", or
       similar datatypes for holding very long strings or large	amounts	of
       binary data in a	single field. Some databases support variable length
       long values over	2,000,000,000 bytes in length.

       Since values of that size can't usually be held in memory, and because
       databases can't usually know in advance the length of the longest long
       that will be returned from a "SELECT" statement (unlike other data
       types), some special handling is	required.

       In this situation, the value of the "$h->{LongReadLen}" attribute is
       used to determine how much buffer space to allocate when	fetching such
       fields.	The "$h->{LongTruncOk}"	attribute is used to determine how to
       behave if a fetched value can't fit into	the buffer.

       See the description of "LongReadLen" for	more information.

       When trying to insert long or binary values, placeholders should	be
       used since there	are often limits on the	maximum	size of	an "INSERT"
       statement and the "quote" method	generally can't	cope with binary data.
       See "Placeholders and Bind Values".

   Simple Examples
       Here's a	complete example program to select and fetch some data:

	 my $data_source = "dbi::DriverName:db_name";
	 my $dbh = DBI->connect($data_source, $user, $password)
	     or	die "Can't connect to $data_source: $DBI::errstr";

	 my $sth = $dbh->prepare( q{
		 SELECT	name, phone
		 FROM mytelbook
	 }) or die "Can't prepare statement: $DBI::errstr";

	 my $rc	= $sth->execute
	     or	die "Can't execute statement: $DBI::errstr";

	 print "Query will return $sth->{NUM_OF_FIELDS}	fields.\n\n";
	 print "Field names: @{	$sth->{NAME} }\n";

	 while (($name,	$phone)	= $sth->fetchrow_array)	{
	     print "$name: $phone\n";
	 }
	 # check for problems which may	have terminated	the fetch early
	 die $sth->errstr if $sth->err;

	 $dbh->disconnect;

       Here's a	complete example program to insert some	data from a file.
       (This example uses "RaiseError" to avoid	needing	to check each call).

	 my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
	     RaiseError	=> 1, AutoCommit => 0
	 });

	 my $sth = $dbh->prepare( q{
	     INSERT INTO table (name, phone) VALUES (?,	?)
	 });

	 open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
	 while (<FH>) {
	     chomp;
	     my	($name,	$phone)	= split	/,/;
	     $sth->execute($name, $phone);
	 }
	 close FH;

	 $dbh->commit;
	 $dbh->disconnect;

       Here's how to convert fetched NULLs (undefined values) into empty
       strings:

	 while($row = $sth->fetchrow_arrayref) {
	   # this is a fast and	simple way to deal with	nulls:
	   foreach (@$row) { $_	= '' unless defined }
	   print "@$row\n";
	 }

       The "q{...}" style quoting used in these	examples avoids	clashing with
       quotes that may be used in the SQL statement. Use the double-quote like
       "qq{...}" operator if you want to interpolate variables into the
       string.	See "Quote and Quote-like Operators" in	perlop for more
       details.

   Threads and Thread Safety
       Perl 5.7	and later support a new	threading model	called iThreads.  (The
       old "5.005 style" threads are not supported by the DBI.)

       In the iThreads model each thread has its own copy of the perl
       interpreter.  When a new	thread is created the original perl
       interpreter is 'cloned' to create a new copy for	the new	thread.

       If the DBI and drivers are loaded and handles created before the	thread
       is created then it will get a cloned copy of the	DBI, the drivers and
       the handles.

       However,	the internal pointer data within the handles will refer	to the
       DBI and drivers in the original interpreter. Using those	handles	in the
       new interpreter thread is not safe, so the DBI detects this and croaks
       on any method call using	handles	that don't belong to the current
       thread (except for DESTROY).

       Because of this (possibly temporary) restriction, newly created threads
       must make their own connections to the database.	Handles	can't be
       shared across threads.

       But BEWARE, some	underlying database APIs (the code the DBD driver uses
       to talk to the database,	often supplied by the database vendor) are not
       thread safe. If it's not	thread safe, then allowing more	than one
       thread to enter the code	at the same time may cause subtle/serious
       problems. In some cases allowing	more than one thread to	enter the
       code, even if not at the	same time, can cause problems. You have	been
       warned.

       Using DBI with perl threads is not yet recommended for production
       environments. For more information see
       <http://www.perlmonks.org/index.pl?node_id=288022>

       Note: There is a	bug in perl 5.8.2 when configured with threads and
       debugging enabled (bug #24463) which causes a DBI test to fail.

   Signal Handling and Canceling Operations
       [The following only applies to systems with unix-like signal handling.
       I'd welcome additions for other systems,	especially Windows.]

       The first thing to say is that signal handling in Perl versions less
       than 5.8	is not safe. There is always a small risk of Perl crashing
       and/or core dumping when, or after, handling a signal because the
       signal could arrive and be handled while	internal data structures are
       being changed. If the signal handling code used those same internal
       data structures it could	cause all manner of subtle and not-so-subtle
       problems.  The risk was reduced with 5.4.4 but was still	present	in all
       perls up	through	5.8.0.

       Beginning in perl 5.8.0 perl implements 'safe' signal handling if your
       system has the POSIX sigaction()	routine. Now when a signal is
       delivered perl just makes a note	of it but does not run the %SIG
       handler.	The handling is	'deferred' until a 'safe' moment.

       Although	this change made signal	handling safe, it also lead to a
       problem with signals being deferred for longer than you'd like.	If a
       signal arrived while executing a	system call, such as waiting for data
       on a network connection,	the signal is noted and	then the system	call
       that was	executing returns with an EINTR	error code to indicate that it
       was interrupted.	All fine so far.

       The problem comes when the code that made the system call sees the
       EINTR code and decides it's going to call it again. Perl	doesn't	do
       that, but database code sometimes does. If that happens then the	signal
       handler doesn't get called until	later. Maybe much later.

       Fortunately there are ways around this which we'll discuss below.
       Unfortunately they make signals unsafe again.

       The two most common uses	of signals in relation to the DBI are for
       canceling operations when the user types	Ctrl-C (interrupt), and	for
       implementing a timeout using "alarm()" and $SIG{ALRM}.

       Cancel
	   The DBI provides a "cancel" method for statement handles. The
	   "cancel" method should abort	the current operation and is designed
	   to be called	from a signal handler.	For example:

	     $SIG{INT} = sub { $sth->cancel };

	   However, few	drivers	implement this (the DBI	provides a default
	   method that just returns "undef") and, even if implemented, there
	   is still a possibility that the statement handle, and even the
	   parent database handle, will	not be usable afterwards.

	   If "cancel" returns true, then it has successfully invoked the
	   database engine's own cancel	function.  If it returns false,	then
	   "cancel" failed. If it returns "undef", then	the database driver
	   does	not have cancel	implemented - very few do.

       Timeout
	   The traditional way to implement a timeout is to set	$SIG{ALRM} to
	   refer to some code that will	be executed when an ALRM signal
	   arrives and then to call alarm($seconds) to schedule	an ALRM	signal
	   to be delivered $seconds in the future. For example:

	     my	$failed;
	     eval {
	       local $SIG{ALRM}	= sub {	die "TIMEOUT\n"	}; # N.B. \n required
	       eval {
		 alarm($seconds);
		 ... code to execute with timeout here (which may die) ...
		 1;
	       } or $failed = 1;
	       # outer eval catches alarm that might fire JUST before this alarm(0)
	       alarm(0);  # cancel alarm (if code ran fast)
	       die "$@"	if $failed;
	       1;
	     } or $failed = 1;
	     if	( $failed ) {
	       if ( defined $@ and $@ eq "TIMEOUT\n" ) { ... }
	       else { ... } # some other error
	     }

	   The first (outer) eval is used to avoid the unlikely	but possible
	   chance that the "code to execute" dies and the alarm	fires before
	   it is cancelled. Without the	outer eval, if this happened your
	   program will	die if you have	no ALRM	handler	or a non-local alarm
	   handler will	be called.

	   Unfortunately, as described above, this won't always	work as
	   expected, depending on your perl version and	the underlying
	   database code.

	   With	Oracle for instance (DBD::Oracle), if the system which hosts
	   the database	is down	the DBI->connect() call	will hang for several
	   minutes before returning an error.

       The solution on these systems is	to use the "POSIX::sigaction()"
       routine to gain low level access	to how the signal handler is
       installed.

       The code	would look something like this (for the	DBD-Oracle connect()):

	  use POSIX qw(:signal_h);

	  my $mask = POSIX::SigSet->new( SIGALRM ); # signals to mask in the handler
	  my $action = POSIX::SigAction->new(
	      sub { die	"connect timeout\n" },	      #	the handler code ref
	      $mask,
	      #	not using (perl	5.8.2 and later) 'safe'	switch or sa_flags
	  );
	  my $oldaction	= POSIX::SigAction->new();
	  sigaction( SIGALRM, $action, $oldaction );
	  my $dbh;
	  my $failed;
	  eval {
	     eval {
	       alarm(5); # seconds before time out
	       $dbh = DBI->connect("dbi:Oracle:$dsn" ... );
	       1;
	     } or $failed = 1;
	     alarm(0); # cancel	alarm (if connect worked fast)
	     die "$@\n"	if $failed; # connect died
	     1;
	  } or $failed = 1;
	  sigaction( SIGALRM, $oldaction );  # restore original	signal handler
	  if ( $failed ) {
	    if ( defined $@ and	$@ eq "connect timeout\n" ) {...}
	    else { # connect died }
	  }

       See previous example for	the reasoning around the double	eval.

       Similar techniques can be used for canceling statement execution.

       Unfortunately, this solution is somewhat	messy, and it does not work
       with perl versions less than perl 5.8 where "POSIX::sigaction()"
       appears to be broken.

       For a cleaner implementation that works across perl versions, see
       Lincoln Baxter's	Sys::SigAction module at Sys::SigAction.  The
       documentation for Sys::SigAction	includes an longer discussion of this
       problem,	and a DBD::Oracle test script.

       Be sure to read all the signal handling sections	of the perlipc manual.

       And finally, two	more points to keep firmly in mind. Firstly, remember
       that what we've done here is essentially	revert to old style unsafe
       handling	of these signals. So do	as little as possible in the handler.
       Ideally just die(). Secondly, the handles in use	at the time the	signal
       is handled may not be safe to use afterwards.

   Subclassing the DBI
       DBI can be subclassed and extended just like any	other object oriented
       module.	Before we talk about how to do that, it's important to be
       clear about the various DBI classes and how they	work together.

       By default "$dbh	= DBI->connect(...)" returns a $dbh blessed into the
       "DBI::db" class.	 And the "$dbh->prepare" method	returns	an $sth
       blessed into the	"DBI::st" class	(actually it simply changes the	last
       four characters of the calling handle class to be "::st").

       The leading '"DBI"' is known as the 'root class'	and the	extra '"::db"'
       or '"::st"' are the 'handle type	suffixes'. If you want to subclass the
       DBI you'll need to put your overriding methods into the appropriate
       classes.	 For example, if you want to use a root	class of "MySubDBI"
       and override the	do(), prepare()	and execute() methods, then your do()
       and prepare() methods should be in the "MySubDBI::db" class and the
       execute() method	should be in the "MySubDBI::st"	class.

       To setup	the inheritance	hierarchy the @ISA variable in "MySubDBI::db"
       should include "DBI::db"	and the	@ISA variable in "MySubDBI::st"	should
       include "DBI::st".  The "MySubDBI" root class itself isn't currently
       used for	anything visible and so, apart from setting @ISA to include
       "DBI", it can be	left empty.

       So, having put your overriding methods into the right classes, and
       setup the inheritance hierarchy,	how do you get the DBI to use them?
       You have	two choices, either a static method call using the name	of
       your subclass:

	 $dbh =	MySubDBI->connect(...);

       or specifying a "RootClass" attribute:

	 $dbh =	DBI->connect(..., { RootClass => 'MySubDBI' });

       If both forms are used then the attribute takes precedence.

       The only	differences between the	two are	that using an explicit
       RootClass attribute will	a) make	the DBI	automatically attempt to load
       a module	by that	name if	the class doesn't exist, and b)	won't call
       your MySubDBI::connect()	method,	if you have one.

       When subclassing	is being used then, after a successful new connect,
       the DBI->connect	method automatically calls:

	 $dbh->connected($dsn, $user, $pass, \%attr);

       The default method does nothing.	The call is made just to simplify any
       post-connection setup that your subclass	may want to perform.  The
       parameters are the same as passed to DBI->connect.  If your subclass
       supplies	a connected method, it should be part of the MySubDBI::db
       package.

       One more	thing to note: you must	let the	DBI do the handle creation.
       If you want to override the connect() method in your *::dr class	then
       it must still call SUPER::connect to get	a $dbh to work with.
       Similarly, an overridden	prepare() method in *::db must still call
       SUPER::prepare to get a $sth.  If you try to create your	own handles
       using bless() then you'll find the DBI will reject them with an "is not
       a DBI handle (has no magic)" error.

       Here's a	brief example of a DBI subclass.  A more thorough example can
       be found	in t/subclass.t	in the DBI distribution.

	 package MySubDBI;

	 use strict;

	 use DBI;
	 use vars qw(@ISA);
	 @ISA =	qw(DBI);

	 package MySubDBI::db;
	 use vars qw(@ISA);
	 @ISA =	qw(DBI::db);

	 sub prepare {
	   my ($dbh, @args) = @_;
	   my $sth = $dbh->SUPER::prepare(@args)
	       or return;
	   $sth->{private_mysubdbi_info} = { foo => 'bar' };
	   return $sth;
	 }

	 package MySubDBI::st;
	 use vars qw(@ISA);
	 @ISA =	qw(DBI::st);

	 sub fetch {
	   my ($sth, @args) = @_;
	   my $row = $sth->SUPER::fetch(@args)
	       or return;
	   do_something_magical_with_row_data($row)
	       or return $sth->set_err(1234, "The magic	failed", undef,	"fetch");
	   return $row;
	 }

       When calling a SUPER::method that returns a handle, be careful to check
       the return value	before trying to do other things with it in your
       overridden method. This is especially important if you want to set a
       hash attribute on the handle, as	Perl's autovivification	will bite you
       by (in)conveniently creating an unblessed hashref, which	your method
       will then return	with usually baffling results later on like the	error
       "dbih_getcom handle HASH(0xa4451a8) is not a DBI	handle (has no magic".
       It's best to check right	after the call and return undef	immediately on
       error, just like	DBI would and just like	the example above.

       If your method needs to record an error it should call the set_err()
       method with the error code and error string, as shown in	the example
       above. The error	code and error string will be recorded in the handle
       and available via "$h->err" and $DBI::errstr etc.  The set_err()	method
       always returns an undef or empty	list as	appropriate. Since your	method
       should nearly always return an undef or empty list as soon as an	error
       is detected it's	handy to simply	return what set_err() returns, as
       shown in	the example above.

       If the handle has "RaiseError", "PrintError", or	"HandleError" etc. set
       then the	set_err() method will honour them. This	means that if
       "RaiseError" is set then	set_err() won't	return in the normal way but
       will 'throw an exception' that can be caught with an "eval" block.

       You can stash private data into DBI handles via "$h->{private_..._*}".
       See the entry under "ATTRIBUTES COMMON TO ALL HANDLES" for info and
       important caveats.

   Memory Leaks
       When tracking down memory leaks using tools like	Devel::Leak you'll
       find that some DBI internals are	reported as 'leaking' memory.  This is
       very unlikely to	be a real leak.	 The DBI has various caches to improve
       performance and the apparrent leaks are simply the normal operation of
       these caches.

       The most	frequent sources of the	apparrent leaks	are "ChildHandles",
       "prepare_cached"	and "connect_cached".

       For example
       http://stackoverflow.com/questions/13338308/perl-dbi-memory-leak

       Given how widely	the DBI	is used, you can rest assured that if a	new
       release of the DBI did have a real leak it would	be discovered,
       reported, and fixed immediately.	The leak you're	looking	for is
       probably	elsewhere. Good	luck!

TRACING
       The DBI has a powerful tracing mechanism	built in. It enables you to
       see what's going	on 'behind the scenes',	both within the	DBI and	the
       drivers you're using.

   Trace Settings
       Which details are written to the	trace output is	controlled by a
       combination of a	trace level, an	integer	from 0 to 15, and a set	of
       trace flags that	are either on or off. Together these are known as the
       trace settings and are stored together in a single integer.  For	normal
       use you only need to set	the trace level, and generally only to a value
       between 1 and 4.

       Each handle has its own trace settings, and so does the DBI.  When you
       call a method the DBI merges the	handles	settings into its own for the
       duration	of the call: the trace flags of	the handle are OR'd into the
       trace flags of the DBI, and if the handle has a higher trace level then
       the DBI trace level is raised to	match it.  The previous	DBI trace
       settings	are restored when the called method returns.

   Trace Levels
       Trace levels are	as follows:

	 0 - Trace disabled.
	 1 - Trace top-level DBI method	calls returning	with results or	errors.
	 2 - As	above, adding tracing of top-level method entry	with parameters.
	 3 - As	above, adding some high-level information from the driver
	     and some internal information from	the DBI.
	 4 - As	above, adding more detailed information	from the driver.
	     This is the first level to	trace all the rows being fetched.
	 5 to 15 - As above but	with more and more internal information.

       Trace level 1 is	best for a simple overview of what's happening.	 Trace
       levels 2	thru 4 a good choice for general purpose tracing.  Levels 5
       and above are best reserved for investigating a specific	problem, when
       you need	to see "inside"	the driver and DBI.

       The trace output	is detailed and	typically very useful. Much of the
       trace output is formatted using the "neat" function, so strings in the
       trace output may	be edited and truncated	by that	function.

   Trace Flags
       Trace flags are used to enable tracing of specific activities within
       the DBI and drivers. The	DBI defines some trace flags and drivers can
       define others. DBI trace	flag names begin with a	capital	letter and
       driver specific names begin with	a lowercase letter, as usual.

       Currently the DBI defines these trace flags:

	 ALL - turn on all DBI and driver flags	(not recommended)
	 SQL - trace SQL statements executed
	       (not yet	implemented in DBI but implemented in some DBDs)
	 CON - trace connection	process
	 ENC - trace encoding (unicode translations etc)
	       (not yet	implemented in DBI but implemented in some DBDs)
	 DBD - trace only DBD messages
	       (not implemented	by all DBDs yet)
	 TXN - trace transactions
	       (not implemented	in all DBDs yet)

       The "parse_trace_flags" and "parse_trace_flag" methods are used to
       convert trace flag names	into the corresponding integer bit flags.

   Enabling Trace
       The "$h->trace" method sets the trace settings for a handle and
       "DBI->trace" does the same for the DBI.

       In addition to the "trace" method, you can enable the same trace
       information, and	direct the output to a file, by	setting	the
       "DBI_TRACE" environment variable	before starting	Perl.  See "DBI_TRACE"
       for more	information.

       Finally,	you can	set, or	get, the trace settings	for a handle using the
       "TraceLevel" attribute.

       All of those methods use	parse_trace_flags() and	so allow you set both
       the trace level and multiple trace flags	by using a string containing
       the trace level and/or flag names separated by vertical bar (""|"") or
       comma ("","") characters. For example:

	 local $h->{TraceLevel}	= "3|SQL|foo";

   Trace Output
       Initially trace output is written to "STDERR".  Both the	"$h->trace"
       and "DBI->trace"	methods	take an	optional $trace_file parameter,	which
       may be either the name of a file	to be opened by	DBI in append mode, or
       a reference to an existing writable (possibly layered) filehandle. If
       $trace_file is a	filename, and can be opened in append mode, or
       $trace_file is a	writable filehandle, then all trace output (currently
       including that from other handles) is redirected	to that	file. A
       warning is generated if $trace_file can't be opened or is not writable.

       Further calls to	trace()	without	$trace_file do not alter where the
       trace output is sent. If	$trace_file is undefined, then trace output is
       sent to "STDERR"	and, if	the prior trace	was opened with	$trace_file as
       a filename, the previous	trace file is closed; if $trace_file was a
       filehandle, the filehandle is not closed.

       NOTE: If	$trace_file is specified as a filehandle, the filehandle
       should not be closed until all DBI operations are completed, or the
       application has reset the trace file via	another	call to	"trace()" that
       changes the trace file.

   Tracing to Layered Filehandles
       NOTE:

       o   Tied	filehandles are	not currently supported, as tie	operations are
	   not available to the	PerlIO methods used by the DBI.

       o   PerlIO layer	support	requires Perl version 5.8 or higher.

       As of version 5.8, Perl provides	the ability to layer various
       "disciplines" on	an open	filehandle via the PerlIO module.

       A simple	example	of using PerlIO	layers is to use a scalar as the
       output:

	   my $scalar =	'';
	   open( my $fh, "+>:scalar", \$scalar );
	   $dbh->trace(	2, $fh );

       Now all trace output is simply appended to $scalar.

       A more complex application of tracing to	a layered filehandle is	the
       use of a	custom layer (Refer to Perlio::via for details on creating
       custom PerlIO layers.). Consider	an application with the	following
       logger module:

	   package MyFancyLogger;

	   sub new
	   {
	       my $self	= {};
	       my $fh;
	       open $fh, '>', 'fancylog.log';
	       $self->{_fh} = $fh;
	       $self->{_buf} = '';
	       return bless $self, shift;
	   }

	   sub log
	   {
	       my $self	= shift;
	       return unless exists $self->{_fh};
	       my $fh =	$self->{_fh};
	       $self->{_buf} .=	shift;
	   #
	   # DBI feeds us pieces at a time, so accumulate a complete line
	   # before outputing
	   #
	       print $fh "At ",	scalar localtime(), ':', $self->{_buf},	"\n" and
	       $self->{_buf} = ''
		   if $self->{_buf}=~tr/\n//;
	   }

	   sub close {
	       my $self	= shift;
	       return unless exists $self->{_fh};
	       my $fh =	$self->{_fh};
	       print $fh "At ",	scalar localtime(), ':', $self->{_buf},	"\n" and
	       $self->{_buf} = ''
		   if $self->{_buf};
	       close $fh;
	       delete $self->{_fh};
	   }

	   1;

       To redirect DBI traces to this logger requires creating a package for
       the layer:

	   package PerlIO::via::MyFancyLogLayer;

	   sub PUSHED
	   {
	       my ($class,$mode,$fh) = @_;
	       my $logger;
	       return bless \$logger,$class;
	   }

	   sub OPEN {
	       my ($self, $path, $mode,	$fh) = @_;
	       #
	       # $path is actually our logger object
	       #
	       $$self =	$path;
	       return 1;
	   }

	   sub WRITE
	   {
	       my ($self, $buf,	$fh) = @_;
	       $$self->log($buf);
	       return length($buf);
	   }

	   sub CLOSE {
	       my $self	= shift;
	       $$self->close();
	       return 0;
	   }

	   1;

       The application can then	cause DBI traces to be routed to the logger
       using

	   use PerlIO::via::MyFancyLogLayer;

	   open	my $fh,	'>:via(MyFancyLogLayer)', MyFancyLogger->new();

	   $dbh->trace('SQL', $fh);

       Now all trace output will be processed by MyFancyLogger's log() method.

   Trace Content
       Many of the values embedded in trace output are formatted using the
       neat() utility function.	This means they	may be quoted, sanitized, and
       possibly	truncated if longer than $DBI::neat_maxlen. See	"neat" for
       more details.

   Tracing Tips
       You can add tracing to your own application code	using the "trace_msg"
       method.

       It can sometimes	be handy to compare trace files	from two different
       runs of the same	script.	However	using a	tool like "diff" on the
       original	log output doesn't work	well because the trace file is full of
       object addresses	that may differ	on each	run.

       The DBI includes	a handy	utility	called dbilogstrip that	can be used to
       'normalize' the log content. It can be used as a	filter like this:

	   DBI_TRACE=2 perl yourscript.pl ...args1... 2>&1 | dbilogstrip > dbitrace1.log
	   DBI_TRACE=2 perl yourscript.pl ...args2... 2>&1 | dbilogstrip > dbitrace2.log
	   diff	-u dbitrace1.log dbitrace2.log

       See dbilogstrip for more	information.

DBI ENVIRONMENT	VARIABLES
       The DBI module recognizes a number of environment variables, but	most
       of them should not be used most of the time.  It	is better to be
       explicit	about what you are doing to avoid the need for environment
       variables, especially in	a web serving system where web servers are
       stingy about which environment variables	are available.

   DBI_DSN
       The DBI_DSN environment variable	is used	by DBI->connect	if you do not
       specify a data source when you issue the	connect.  It should have a
       format such as "dbi:Driver:databasename".

   DBI_DRIVER
       The DBI_DRIVER environment variable is used to fill in the database
       driver name in DBI->connect if the data source string starts "dbi::"
       (thereby	omitting the driver).  If DBI_DSN omits	the driver name,
       DBI_DRIVER can fill the gap.

   DBI_AUTOPROXY
       The DBI_AUTOPROXY environment variable takes a string value that	starts
       "dbi:Proxy:" and	is typically followed by "hostname=...;port=...".  It
       is used to alter	the behaviour of DBI->connect.	For full details, see
       DBI::Proxy documentation.

   DBI_USER
       The DBI_USER environment	variable takes a string	value that is used as
       the user	name if	the DBI->connect call is given undef (as distinct from
       an empty	string)	as the username	argument.  Be wary of the security
       implications of using this.

   DBI_PASS
       The DBI_PASS environment	variable takes a string	value that is used as
       the password if the DBI->connect	call is	given undef (as	distinct from
       an empty	string)	as the password	argument.  Be extra wary of the
       security	implications of	using this.

   DBI_DBNAME (obsolete)
       The DBI_DBNAME environment variable takes a string value	that is	used
       only when the obsolescent style of DBI->connect (with driver name as
       fourth parameter) is used, and when no value is provided	for the	first
       (database name) argument.

   DBI_TRACE
       The DBI_TRACE environment variable specifies the	global default trace
       settings	for the	DBI at startup.	Can also be used to direct trace
       output to a file. When the DBI is loaded	it does:

	 DBI->trace(split /=/, $ENV{DBI_TRACE},	2) if $ENV{DBI_TRACE};

       So if "DBI_TRACE" contains an ""="" character then what follows it is
       used as the name	of the file to append the trace	to.

       output appended to that file. If	the name begins	with a number followed
       by an equal sign	("="), then the	number and the equal sign are stripped
       off from	the name, and the number is used to set	the trace level. For
       example:

	 DBI_TRACE=1=dbitrace.log perl your_test_script.pl

       On Unix-like systems using a Bourne-like	shell, you can do this easily
       on the command line:

	 DBI_TRACE=2 perl your_test_script.pl

       See "TRACING" for more information.

   PERL_DBI_DEBUG (obsolete)
       An old variable that should no longer be	used; equivalent to DBI_TRACE.

   DBI_PROFILE
       The DBI_PROFILE environment variable can	be used	to enable profiling of
       DBI method calls. See DBI::Profile for more information.

   DBI_PUREPERL
       The DBI_PUREPERL	environment variable can be used to enable the use of
       DBI::PurePerl.  See DBI::PurePerl for more information.

WARNING	AND ERROR MESSAGES
   Fatal Errors
       Can't call method "prepare" without a package or	object reference
	   The $dbh handle you're using	to call	"prepare" is probably
	   undefined because the preceding "connect" failed. You should	always
	   check the return status of DBI methods, or use the "RaiseError"
	   attribute.

       Can't call method "execute" without a package or	object reference
	   The $sth handle you're using	to call	"execute" is probably
	   undefined because the preceding "prepare" failed. You should	always
	   check the return status of DBI methods, or use the "RaiseError"
	   attribute.

       DBI/DBD internal	version	mismatch
	   The DBD driver module was built with	a different version of DBI
	   than	the one	currently being	used.  You should rebuild the DBD
	   module under	the current version of DBI.

	   (Some rare platforms	require	"static	linking". On those platforms,
	   there may be	an old DBI or DBD driver version actually embedded in
	   the Perl executable being used.)

       DBD driver has not implemented the AutoCommit attribute
	   The DBD driver implementation is incomplete.	Consult	the author.

       Can't [sg]et %s->{%s}: unrecognised attribute
	   You attempted to set	or get an unknown attribute of a handle.  Make
	   sure	you have spelled the attribute name correctly; case is
	   significant (e.g., "Autocommit" is not the same as "AutoCommit").

Pure-Perl DBI
       A pure-perl emulation of	the DBI	is included in the distribution	for
       people using pure-perl drivers who, for whatever	reason,	can't install
       the compiled DBI. See DBI::PurePerl.

SEE ALSO
   Driver and Database Documentation
       Refer to	the documentation for the DBD driver that you are using.

       Refer to	the SQL	Language Reference Manual for the database engine that
       you are using.

   ODBC	and SQL/CLI Standards Reference	Information
       More detailed information about the semantics of	certain	DBI methods
       that are	based on ODBC and SQL/CLI standards is available on-line via
       microsoft.com, for ODBC,	and www.jtc1sc32.org for the SQL/CLI standard:

	DBI method	  ODBC function	    SQL/CLI Working Draft
	----------	  -------------	    ---------------------
	column_info	  SQLColumns	    Page 124
	foreign_key_info  SQLForeignKeys    Page 163
	get_info	  SQLGetInfo	    Page 214
	primary_key_info  SQLPrimaryKeys    Page 254
	table_info	  SQLTables	    Page 294
	type_info	  SQLGetTypeInfo    Page 239
	statistics_info	  SQLStatistics

       To find documentation on	the ODBC function you can use the MSDN search
       facility	at:

	   http://msdn.microsoft.com/Search

       and search for something	like "SQLColumns returns".

       And for SQL/CLI standard	information on SQLColumns you'd	read page 124
       of the (very large) SQL/CLI Working Draft available from:

	 http://jtc1sc32.org/doc/N0701-0750/32N0744T.pdf

   Standards Reference Information
       A hyperlinked, browsable	version	of the BNF syntax for SQL92 (plus
       Oracle 7	SQL and	PL/SQL)	is available here:

	 http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html

       You can find more information about SQL standards online	by searching
       for the appropriate standard names and numbers. For example, searching
       for "ANSI/ISO/IEC International Standard	(IS) Database Language SQL -
       Part 1: SQL/Framework" you'll find a copy at:

	 ftp://ftp.iks-jena.de/mitarb/lutz/standards/sql/ansi-iso-9075-1-1999.pdf

   Books and Articles
       Programming the Perl DBI, by Alligator Descartes	and Tim	Bunce.
       <http://books.perl.org/book/154>

       Programming Perl	3rd Ed.	by Larry Wall, Tom Christiansen	& Jon Orwant.
       <http://books.perl.org/book/134>

       Learning	Perl by	Randal Schwartz.  <http://books.perl.org/book/101>

       Details of many other books related to perl can be found	at
       <http://books.perl.org>

   Perl	Modules
       Index of	DBI related modules available from CPAN:

	L<https://metacpan.org/search?q=DBD%3A%3A>
	L<https://metacpan.org/search?q=DBIx%3A%3A>
	L<https://metacpan.org/search?q=DBI>

       For a good comparison of	RDBMS-OO mappers and some OO-RDBMS mappers
       (including Class::DBI, Alzabo, and DBIx::RecordSet in the former
       category	and Tangram and	SPOPS in the latter) see the Perl Object-
       Oriented	Persistence project pages at:

	http://poop.sourceforge.net

       A similar page for Java toolkits	can be found at:

	http://c2.com/cgi-bin/wiki?ObjectRelationalToolComparison

   Mailing List
       The dbi-users mailing list is the primary means of communication	among
       users of	the DBI	and its	related	modules. For details send email	to:

	L<dbi-users-help@perl.org>

       There are typically between 700 and 900 messages	per month.  You	have
       to subscribe in order to	be able	to post. However you can opt for a
       'post-only' subscription.

       Mailing list archives (of variable quality) are held at:

	http://groups.google.com/groups?group=perl.dbi.users
	http://www.xray.mpe.mpg.de/mailing-lists/dbi/
	http://www.mail-archive.com/dbi-users%40perl.org/

   Assorted Related Links
       The DBI "Home Page":

	http://dbi.perl.org/

       Other DBI related links:

	http://www.perlmonks.org/?node=DBI%20recipes
	http://www.perlmonks.org/?node=Speeding%20up%20the%20DBI

       Other database related links:

	http://www.connectionstrings.com/

       Security, especially the	"SQL Injection"	attack:

	http://bobby-tables.com/
	http://online.securityfocus.com/infocus/1644

   FAQ
       See <http://faq.dbi-support.com/>

AUTHORS
       DBI by Tim Bunce, <http://www.tim.bunce.name>

       This pod	text by	Tim Bunce, J. Douglas Dunlop, Jonathan Leffler and
       others.	Perl by	Larry Wall and the "perl5-porters".

COPYRIGHT
       The DBI module is Copyright (c) 1994-2012 Tim Bunce. Ireland.  All
       rights reserved.

       You may distribute under	the terms of either the	GNU General Public
       License or the Artistic License,	as specified in	the Perl 5.10.0	README
       file.

SUPPORT	/ WARRANTY
       The DBI is free Open Source software. IT	COMES WITHOUT WARRANTY OF ANY
       KIND.

   Support
       My consulting company, Data Plan	Services, offers annual	and multi-
       annual support contracts	for the	DBI. These provide sustained support
       for DBI development, and	sustained value	for you	in return.  Contact me
       for details.

   Sponsor Enhancements
       If your company would benefit from a specific new DBI feature, please
       consider	sponsoring its development.  Work is performed rapidly,	and
       usually on a fixed-price	payment-on-delivery basis.  Contact me for
       details.

       Using such targeted financing allows you	to contribute to DBI
       development, and	rapidly	get something specific and valuable in return.

ACKNOWLEDGEMENTS
       I would like to acknowledge the valuable	contributions of the many
       people I	have worked with on the	DBI project, especially	in the early
       years (1992-1994). In no	particular order: Kevin	Stock, Buzz Moschetti,
       Kurt Andersen, Ted Lemon, William Hails,	Garth Kennedy, Michael
       Peppler,	Neil S.	Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander,
       Forrest D Whitcher, Larry Wall, Jeff Fried, Roy Johnson,	Paul Hudson,
       Georg Rehfeld, Steve Sizemore, Ron Pool,	Jon Meek, Tom Christiansen,
       Steve Baumgarten, Randal	Schwartz, and a	whole lot more.

       Then, of	course,	there are the poor souls who have struggled through
       untold and undocumented obstacles to actually implement DBI drivers.
       Among their ranks are Jochen Wiedmann, Alligator	Descartes, Jonathan
       Leffler,	Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo,
       Davide Migliavacca, Jan Pazdziora, Peter	Haworth, Edmund	Mergl, Steve
       Williams, Thomas	Lowery,	and Phlip Plumlee. Without them, the DBI would
       not be the practical reality it is today.  I'm also especially grateful
       to Alligator Descartes for starting work	on the first edition of	the
       "Programming the	Perl DBI" book and letting me jump on board.

       The DBI and DBD::Oracle were originally developed while I was Technical
       Director	(CTO) of the Paul Ingram Group in the UK.  So I'd especially
       like to thank Paul for his generosity and vision	in supporting this
       work for	many years.

       A couple	of specific DBI	features have been sponsored by	enlightened
       companies:

       The development of the swap_inner_handle() method was sponsored by
       BizRate.com (<http://BizRate.com>)

       The development of DBD::Gofer and related modules was sponsored by
       Shopzilla.com (<http://Shopzilla.com>), where I currently work.

CONTRIBUTING
       As you can see above, many people have contributed to the DBI and
       drivers in many ways over many years.

       If you'd	like to	help then see <http://dbi.perl.org/contributing>.

       If you'd	like the DBI to	do something new or different then a good way
       to make that happen is to do it yourself	and send me a patch to the
       source code that	shows the changes. (But	read "Speak before you patch"
       below.)

   Browsing the	source code repository
       Use https://github.com/perl5-dbi/dbi

   How to create a patch using Git
       The DBI source code is maintained using Git.  To	access the source
       you'll need to install a	Git client. Then, to get the source code, do:

	 git clone https://github.com/perl5-dbi/dbi.git	DBI-git

       The source code will now	be available in	the new	subdirectory
       "DBI-git".

       When you	want to	synchronize later, issue the command

	 git pull --all

       Make your changes, test them, test them again until everything passes.
       If there	are no tests for the new feature you added or a	behaviour
       change, the change should include a new test. Then commit the changes.
       Either use

	 git gui

       or

	 git commit -a -m 'Message to my changes'

       If you get any conflicts	reported you'll	need to	fix them first.

       Then generate the patch file to be mailed:

	 git format-patch -1 --attach

       which will create a file	0001-*.patch (where * relates to the commit
       message).  Read the patch file, as a sanity check, and then email it to
       dbi-dev@perl.org.

       If you have a github <https://github.com> account, you can also fork
       the repository, commit your changes to the forked repository and	then
       do a pull request.

   How to create a patch without Git
       Unpack a	fresh copy of the distribution:

	 wget http://cpan.metacpan.org/authors/id/T/TI/TIMB/DBI-1.627.tar.gz
	 tar xfz DBI-1.627.tar.gz

       Rename the newly	created	top level directory:

	 mv DBI-1.627 DBI-1.627.your_foo

       Edit the	contents of DBI-1.627.your_foo/* till it does what you want.

       Test your changes and then remove all temporary files:

	 make test && make distclean

       Go back to the directory	you originally unpacked	the distribution:

	 cd ..

       Unpack another copy of the original distribution	you started with:

	 tar xfz DBI-1.627.tar.gz

       Then create a patch file	by performing a	recursive "diff" on the	two
       top level directories:

	 diff -purd DBI-1.627 DBI-1.627.your_foo > DBI-1.627.your_foo.patch

   Speak before	you patch
       For anything non-trivial	or possibly controversial it's a good idea to
       discuss (on dbi-dev@perl.org) the changes you propose before actually
       spending	time working on	them. Otherwise	you run	the risk of them being
       rejected	because	they don't fit into some larger	plans you may not be
       aware of.

       You can also reach the developers on IRC	(chat).	If they	are on-line,
       the most	likely place to	talk to	them is	the #dbi channel on
       irc.perl.org

TRANSLATIONS
       A German	translation of this manual (possibly slightly out of date) is
       available, thanks to O'Reilly, at:

	 http://www.oreilly.de/catalog/perldbiger/

OTHER RELATED WORK AND PERL MODULES
       Apache::DBI
	   To be used with the Apache daemon together with an embedded Perl
	   interpreter like "mod_perl".	Establishes a database connection
	   which remains open for the lifetime of the HTTP daemon. This	way
	   the CGI connect and disconnect for every database access becomes
	   superfluous.

       SQL Parser
	   See also the	SQL::Statement module, SQL parser and engine.

perl v5.32.0			  2020-01-31				DBI(3)

NAME | SYNOPSIS | DESCRIPTION | THE DBI PACKAGE AND CLASS | METHODS COMMON TO ALL HANDLES | ATTRIBUTES COMMON TO ALL HANDLES | DBI DATABASE HANDLE OBJECTS | DBI STATEMENT HANDLE OBJECTS | FURTHER INFORMATION | TRACING | DBI ENVIRONMENT VARIABLES | WARNING AND ERROR MESSAGES | Pure-Perl DBI | SEE ALSO | AUTHORS | COPYRIGHT | SUPPORT / WARRANTY | ACKNOWLEDGEMENTS | CONTRIBUTING | TRANSLATIONS | OTHER RELATED WORK AND PERL MODULES

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