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MYISAMCHK(1)		     MySQL Database System		  MYISAMCHK(1)

NAME
       myisamchk - MyISAM table-maintenance utility

SYNOPSIS
       myisamchk [options] tbl_name ...

DESCRIPTION
       The myisamchk utility gets information about your database tables or
       checks, repairs,	or optimizes them.  myisamchk works with MyISAM	tables
       (tables that have .MYD and .MYI files for storing data and indexes).

       You can also use	the CHECK TABLE	and REPAIR TABLE statements to check
       and repair MyISAM tables. See Section 13.7.2.2, "CHECK TABLE
       Statement", and Section 13.7.2.5, "REPAIR TABLE Statement".

       The use of myisamchk with partitioned tables is not supported.

	   Caution
	   It is best to make a	backup of a table before performing a table
	   repair operation; under some	circumstances the operation might
	   cause data loss. Possible causes include but	are not	limited	to
	   file	system errors.

       Invoke myisamchk	like this:

	   shell> myisamchk [options] tbl_name ...

       The options specify what	you want myisamchk to do. They are described
       in the following	sections. You can also get a list of options by
       invoking	myisamchk --help.

       With no options,	myisamchk simply checks	your table as the default
       operation. To get more information or to	tell myisamchk to take
       corrective action, specify options as described in the following
       discussion.

       tbl_name	is the database	table you want to check	or repair. If you run
       myisamchk somewhere other than in the database directory, you must
       specify the path	to the database	directory, because myisamchk has no
       idea where the database is located. In fact, myisamchk does not
       actually	care whether the files you are working on are located in a
       database	directory. You can copy	the files that correspond to a
       database	table into some	other location and perform recovery operations
       on them there.

       You can name several tables on the myisamchk command line if you	wish.
       You can also specify a table by naming its index	file (the file with
       the .MYI	suffix). This enables you to specify all tables	in a directory
       by using	the pattern *.MYI. For example,	if you are in a	database
       directory, you can check	all the	MyISAM tables in that directory	like
       this:

	   shell> myisamchk *.MYI

       If you are not in the database directory, you can check all the tables
       there by	specifying the path to the directory:

	   shell> myisamchk /path/to/database_dir/*.MYI

       You can even check all tables in	all databases by specifying a wildcard
       with the	path to	the MySQL data directory:

	   shell> myisamchk /path/to/datadir/*/*.MYI

       The recommended way to quickly check all	MyISAM tables is:

	   shell> myisamchk --silent --fast /path/to/datadir/*/*.MYI

       If you want to check all	MyISAM tables and repair any that are
       corrupted, you can use the following command:

	   shell> myisamchk --silent --force --fast --update-state \
		     --key_buffer_size=64M --myisam_sort_buffer_size=64M \
		     --read_buffer_size=1M --write_buffer_size=1M \
		     /path/to/datadir/*/*.MYI

       This command assumes that you have more than 64MB free. For more
       information about memory	allocation with	myisamchk, see the section
       called "MYISAMCHK MEMORY	USAGE".

       For additional information about	using myisamchk, see Section 7.6,
       "MyISAM Table Maintenance and Crash Recovery".

	   Important
	   You must ensure that	no other program is using the tables while you
	   are running myisamchk. The most effective means of doing so is to
	   shut	down the MySQL server while running myisamchk, or to lock all
	   tables that myisamchk is being used on.

	   Otherwise, when you run myisamchk, it may display the following
	   error message:

	       warning:	clients	are using or haven't closed the	table properly

	   This	means that you are trying to check a table that	has been
	   updated by another program (such as the mysqld server) that hasn't
	   yet closed the file or that has died	without	closing	the file
	   properly, which can sometimes lead to the corruption	of one or more
	   MyISAM tables.

	   If mysqld is	running, you must force	it to flush any	table
	   modifications that are still	buffered in memory by using FLUSH
	   TABLES. You should then ensure that no one is using the tables
	   while you are running myisamchk

	   However, the	easiest	way to avoid this problem is to	use CHECK
	   TABLE instead of myisamchk to check tables. See Section 13.7.2.2,
	   "CHECK TABLE	Statement".

       myisamchk supports the following	options, which can be specified	on the
       command line or in the [myisamchk] group	of an option file. For
       information about option	files used by MySQL programs, see
       Section 4.2.2.2,	"Using Option Files".

MYISAMCHK GENERAL OPTIONS
       The options described in	this section can be used for any type of table
       maintenance operation performed by myisamchk. The sections following
       this one	describe options that pertain only to specific operations,
       such as table checking or repairing.

       o   --help, -?  Display a help message and exit.	Options	are grouped by
	   type	of operation.

       o   --HELP, -H Display a	help message and exit. Options are presented
	   in a	single list.

       o   --debug=debug_options, -# debug_options Write a debugging log. A
	   typical debug_options string	is d:t:o,file_name. The	default	is
	   d:t:o,/tmp/myisamchk.trace.

	   This	option is available only if MySQL was built using WITH_DEBUG.
	   MySQL release binaries provided by Oracle are not built using this
	   option.

       o   --defaults-extra-file=file_name Read	this option file after the
	   global option file but (on Unix) before the user option file. If
	   the file does not exist or is otherwise inaccessible, an error
	   occurs.  file_name is interpreted relative to the current directory
	   if given as a relative path name rather than	a full path name.

	   For additional information about this and other option-file
	   options, see	Section	4.2.2.3, "Command-Line Options that Affect
	   Option-File Handling".

       o   --defaults-file=file_name Use only the given	option file. If	the
	   file	does not exist or is otherwise inaccessible, an	error occurs.
	   file_name is	interpreted relative to	the current directory if given
	   as a	relative path name rather than a full path name.

	   For additional information about this and other option-file
	   options, see	Section	4.2.2.3, "Command-Line Options that Affect
	   Option-File Handling".

       o   --defaults-group-suffix=str Read not	only the usual option groups,
	   but also groups with	the usual names	and a suffix of	str. For
	   example, myisamchk normally reads the [myisamchk] group. If the
	   --defaults-group-suffix=_other option is given, myisamchk also
	   reads the [myisamchk_other] group.

	   For additional information about this and other option-file
	   options, see	Section	4.2.2.3, "Command-Line Options that Affect
	   Option-File Handling".

       o   --no-defaults Do not	read any option	files. If program startup
	   fails due to	reading	unknown	options	from an	option file,
	   --no-defaults can be	used to	prevent	them from being	read.

	   The exception is that the .mylogin.cnf file,	if it exists, is read
	   in all cases. This permits passwords	to be specified	in a safer way
	   than	on the command line even when --no-defaults is used.
	   (.mylogin.cnf is created by the mysql_config_editor utility.	See
	   mysql_config_editor(1).)

	   For additional information about this and other option-file
	   options, see	Section	4.2.2.3, "Command-Line Options that Affect
	   Option-File Handling".

       o   --print-defaults Print the program name and all options that	it
	   gets	from option files.

	   For additional information about this and other option-file
	   options, see	Section	4.2.2.3, "Command-Line Options that Affect
	   Option-File Handling".

       o   --silent, -s	Silent mode. Write output only when errors occur. You
	   can use -s twice (-ss) to make myisamchk very silent.

       o   --verbose, -v Verbose mode. Print more information about what the
	   program does. This can be used with -d and -e. Use -v multiple
	   times (-vv, -vvv) for even more output.

       o   --version, -V Display version information and exit.

       o   --wait, -w Instead of terminating with an error if the table	is
	   locked, wait	until the table	is unlocked before continuing. If you
	   are running mysqld with external locking disabled, the table	can be
	   locked only by another myisamchk command.

       You can also set	the following variables	by using --var_name=value
       syntax:

       +-----------------------+-------------------+
       |Variable	       | Default Value	   |
       +-----------------------+-------------------+
       |decode_bits	       | 9		   |
       +-----------------------+-------------------+
       |ft_max_word_len	       | version-dependent |
       +-----------------------+-------------------+
       |ft_min_word_len	       | 4		   |
       +-----------------------+-------------------+
       |ft_stopword_file       | built-in list	   |
       +-----------------------+-------------------+
       |key_buffer_size	       | 523264		   |
       +-----------------------+-------------------+
       |myisam_block_size      | 1024		   |
       +-----------------------+-------------------+
       |myisam_sort_key_blocks | 16		   |
       +-----------------------+-------------------+
       |read_buffer_size       | 262136		   |
       +-----------------------+-------------------+
       |sort_buffer_size       | 2097144	   |
       +-----------------------+-------------------+
       |sort_key_blocks	       | 16		   |
       +-----------------------+-------------------+
       |stats_method	       | nulls_unequal	   |
       +-----------------------+-------------------+
       |write_buffer_size      | 262136		   |
       +-----------------------+-------------------+

       The possible myisamchk variables	and their default values can be
       examined	with myisamchk --help:

       myisam_sort_buffer_size is used when the	keys are repaired by sorting
       keys, which is the normal case when you use --recover.
       sort_buffer_size	is a deprecated	synonym	for myisam_sort_buffer_size.

       key_buffer_size is used when you	are checking the table with
       --extend-check or when the keys are repaired by inserting keys row by
       row into	the table (like	when doing normal inserts). Repairing through
       the key buffer is used in the following cases:

       o   You use --safe-recover.

       o   The temporary files needed to sort the keys would be	more than
	   twice as big	as when	creating the key file directly.	This is	often
	   the case when you have large	key values for CHAR, VARCHAR, or TEXT
	   columns, because the	sort operation needs to	store the complete key
	   values as it	proceeds. If you have lots of temporary	space and you
	   can force myisamchk to repair by sorting, you can use the
	   --sort-recover option.

       Repairing through the key buffer	takes much less	disk space than	using
       sorting,	but is also much slower.

       If you want a faster repair, set	the key_buffer_size and
       myisam_sort_buffer_size variables to about 25% of your available
       memory. You can set both	variables to large values, because only	one of
       them is used at a time.

       myisam_block_size is the	size used for index blocks.

       stats_method influences how NULL	values are treated for index
       statistics collection when the --analyze	option is given. It acts like
       the myisam_stats_method system variable.	For more information, see the
       description of myisam_stats_method in Section 5.1.7, "Server System
       Variables", and Section 8.3.7, "InnoDB and MyISAM Index Statistics
       Collection".

       ft_min_word_len and ft_max_word_len indicate the	minimum	and maximum
       word length for FULLTEXT	indexes	on MyISAM tables.  ft_stopword_file
       names the stopword file.	These need to be set under the following
       circumstances.

       If you use myisamchk to perform an operation that modifies table
       indexes (such as	repair or analyze), the	FULLTEXT indexes are rebuilt
       using the default full-text parameter values for	minimum	and maximum
       word length and the stopword file unless	you specify otherwise. This
       can result in queries failing.

       The problem occurs because these	parameters are known only by the
       server. They are	not stored in MyISAM index files. To avoid the problem
       if you have modified the	minimum	or maximum word	length or the stopword
       file in the server, specify the same ft_min_word_len, ft_max_word_len,
       and ft_stopword_file values to myisamchk	that you use for mysqld. For
       example,	if you have set	the minimum word length	to 3, you can repair a
       table with myisamchk like this:

	   shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

       To ensure that myisamchk	and the	server use the same values for
       full-text parameters, you can place each	one in both the	[mysqld] and
       [myisamchk] sections of an option file:

	   [mysqld]
	   ft_min_word_len=3
	   [myisamchk]
	   ft_min_word_len=3

       An alternative to using myisamchk is to use the REPAIR TABLE, ANALYZE
       TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed
       by the server, which knows the proper full-text parameter values	to
       use.

MYISAMCHK CHECK	OPTIONS
       myisamchk supports the following	options	for table checking operations:

       o   --check, -c Check the table for errors. This	is the default
	   operation if	you specify no option that selects an operation	type
	   explicitly.

       o   --check-only-changed, -C Check only tables that have	changed	since
	   the last check.

       o   --extend-check, -e Check the	table very thoroughly. This is quite
	   slow	if the table has many indexes. This option should only be used
	   in extreme cases. Normally, myisamchk or myisamchk --medium-check
	   should be able to determine whether there are any errors in the
	   table.

	   If you are using --extend-check and have plenty of memory, setting
	   the key_buffer_size variable	to a large value helps the repair
	   operation run faster.

	   See also the	description of this option under table repair options.

	   For a description of	the output format, see the section called
	   "OBTAINING TABLE INFORMATION	WITH MYISAMCHK".

       o   --fast, -F Check only tables	that haven't been closed properly.

       o   --force, -f Do a repair operation automatically if myisamchk	finds
	   any errors in the table. The	repair type is the same	as that
	   specified with the --recover	or -r option.

       o   --information, -i Print informational statistics about the table
	   that	is checked.

       o   --medium-check, -m Do a check that is faster	than an	--extend-check
	   operation. This finds only 99.99% of	all errors, which should be
	   good	enough in most cases.

       o   --read-only,	-T Do not mark the table as checked. This is useful if
	   you use myisamchk to	check a	table that is in use by	some other
	   application that does not use locking, such as mysqld when run with
	   external locking disabled.

       o   --update-state, -U Store information	in the .MYI file to indicate
	   when	the table was checked and whether the table crashed. This
	   should be used to get full benefit of the --check-only-changed
	   option, but you shouldn't use this option if	the mysqld server is
	   using the table and you are running it with external	locking
	   disabled.

MYISAMCHK REPAIR OPTIONS
       myisamchk supports the following	options	for table repair operations
       (operations performed when an option such as --recover or
       --safe-recover is given):

       o   --backup, -B	Make a backup of the .MYD file as file_name-time.BAK

       o   --character-sets-dir=dir_name The directory where character sets
	   are installed. See Section 10.15, "Character	Set Configuration".

       o   --correct-checksum Correct the checksum information for the table.

       o   --data-file-length=len, -D len The maximum length of	the data file
	   (when re-creating data file when it is "full").

       o   --extend-check, -e

	   Do a	repair that tries to recover every possible row	from the data
	   file. Normally, this	also finds a lot of garbage rows. Do not use
	   this	option unless you are desperate.

	   See also the	description of this option under table checking
	   options.

	   For a description of	the output format, see the section called
	   "OBTAINING TABLE INFORMATION	WITH MYISAMCHK".

       o   --force, -f

	   Overwrite old intermediate files (files with	names like
	   tbl_name.TMD) instead of aborting.

       o   --keys-used=val, -k val For myisamchk, the option value is a	bit
	   value that indicates	which indexes to update. Each binary bit of
	   the option value corresponds	to a table index, where	the first
	   index is bit	0. An option value of 0	disables updates to all
	   indexes, which can be used to get faster inserts. Deactivated
	   indexes can be reactivated by using myisamchk -r.

       o   --no-symlinks, -l Do	not follow symbolic links. Normally myisamchk
	   repairs the table that a symlink points to. This option does	not
	   exist as of MySQL 4.0 because versions from 4.0 on do not remove
	   symlinks during repair operations.

       o   --max-record-length=len Skip	rows larger than the given length if
	   myisamchk cannot allocate memory to hold them.

       o   --parallel-recover, -p Use the same technique as -r and -n, but
	   create all the keys in parallel, using different threads.  This is
	   beta-quality	code. Use at your own risk!

       o   --quick, -q Achieve a faster	repair by modifying only the index
	   file, not the data file. You	can specify this option	twice to force
	   myisamchk to	modify the original data file in case of duplicate
	   keys.

       o   --recover, -r Do a repair that can fix almost any problem except
	   unique keys that are	not unique (which is an	extremely unlikely
	   error with MyISAM tables). If you want to recover a table, this is
	   the option to try first. You	should try --safe-recover only if
	   myisamchk reports that the table cannot be recovered	using
	   --recover. (In the unlikely case that --recover fails, the data
	   file	remains	intact.)

	   If you have lots of memory, you should increase the value of
	   myisam_sort_buffer_size.

       o   --safe-recover, -o Do a repair using	an old recovery	method that
	   reads through all rows in order and updates all index trees based
	   on the rows found. This is an order of magnitude slower than
	   --recover, but can handle a couple of very unlikely cases that
	   --recover cannot. This recovery method also uses much less disk
	   space than --recover. Normally, you should repair first using
	   --recover, and then with --safe-recover only	if --recover fails.

	   If you have lots of memory, you should increase the value of
	   key_buffer_size.

       o   --set-collation=name	Specify	the collation to use for sorting table
	   indexes. The	character set name is implied by the first part	of the
	   collation name.

       o   --sort-recover, -n Force myisamchk to use sorting to	resolve	the
	   keys	even if	the temporary files would be very large.

       o   --tmpdir=dir_name, -t dir_name The path of the directory to be used
	   for storing temporary files.	If this	is not set, myisamchk uses the
	   value of the	TMPDIR environment variable.  --tmpdir can be set to a
	   list	of directory paths that	are used successively in round-robin
	   fashion for creating	temporary files. The separator character
	   between directory names is the colon	(:) on Unix and	the semicolon
	   (;) on Windows.

       o   --unpack, -u	Unpack a table that was	packed with myisampack.

OTHER MYISAMCHK	OPTIONS
       myisamchk supports the following	options	for actions other than table
       checks and repairs:

       o   --analyze, -a Analyze the distribution of key values. This improves
	   join	performance by enabling	the join optimizer to better choose
	   the order in	which to join the tables and which indexes it should
	   use.	To obtain information about the	key distribution, use a
	   myisamchk --description --verbose tbl_name command or the SHOW
	   INDEX FROM tbl_name statement.

       o   --block-search=offset, -b offset Find the record that a block at
	   the given offset belongs to.

       o   --description, -d Print some	descriptive information	about the
	   table. Specifying the --verbose option once or twice	produces
	   additional information. See the section called "OBTAINING TABLE
	   INFORMATION WITH MYISAMCHK".

       o   --set-auto-increment[=value], -A[value] Force AUTO_INCREMENT
	   numbering for new records to	start at the given value (or higher,
	   if there are	existing records with AUTO_INCREMENT values this
	   large). If value is not specified, AUTO_INCREMENT numbers for new
	   records begin with the largest value	currently in the table,	plus
	   one.

       o   --sort-index, -S Sort the index tree	blocks in high-low order. This
	   optimizes seeks and makes table scans that use indexes faster.

       o   --sort-records=N, -R	N Sort records according to a particular
	   index. This makes your data much more localized and may speed up
	   range-based SELECT and ORDER	BY operations that use this index.
	   (The	first time you use this	option to sort a table,	it may be very
	   slow.) To determine a table's index numbers,	use SHOW INDEX,	which
	   displays a table's indexes in the same order	that myisamchk sees
	   them. Indexes are numbered beginning	with 1.

	   If keys are not packed (PACK_KEYS=0), they have the same length, so
	   when	myisamchk sorts	and moves records, it just overwrites record
	   offsets in the index. If keys are packed (PACK_KEYS=1), myisamchk
	   must	unpack key blocks first, then re-create	indexes	and pack the
	   key blocks again. (In this case, re-creating	indexes	is faster than
	   updating offsets for	each index.)

OBTAINING TABLE	INFORMATION WITH MYISAMCHK
       To obtain a description of a MyISAM table or statistics about it, use
       the commands shown here.	The output from	these commands is explained
       later in	this section.

       o   myisamchk -d	tbl_name

	   Runs	myisamchk in "describe mode" to	produce	a description of your
	   table. If you start the MySQL server	with external locking
	   disabled, myisamchk may report an error for a table that is updated
	   while it runs. However, because myisamchk does not change the table
	   in describe mode, there is no risk of destroying data.

       o   myisamchk -dv tbl_name

	   Adding -v runs myisamchk in verbose mode so that it produces	more
	   information about the table.	Adding -v a second time	produces even
	   more	information.

       o   myisamchk -eis tbl_name

	   Shows only the most important information from a table. This
	   operation is	slow because it	must read the entire table.

       o   myisamchk -eiv tbl_name

	   This	is like	-eis, but tells	you what is being done.

       The tbl_name argument can be either the name of a MyISAM	table or the
       name of its index file, as described in myisamchk(1). Multiple tbl_name
       arguments can be	given.

       Suppose that a table named person has the following structure. (The
       MAX_ROWS	table option is	included so that in the	example	output from
       myisamchk shown later, some values are smaller and fit the output
       format more easily.)

	   CREATE TABLE	person
	   (
	     id		INT NOT	NULL AUTO_INCREMENT,
	     last_name	VARCHAR(20) NOT	NULL,
	     first_name	VARCHAR(20) NOT	NULL,
	     birth	DATE,
	     death	DATE,
	     PRIMARY KEY (id),
	     INDEX (last_name, first_name),
	     INDEX (birth)
	   ) MAX_ROWS =	1000000	ENGINE=MYISAM;

       Suppose also that the table has these data and index file sizes:

	   -rw-rw----  1 mysql	mysql  9347072 Aug 19 11:47 person.MYD
	   -rw-rw----  1 mysql	mysql  6066176 Aug 19 11:47 person.MYI

       Example of myisamchk -dvv output:

	   MyISAM file:		person
	   Record format:	Packed
	   Character set:	latin1_swedish_ci (8)
	   File-version:	1
	   Creation time:	2009-08-19 16:47:41
	   Recover time:	2009-08-19 16:47:56
	   Status:		checked,analyzed,optimized keys
	   Auto	increment key:		    1  Last value:		  306688
	   Data	records:	       306688  Deleted blocks:		       0
	   Datafile parts:	       306688  Deleted data:		       0
	   Datafile pointer (bytes):	    4  Keyfile pointer (bytes):	       3
	   Datafile length:	      9347072  Keyfile length:		 6066176
	   Max datafile	length:	   4294967294  Max keyfile length:   17179868159
	   Recordlength:		   54
	   table description:
	   Key Start Len Index	 Type		      Rec/key	      Root  Blocksize
	   1   2     4	 unique	 long			    1	     99328	 1024
	   2   6     20	 multip. varchar prefix		  512	   3563520	 1024
	       27    20		 varchar		  512
	   3   48    3	 multip. uint24	NULL	       306688	   6065152	 1024
	   Field Start Length Nullpos Nullbit Type
	   1	 1     1
	   2	 2     4		      no zeros
	   3	 6     21		      varchar
	   4	 27    21		      varchar
	   5	 48    3      1	      1	      no zeros
	   6	 51    3      1	      2	      no zeros

       Explanations for	the types of information myisamchk produces are	given
       here.  "Keyfile"	refers to the index file.  "Record" and	"row" are
       synonymous, as are "field" and "column."

       The initial part	of the table description contains these	values:

       o   MyISAM file

	   Name	of the MyISAM (index) file.

       o   Record format

	   The format used to store table rows.	The preceding examples use
	   Fixed length. Other possible	values are Compressed and Packed.
	   (Packed corresponds to what SHOW TABLE STATUS reports as Dynamic.)

       o   Chararacter set

	   The table default character set.

       o   File-version

	   Version of MyISAM format. Always 1.

       o   Creation time

	   When	the data file was created.

       o   Recover time

	   When	the index/data file was	last reconstructed.

       o   Status

	   Table status	flags. Possible	values are crashed, open, changed,
	   analyzed, optimized keys, and sorted	index pages.

       o   Auto	increment key, Last value

	   The key number associated the table's AUTO_INCREMENT	column,	and
	   the most recently generated value for this column. These fields do
	   not appear if there is no such column.

       o   Data	records

	   The number of rows in the table.

       o   Deleted blocks

	   How many deleted blocks still have reserved space. You can optimize
	   your	table to minimize this space. See Section 7.6.4, "MyISAM Table
	   Optimization".

       o   Datafile parts

	   For dynamic-row format, this	indicates how many data	blocks there
	   are.	For an optimized table without fragmented rows,	this is	the
	   same	as Data	records.

       o   Deleted data

	   How many bytes of unreclaimed deleted data there are. You can
	   optimize your table to minimize this	space. See Section 7.6.4,
	   "MyISAM Table Optimization".

       o   Datafile pointer

	   The size of the data	file pointer, in bytes.	It is usually 2, 3, 4,
	   or 5	bytes. Most tables manage with 2 bytes,	but this cannot	be
	   controlled from MySQL yet. For fixed	tables,	this is	a row address.
	   For dynamic tables, this is a byte address.

       o   Keyfile pointer

	   The size of the index file pointer, in bytes. It is usually 1, 2,
	   or 3	bytes. Most tables manage with 2 bytes,	but this is calculated
	   automatically by MySQL. It is always	a block	address.

       o   Max datafile	length

	   How long the	table data file	can become, in bytes.

       o   Max keyfile length

	   How long the	table index file can become, in	bytes.

       o   Recordlength

	   How much space each row takes, in bytes.

       The table description part of the output	includes a list	of all keys in
       the table. For each key,	myisamchk displays some	low-level information:

       o   Key

	   This	key's number. This value is shown only for the first column of
	   the key. If this value is missing, the line corresponds to the
	   second or later column of a multiple-column key. For	the table
	   shown in the	example, there are two table description lines for the
	   second index. This indicates	that it	is a multiple-part index with
	   two parts.

       o   Start

	   Where in the	row this portion of the	index starts.

       o   Len

	   How long this portion of the	index is. For packed numbers, this
	   should always be the	full length of the column. For strings,	it may
	   be shorter than the full length of the indexed column, because you
	   can index a prefix of a string column. The total length of a
	   multiple-part key is	the sum	of the Len values for all key parts.

       o   Index

	   Whether a key value can exist multiple times	in the index. Possible
	   values are unique or	multip.	 (multiple).

       o   Type

	   What	data type this portion of the index has. This is a MyISAM data
	   type	with the possible values packed, stripped, or empty.

       o   Root

	   Address of the root index block.

       o   Blocksize

	   The size of each index block. By default this is 1024, but the
	   value may be	changed	at compile time	when MySQL is built from
	   source.

       o   Rec/key

	   This	is a statistical value used by the optimizer. It tells how
	   many	rows there are per value for this index. A unique index	always
	   has a value of 1. This may be updated after a table is loaded (or
	   greatly changed) with myisamchk -a. If this is not updated at all,
	   a default value of 30 is given.

       The last	part of	the output provides information	about each column:

       o   Field

	   The column number.

       o   Start

	   The byte position of	the column within table	rows.

       o   Length

	   The length of the column in bytes.

       o   Nullpos, Nullbit

	   For columns that can	be NULL, MyISAM	stores NULL values as a	flag
	   in a	byte. Depending	on how many nullable columns there are,	there
	   can be one or more bytes used for this purpose. The Nullpos and
	   Nullbit values, if nonempty,	indicate which byte and	bit contains
	   that	flag indicating	whether	the column is NULL.

	   The position	and number of bytes used to store NULL flags is	shown
	   in the line for field 1. This is why	there are six Field lines for
	   the person table even though	it has only five columns.

       o   Type

	   The data type. The value may	contain	any of the following
	   descriptors:

	   o   constant

	       All rows	have the same value.

	   o   no endspace

	       Do not store endspace.

	   o   no endspace, not_always

	       Do not store endspace and do not	do endspace compression	for
	       all values.

	   o   no endspace, no empty

	       Do not store endspace. Do not store empty values.

	   o   table-lookup

	       The column was converted	to an ENUM.

	   o   zerofill(N)

	       The most	significant N bytes in the value are always 0 and are
	       not stored.

	   o   no zeros

	       Do not store zeros.

	   o   always zero

	       Zero values are stored using one	bit.

       o   Huff	tree

	   The number of the Huffman tree associated with the column.

       o   Bits

	   The number of bits used in the Huffman tree.

       The Huff	tree and Bits fields are displayed if the table	has been
       compressed with myisampack. See myisampack(1), for an example of	this
       information.

       Example of myisamchk -eiv output:

	   Checking MyISAM file: person
	   Data	records:  306688   Deleted blocks:	 0
	   - check file-size
	   - check record delete-chain
	   No recordlinks
	   - check key delete-chain
	   block_size 1024:
	   - check index reference
	   - check data	record references index: 1
	   Key:	 1:  Keyblocks used:  98%  Packed:    0%  Max levels:  3
	   - check data	record references index: 2
	   Key:	 2:  Keyblocks used:  99%  Packed:   97%  Max levels:  3
	   - check data	record references index: 3
	   Key:	 3:  Keyblocks used:  98%  Packed:  -14%  Max levels:  3
	   Total:    Keyblocks used:  98%  Packed:   89%
	   - check records and index references*** LOTS	OF ROW NUMBERS DELETED ***
	   Records:	       306688  M.recordlength:	     25	 Packed:	    83%
	   Recordspace used:	   97% Empty space:	      2% Blocks/Record:	  1.00
	   Record blocks:      306688  Delete blocks:	      0
	   Record data:	      7934464  Deleted data:	      0
	   Lost	space:	       256512  Linkdata:	1156096
	   User	time 43.08, System time	1.68
	   Maximum resident set	size 0,	Integral resident set size 0
	   Non-physical	pagefaults 0, Physical pagefaults 0, Swaps 0
	   Blocks in 0 out 7, Messages in 0 out	0, Signals 0
	   Voluntary context switches 0, Involuntary context switches 0
	   Maximum memory usage: 1046926 bytes (1023k)

       myisamchk -eiv output includes the following information:

       o   Data	records

	   The number of rows in the table.

       o   Deleted blocks

	   How many deleted blocks still have reserved space. You can optimize
	   your	table to minimize this space. See Section 7.6.4, "MyISAM Table
	   Optimization".

       o   Key

	   The key number.

       o   Keyblocks used

	   What	percentage of the keyblocks are	used. When a table has just
	   been	reorganized with myisamchk, the	values are very	high (very
	   near	theoretical maximum).

       o   Packed

	   MySQL tries to pack key values that have a common suffix. This can
	   only	be used	for indexes on CHAR and	VARCHAR	columns. For long
	   indexed strings that	have similar leftmost parts, this can
	   significantly reduce	the space used.	In the preceding example, the
	   second key is 40 bytes long and a 97% reduction in space is
	   achieved.

       o   Max levels

	   How deep the	B-tree for this	key is.	Large tables with long key
	   values get high values.

       o   Records

	   How many rows are in	the table.

       o   M.recordlength

	   The average row length. This	is the exact row length	for tables
	   with	fixed-length rows, because all rows have the same length.

       o   Packed

	   MySQL strips	spaces from the	end of strings.	The Packed value
	   indicates the percentage of savings achieved	by doing this.

       o   Recordspace used

	   What	percentage of the data file is used.

       o   Empty space

	   What	percentage of the data file is unused.

       o   Blocks/Record

	   Average number of blocks per	row (that is, how many links a
	   fragmented row is composed of). This	is always 1.0 for fixed-format
	   tables. This	value should stay as close to 1.0 as possible. If it
	   gets	too large, you can reorganize the table. See Section 7.6.4,
	   "MyISAM Table Optimization".

       o   Recordblocks

	   How many blocks (links) are used. For fixed-format tables, this is
	   the same as the number of rows.

       o   Deleteblocks

	   How many blocks (links) are deleted.

       o   Recorddata

	   How many bytes in the data file are used.

       o   Deleted data

	   How many bytes in the data file are deleted (unused).

       o   Lost	space

	   If a	row is updated to a shorter length, some space is lost.	This
	   is the sum of all such losses, in bytes.

       o   Linkdata

	   When	the dynamic table format is used, row fragments	are linked
	   with	pointers (4 to 7 bytes each).  Linkdata	is the sum of the
	   amount of storage used by all such pointers.

MYISAMCHK MEMORY USAGE
       Memory allocation is important when you run myisamchk.  myisamchk uses
       no more memory than its memory-related variables	are set	to. If you are
       going to	use myisamchk on very large tables, you	should first decide
       how much	memory you want	it to use. The default is to use only about
       3MB to perform repairs. By using	larger values, you can get myisamchk
       to operate faster. For example, if you have more	than 512MB RAM
       available, you could use	options	such as	these (in addition to any
       other options you might specify):

	   shell> myisamchk --myisam_sort_buffer_size=256M \
		      --key_buffer_size=512M \
		      --read_buffer_size=64M \
		      --write_buffer_size=64M ...

       Using --myisam_sort_buffer_size=16M is probably enough for most cases.

       Be aware	that myisamchk uses temporary files in TMPDIR. If TMPDIR
       points to a memory file system, out of memory errors can	easily occur.
       If this happens,	run myisamchk with the --tmpdir=dir_name option	to
       specify a directory located on a	file system that has more space.

       When performing repair operations, myisamchk also needs a lot of	disk
       space:

       o   Twice the size of the data file (the	original file and a copy).
	   This	space is not needed if you do a	repair with --quick; in	this
	   case, only the index	file is	re-created.  This space	must be
	   available on	the same file system as	the original data file,	as the
	   copy	is created in the same directory as the	original.

       o   Space for the new index file	that replaces the old one. The old
	   index file is truncated at the start	of the repair operation, so
	   you usually ignore this space. This space must be available on the
	   same	file system as the original data file.

       o   When	using --recover	or --sort-recover (but not when	using
	   --safe-recover), you	need space on disk for sorting.	This space is
	   allocated in	the temporary directory	(specified by TMPDIR or
	   --tmpdir=dir_name). The following formula yields the	amount of
	   space required:

	       (largest_key + row_pointer_length) * number_of_rows * 2

	   You can check the length of the keys	and the	row_pointer_length
	   with	myisamchk -dv tbl_name (see the	section	called "OBTAINING
	   TABLE INFORMATION WITH MYISAMCHK"). The row_pointer_length and
	   number_of_rows values are the Datafile pointer and Data records
	   values in the table description. To determine the largest_key
	   value, check	the Key	lines in the table description.	The Len	column
	   indicates the number	of bytes for each key part. For	a
	   multiple-column index, the key size is the sum of the Len values
	   for all key parts.

       If you have a problem with disk space during repair, you	can try
       --safe-recover instead of --recover.

COPYRIGHT
       Copyright (C) 1997, 2020, Oracle	and/or its affiliates.

       This documentation is free software; you	can redistribute it and/or
       modify it only under the	terms of the GNU General Public	License	as
       published by the	Free Software Foundation; version 2 of the License.

       This documentation is distributed in the	hope that it will be useful,
       but WITHOUT ANY WARRANTY; without even the implied warranty of
       MERCHANTABILITY or FITNESS FOR A	PARTICULAR PURPOSE. See	the GNU
       General Public License for more details.

       You should have received	a copy of the GNU General Public License along
       with the	program; if not, write to the Free Software Foundation,	Inc.,
       51 Franklin Street, Fifth Floor,	Boston,	MA 02110-1301 USA or see
       http://www.gnu.org/licenses/.

SEE ALSO
       For more	information, please refer to the MySQL Reference Manual, which
       may already be installed	locally	and which is also available online at
       http://dev.mysql.com/doc/.

AUTHOR
       Oracle Corporation (http://dev.mysql.com/).

MySQL 5.7			  06/02/2020			  MYISAMCHK(1)

NAME | SYNOPSIS | DESCRIPTION | MYISAMCHK GENERAL OPTIONS | MYISAMCHK CHECK OPTIONS | MYISAMCHK REPAIR OPTIONS | OTHER MYISAMCHK OPTIONS | OBTAINING TABLE INFORMATION WITH MYISAMCHK | MYISAMCHK MEMORY USAGE | COPYRIGHT | SEE ALSO | AUTHOR

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