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DECLARE(7)		PostgreSQL 9.6.3 Documentation		    DECLARE(7)

NAME
       DECLARE - define	a cursor

SYNOPSIS
       DECLARE name [ BINARY ] [ INSENSITIVE ] [ [ NO ]	SCROLL ]
	   CURSOR [ { WITH | WITHOUT } HOLD ] FOR query

DESCRIPTION
       DECLARE allows a	user to	create cursors,	which can be used to retrieve
       a small number of rows at a time	out of a larger	query. After the
       cursor is created, rows are fetched from	it using FETCH(7).

	   Note
	   This	page describes usage of	cursors	at the SQL command level. If
	   you are trying to use cursors inside	a PL/pgSQL function, the rules
	   are different -- see	Section	41.7, "Cursors", in the	documentation.

PARAMETERS
       name
	   The name of the cursor to be	created.

       BINARY
	   Causes the cursor to	return data in binary rather than in text
	   format.

       INSENSITIVE
	   Indicates that data retrieved from the cursor should	be unaffected
	   by updates to the table(s) underlying the cursor that occur after
	   the cursor is created. In PostgreSQL, this is the default behavior;
	   so this key word has	no effect and is only accepted for
	   compatibility with the SQL standard.

       SCROLL
       NO SCROLL
	   SCROLL specifies that the cursor can	be used	to retrieve rows in a
	   nonsequential fashion (e.g.,	backward). Depending upon the
	   complexity of the query's execution plan, specifying	SCROLL might
	   impose a performance	penalty	on the query's execution time.	NO
	   SCROLL specifies that the cursor cannot be used to retrieve rows in
	   a nonsequential fashion. The	default	is to allow scrolling in some
	   cases; this is not the same as specifying SCROLL. See NOTES for
	   details.

       WITH HOLD
       WITHOUT HOLD
	   WITH	HOLD specifies that the	cursor can continue to be used after
	   the transaction that	created	it successfully	commits.  WITHOUT HOLD
	   specifies that the cursor cannot be used outside of the transaction
	   that	created	it. If neither WITHOUT HOLD nor	WITH HOLD is
	   specified, WITHOUT HOLD is the default.

       query
	   A SELECT(7) or VALUES(7) command which will provide the rows	to be
	   returned by the cursor.

       The key words BINARY, INSENSITIVE, and SCROLL can appear	in any order.

NOTES
       Normal cursors return data in text format, the same as a	SELECT would
       produce.	The BINARY option specifies that the cursor should return data
       in binary format. This reduces conversion effort	for both the server
       and client, at the cost of more programmer effort to deal with
       platform-dependent binary data formats. As an example, if a query
       returns a value of one from an integer column, you would	get a string
       of 1 with a default cursor, whereas with	a binary cursor	you would get
       a 4-byte	field containing the internal representation of	the value (in
       big-endian byte order).

       Binary cursors should be	used carefully.	Many applications, including
       psql, are not prepared to handle	binary cursors and expect data to come
       back in the text	format.

	   Note
	   When	the client application uses the	"extended query" protocol to
	   issue a FETCH command, the Bind protocol message specifies whether
	   data	is to be retrieved in text or binary format. This choice
	   overrides the way that the cursor is	defined. The concept of	a
	   binary cursor as such is thus obsolete when using extended query
	   protocol -- any cursor can be treated as either text	or binary.

       Unless WITH HOLD	is specified, the cursor created by this command can
       only be used within the current transaction. Thus, DECLARE without WITH
       HOLD is useless outside a transaction block: the	cursor would survive
       only to the completion of the statement.	Therefore PostgreSQL reports
       an error	if such	a command is used outside a transaction	block. Use
       BEGIN(7)	and COMMIT(7) (or ROLLBACK(7)) to define a transaction block.

       If WITH HOLD is specified and the transaction that created the cursor
       successfully commits, the cursor	can continue to	be accessed by
       subsequent transactions in the same session. (But if the	creating
       transaction is aborted, the cursor is removed.) A cursor	created	with
       WITH HOLD is closed when	an explicit CLOSE command is issued on it, or
       the session ends. In the	current	implementation,	the rows represented
       by a held cursor	are copied into	a temporary file or memory area	so
       that they remain	available for subsequent transactions.

       WITH HOLD may not be specified when the query includes FOR UPDATE or
       FOR SHARE.

       The SCROLL option should	be specified when defining a cursor that will
       be used to fetch	backwards. This	is required by the SQL standard.
       However,	for compatibility with earlier versions, PostgreSQL will allow
       backward	fetches	without	SCROLL,	if the cursor's	query plan is simple
       enough that no extra overhead is	needed to support it. However,
       application developers are advised not to rely on using backward
       fetches from a cursor that has not been created with SCROLL. If NO
       SCROLL is specified, then backward fetches are disallowed in any	case.

       Backward	fetches	are also disallowed when the query includes FOR	UPDATE
       or FOR SHARE; therefore SCROLL may not be specified in this case.

	   Caution
	   Scrollable and WITH HOLD cursors may	give unexpected	results	if
	   they	invoke any volatile functions (see Section 36.6, "Function
	   Volatility Categories", in the documentation). When a previously
	   fetched row is re-fetched, the functions might be re-executed,
	   perhaps leading to results different	from the first time. One
	   workaround for such cases is	to declare the cursor WITH HOLD	and
	   commit the transaction before reading any rows from it. This	will
	   force the entire output of the cursor to be materialized in
	   temporary storage, so that volatile functions are executed exactly
	   once	for each row.

       If the cursor's query includes FOR UPDATE or FOR	SHARE, then returned
       rows are	locked at the time they	are first fetched, in the same way as
       for a regular SELECT(7) command with these options. In addition,	the
       returned	rows will be the most up-to-date versions; therefore these
       options provide the equivalent of what the SQL standard calls a
       "sensitive cursor". (Specifying INSENSITIVE together with FOR UPDATE or
       FOR SHARE is an error.)

	   Caution
	   It is generally recommended to use FOR UPDATE if the	cursor is
	   intended to be used with UPDATE ... WHERE CURRENT OF	or DELETE ...
	   WHERE CURRENT OF. Using FOR UPDATE prevents other sessions from
	   changing the	rows between the time they are fetched and the time
	   they	are updated. Without FOR UPDATE, a subsequent WHERE CURRENT OF
	   command will	have no	effect if the row was changed since the	cursor
	   was created.

	   Another reason to use FOR UPDATE is that without it,	a subsequent
	   WHERE CURRENT OF might fail if the cursor query does	not meet the
	   SQL standard's rules	for being "simply updatable" (in particular,
	   the cursor must reference just one table and	not use	grouping or
	   ORDER BY). Cursors that are not simply updatable might work,	or
	   might not, depending	on plan	choice details;	so in the worst	case,
	   an application might	work in	testing	and then fail in production.

	   The main reason not to use FOR UPDATE with WHERE CURRENT OF is if
	   you need the	cursor to be scrollable, or to be insensitive to the
	   subsequent updates (that is,	continue to show the old data).	If
	   this	is a requirement, pay close heed to the	caveats	shown above.

       The SQL standard	only makes provisions for cursors in embedded SQL. The
       PostgreSQL server does not implement an OPEN statement for cursors; a
       cursor is considered to be open when it is declared. However, ECPG, the
       embedded	SQL preprocessor for PostgreSQL, supports the standard SQL
       cursor conventions, including those involving DECLARE and OPEN
       statements.

       You can see all available cursors by querying the pg_cursors system
       view.

EXAMPLES
       To declare a cursor:

	   DECLARE liahona CURSOR FOR SELECT * FROM films;

       See FETCH(7) for	more examples of cursor	usage.

COMPATIBILITY
       The SQL standard	says that it is	implementation-dependent whether
       cursors are sensitive to	concurrent updates of the underlying data by
       default.	In PostgreSQL, cursors are insensitive by default, and can be
       made sensitive by specifying FOR	UPDATE.	Other products may work
       differently.

       The SQL standard	allows cursors only in embedded	SQL and	in modules.
       PostgreSQL permits cursors to be	used interactively.

       Binary cursors are a PostgreSQL extension.

SEE ALSO
       CLOSE(7), FETCH(7), MOVE(7)

PostgreSQL 9.6.3		     2017			    DECLARE(7)

NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | NOTES | EXAMPLES | COMPATIBILITY | SEE ALSO

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