DECLARE(7)		PostgreSQL 9.2.24 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 39.7, "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 35.6, "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 9.2.24		  2017-11-06			    DECLARE(7)