DECLARE(7)                                                SQL Commands                                                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 [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 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 cur-
              sor 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). Depend-
              ing 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
              [declare(7)] 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 nei-
              ther WITHOUT HOLD nor WITH HOLD is specified, WITHOUT HOLD is the default.

       query  A SELECT [select(7)] or VALUES [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 cur-
       sor 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
       [begin(7)] and COMMIT [commit(7)] (or ROLLBACK [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  speci-
       fied in this case.

              Caution:  Scrollable  and WITH HOLD cursors may give unexpected results if they invoke any volatile functions (see
              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 [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 pro-
              duction.

              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 preproces-
       sor 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 [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 [close(7)], FETCH [fetch(7)], MOVE [move(7)]



SQL - Language Statements                                  2011-09-22                                                 DECLARE(7)