declare

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 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 [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 neither 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 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 [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 specified 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 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 [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         2014-02-17                        DECLARE(7)