xenial (7) DECLARE.7.gz

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 40.7, “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. If FOR UPDATE is specified, the cursor
           is guaranteed to be updatable.

           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)