Man Pages

declare(7) - phpMan declare(7) - phpMan

Command: man perldoc info search(apropos)  

DECLARE(7)                       SQL Commands                       DECLARE(7)

       DECLARE - define a cursor

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

       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.

       name   The name of the cursor to be created.

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

              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.


       NO SCROLL
              SCROLL  specifies  that  the cursor can be used to retrieve rows in a nonsequential fashion (e.g., back-
              ward). 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

              WITH  HOLD  specifies that the cursor can continue to be used after the transaction that created it suc-
              cessfully 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

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

       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 transac-
       tion. 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  trans-
       action block.

       If WITH HOLD is specified and the transaction that created the cursor successfully commits, the cursor can con-
       tinue 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  func-
              tions  (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  func-
              tions 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.)


              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 applica-
              tion 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.

       To declare a cursor:

       DECLARE liahona CURSOR FOR SELECT * FROM films;

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

       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

       Binary cursors are a PostgreSQL extension.

       CLOSE [close(7)], FETCH [fetch(7)], MOVE [move(7)]

SQL - Language Statements         2014-02-17                        DECLARE(7)