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 compati-
	      bility 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 com-
	      plexity 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 speci-
	      fied, 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 [roll-
       back(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  sub-
       sequent	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 sim-
       ple  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	documenta-
	      tion).  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					    2010-05-14								DECLARE(7)