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DECLARE() SQL Commands DECLARE()
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)].
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 Post-
greSQL, 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 con-
version 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 repre-
sentation of the value (in big-endian byte order).
Binary cursors should be used carefully. Many applications, including psql, are not pre-
pared 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 Post-
greSQL reports an error if such a command is used outside a transaction block. Use BEGIN
[begin(7)], COMMIT [commit(7)] and ROLLBACK [rollback(7)] to define a transaction block.
If WITH HOLD is specified and the transaction that created the cursor successfully com-
mits, the cursor can continue to be accessed by subsequent transactions in the same ses-
sion. (But if the creating transaction is aborted, the cursor is removed.) A cursor cre-
ated with WITH HOLD is closed when an explicit CLOSE command is issued on it, or the ses-
sion 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 transac-
tions.
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.
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)] com-
mand with these options. In addition, the returned rows will be the most up-to-date ver-
sions; therefore these options provide the equivalent of what the SQL standard calls a
``sensitive cursor''. It is often wise to use FOR UPDATE if the cursor is intended to be
used with UPDATE ... WHERE CURRENT OF or DELETE ... WHERE CURRENT OF, since this will pre-
vent 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 meanwhile.
SCROLL may not be specified when the query includes FOR UPDATE or FOR SHARE.
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 insensi-
tive 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(l)], MOVE [move(l)]
SQL - Language Statements 2011-09-22 DECLARE()
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