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 cur‐
sor 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 unaf‐
fected 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 specify‐
ing 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 plat‐
form-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 for‐
mat. 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)], 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 commits, the cursor can continue to be accessed by subse‐
quent transactions in the same session. (But if the creating transac‐
tion 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. How‐
ever, 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, appli‐
cation 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)] command with these options. In addi‐
tion, 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''. 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 prevent 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 state‐
ments.
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 cur‐
sors 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 dif‐
ferently.
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 2013-02-04 DECLARE()
