DECLARE(7) PostgreSQL 15.7 Documentation DECLARE(7)
NAME
DECLARE - define a cursor
SYNOPSIS
DECLARE name [ BINARY ] [ ASENSITIVE | 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.
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 43.7.
PARAMETERS
name
The name of the cursor to be created.
BINARY
Causes the cursor to return data in binary rather than in text
format.
ASENSITIVE
INSENSITIVE
Cursor sensitivity determines whether changes to the data
underlying the cursor, done in the same transaction, after the
cursor has been declared, are visible in the cursor. INSENSITIVE
means they are not visible, ASENSITIVE means the behavior is
implementation-dependent. A third behavior, SENSITIVE, meaning that
such changes are visible in the cursor, is not available in
PostgreSQL. In PostgreSQL, all cursors are insensitive; so these
key words have no effect and are only accepted for compatibility
with the SQL standard.
Specifying INSENSITIVE together with FOR UPDATE or FOR SHARE is an
error.
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 below
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 or VALUES command which will provide the rows to be
returned by the cursor.
The key words ASENSITIVE, 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 and COMMIT (or ROLLBACK) 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 cursors may give unexpected results if they invoke any
volatile functions (see Section 38.7). When a previously fetched
row is re-fetched, the functions might be re-executed, perhaps
leading to results different from the first time. It's best to
specify NO SCROLL for a query involving volatile functions. If that
is not practical, one workaround is to declare the cursor SCROLL
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 command with these options. In addition, the
returned rows will be the most up-to-date versions.
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 isolated from
concurrent 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 allows cursors only in embedded SQL and in modules.
PostgreSQL permits cursors to be used interactively.
According to the SQL standard, changes made to insensitive cursors by
UPDATE ... WHERE CURRENT OF and DELETE ... WHERE CURRENT OF statements
are visible in that same cursor. PostgreSQL treats these statements
like all other data changing statements in that they are not visible in
insensitive cursors.
Binary cursors are a PostgreSQL extension.
SEE ALSO
CLOSE(7), FETCH(7), MOVE(7)
PostgreSQL 15.7 2024 DECLARE(7)
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