DECLARE — define a cursor
DECLAREname
[ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ] CURSOR [ { WITH | WITHOUT } HOLD ] FORquery
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
.
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 45.7.
name
The name of the cursor to be created. This must be different from any other active cursor name in the session.
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.
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.
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.
Scrollable cursors may give unexpected
results if they invoke any volatile functions (see Section 40.7). 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.
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.
The server data structure underlying an open cursor is called a
portal. Portal names are exposed in the
client protocol: a client can fetch rows directly from an open
portal, if it knows the portal name. When creating a cursor with
DECLARE
, the portal name is the same as the
cursor name.
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 for more examples of cursor usage.
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.