Rather than executing a whole query at once, it is possible to set
up a cursor that encapsulates the query, and then read
the query result a few rows at a time. One reason for doing this is
to avoid memory overrun when the result contains a large number of
rows. (However, PL/pgSQL users do not normally need
to worry about that, since FOR
loops automatically use a cursor
internally to avoid memory problems.) A more interesting usage is to
return a reference to a cursor that a function has created, allowing the
caller to read the rows. This provides an efficient way to return
large row sets from functions.
All access to cursors in PL/pgSQL goes through
cursor variables, which are always of the special data type
refcursor
. One way to create a cursor variable
is just to declare it as a variable of type refcursor
.
Another way is to use the cursor declaration syntax,
which in general is:
name
[ [ NO ] SCROLL ] CURSOR [ (arguments
) ] FORquery
;
(FOR
can be replaced by IS
for
Oracle compatibility.)
If SCROLL
is specified, the cursor will be capable of
scrolling backward; if NO SCROLL
is specified, backward
fetches will be rejected; if neither specification appears, it is
query-dependent whether backward fetches will be allowed.
arguments
, if specified, is a
comma-separated list of pairs
that define names to be
replaced by parameter values in the given query. The actual
values to substitute for these names will be specified later,
when the cursor is opened.
name
datatype
Some examples:
DECLARE curs1 refcursor; curs2 CURSOR FOR SELECT * FROM tenk1; curs3 CURSOR (key integer) FOR SELECT * FROM tenk1 WHERE unique1 = key;
All three of these variables have the data type refcursor
,
but the first can be used with any query, while the second has
a fully specified query already bound to it, and the last
has a parameterized query bound to it. (key
will be
replaced by an integer parameter value when the cursor is opened.)
The variable curs1
is said to be unbound since it is not bound to
any particular query.
The SCROLL
option cannot be used when the cursor's
query uses FOR UPDATE/SHARE
. Also, it is
best to use NO SCROLL
with a query that involves
volatile functions. The implementation of SCROLL
assumes that re-reading the query's output will give consistent
results, which a volatile function might not do.
Before a cursor can be used to retrieve rows, it must be
opened. (This is the equivalent action to the SQL
command DECLARE CURSOR
.) PL/pgSQL has
three forms of the OPEN
statement, two of which use unbound
cursor variables while the third uses a bound cursor variable.
Bound cursor variables can also be used without explicitly opening the cursor,
via the FOR
statement described in
Section 41.7.4.
OPEN FOR
query
OPENunbound_cursorvar
[ [ NO ] SCROLL ] FORquery
;
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor variable (that is, as a simple
refcursor
variable). The query must be a
SELECT
, or something else that returns rows
(such as EXPLAIN
). The query
is treated in the same way as other SQL commands in
PL/pgSQL: PL/pgSQL
variable names are substituted, and the query plan is cached for
possible reuse. When a PL/pgSQL
variable is substituted into the cursor query, the value that is
substituted is the one it has at the time of the OPEN
;
subsequent changes to the variable will not affect the cursor's
behavior.
The SCROLL
and NO SCROLL
options have the same meanings as for a bound cursor.
An example:
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
OPEN FOR EXECUTE
OPENunbound_cursorvar
[ [ NO ] SCROLL ] FOR EXECUTEquery_string
[ USINGexpression
[, ... ] ];
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor variable (that is, as a simple
refcursor
variable). The query is specified as a string
expression, in the same way as in the EXECUTE
command. As usual, this gives flexibility so the query plan can vary
from one run to the next (see Section 41.11.2),
and it also means that variable substitution is not done on the
command string. As with EXECUTE
, parameter values
can be inserted into the dynamic command via
format()
and USING
.
The SCROLL
and
NO SCROLL
options have the same meanings as for a bound
cursor.
An example:
OPEN curs1 FOR EXECUTE format('SELECT * FROM %I WHERE col1 = $1',tabname) USING keyvalue;
In this example, the table name is inserted into the query via
format()
. The comparison value for col1
is inserted via a USING
parameter, so it needs
no quoting.
OPENbound_cursorvar
[ ( [argument_name
:= ]argument_value
[, ...] ) ];
This form of OPEN
is used to open a cursor
variable whose query was bound to it when it was declared. The
cursor cannot be open already. A list of actual argument value
expressions must appear if and only if the cursor was declared to
take arguments. These values will be substituted in the query.
The query plan for a bound cursor is always considered cacheable;
there is no equivalent of EXECUTE
in this case.
Notice that SCROLL
and NO SCROLL
cannot be
specified in OPEN
, as the cursor's scrolling
behavior was already determined.
Argument values can be passed using either positional
or named notation. In positional
notation, all arguments are specified in order. In named notation,
each argument's name is specified using :=
to
separate it from the argument expression. Similar to calling
functions, described in Section 4.3, it
is also allowed to mix positional and named notation.
Examples (these use the cursor declaration examples above):
OPEN curs2; OPEN curs3(42); OPEN curs3(key := 42);
Because variable substitution is done on a bound cursor's query,
there are really two ways to pass values into the cursor: either
with an explicit argument to OPEN
, or implicitly by
referencing a PL/pgSQL variable in the query.
However, only variables declared before the bound cursor was
declared will be substituted into it. In either case the value to
be passed is determined at the time of the OPEN
.
For example, another way to get the same effect as the
curs3
example above is
DECLARE key integer; curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key; BEGIN key := 42; OPEN curs4;
Once a cursor has been opened, it can be manipulated with the statements described here.
These manipulations need not occur in the same function that
opened the cursor to begin with. You can return a refcursor
value out of a function and let the caller operate on the cursor.
(Internally, a refcursor
value is simply the string name
of a so-called portal containing the active query for the cursor. This name
can be passed around, assigned to other refcursor
variables,
and so on, without disturbing the portal.)
All portals are implicitly closed at transaction end. Therefore
a refcursor
value is usable to reference an open cursor
only until the end of the transaction.
FETCH
FETCH [direction
{ FROM | IN } ]cursor
INTOtarget
;
FETCH
retrieves the next row from the
cursor into a target, which might be a row variable, a record
variable, or a comma-separated list of simple variables, just like
SELECT INTO
. If there is no next row, the
target is set to NULL(s). As with SELECT
INTO
, the special variable FOUND
can
be checked to see whether a row was obtained or not.
The direction
clause can be any of the
variants allowed in the SQL FETCH
command except the ones that can fetch
more than one row; namely, it can be
NEXT
,
PRIOR
,
FIRST
,
LAST
,
ABSOLUTE
count
,
RELATIVE
count
,
FORWARD
, or
BACKWARD
.
Omitting direction
is the same
as specifying NEXT
.
In the forms using a count
,
the count
can be any integer-valued
expression (unlike the SQL FETCH
command,
which only allows an integer constant).
direction
values that require moving
backward are likely to fail unless the cursor was declared or opened
with the SCROLL
option.
cursor
must be the name of a refcursor
variable that references an open cursor portal.
Examples:
FETCH curs1 INTO rowvar; FETCH curs2 INTO foo, bar, baz; FETCH LAST FROM curs3 INTO x, y; FETCH RELATIVE -2 FROM curs4 INTO x;
MOVE
MOVE [direction
{ FROM | IN } ]cursor
;
MOVE
repositions a cursor without retrieving
any data. MOVE
works exactly like the
FETCH
command, except it only repositions the
cursor and does not return the row moved to. As with SELECT
INTO
, the special variable FOUND
can
be checked to see whether there was a next row to move to.
Examples:
MOVE curs1; MOVE LAST FROM curs3; MOVE RELATIVE -2 FROM curs4; MOVE FORWARD 2 FROM curs4;
UPDATE/DELETE WHERE CURRENT OF
UPDATEtable
SET ... WHERE CURRENT OFcursor
; DELETE FROMtable
WHERE CURRENT OFcursor
;
When a cursor is positioned on a table row, that row can be updated
or deleted using the cursor to identify the row. There are
restrictions on what the cursor's query can be (in particular,
no grouping) and it's best to use FOR UPDATE
in the
cursor. For more information see the
DECLARE
reference page.
An example:
UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;
CLOSE
CLOSE cursor
;
CLOSE
closes the portal underlying an open
cursor. This can be used to release resources earlier than end of
transaction, or to free up the cursor variable to be opened again.
An example:
CLOSE curs1;
PL/pgSQL functions can return cursors to the caller. This is useful to return multiple rows or columns, especially with very large result sets. To do this, the function opens the cursor and returns the cursor name to the caller (or simply opens the cursor using a portal name specified by or otherwise known to the caller). The caller can then fetch rows from the cursor. The cursor can be closed by the caller, or it will be closed automatically when the transaction closes.
The portal name used for a cursor can be specified by the
programmer or automatically generated. To specify a portal name,
simply assign a string to the refcursor
variable before
opening it. The string value of the refcursor
variable
will be used by OPEN
as the name of the underlying portal.
However, if the refcursor
variable is null,
OPEN
automatically generates a name that does not
conflict with any existing portal, and assigns it to the
refcursor
variable.
A bound cursor variable is initialized to the string value representing its name, so that the portal name is the same as the cursor variable name, unless the programmer overrides it by assignment before opening the cursor. But an unbound cursor variable defaults to the null value initially, so it will receive an automatically-generated unique name, unless overridden.
The following example shows one way a cursor name can be supplied by the caller:
CREATE TABLE test (col text); INSERT INTO test VALUES ('123'); CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS ' BEGIN OPEN $1 FOR SELECT col FROM test; RETURN $1; END; ' LANGUAGE plpgsql; BEGIN; SELECT reffunc('funccursor'); FETCH ALL IN funccursor; COMMIT;
The following example uses automatic cursor name generation:
CREATE FUNCTION reffunc2() RETURNS refcursor AS ' DECLARE ref refcursor; BEGIN OPEN ref FOR SELECT col FROM test; RETURN ref; END; ' LANGUAGE plpgsql; -- need to be in a transaction to use cursors. BEGIN; SELECT reffunc2(); reffunc2 -------------------- <unnamed cursor 1> (1 row) FETCH ALL IN "<unnamed cursor 1>"; COMMIT;
The following example shows one way to return multiple cursors from a single function:
CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$ BEGIN OPEN $1 FOR SELECT * FROM table_1; RETURN NEXT $1; OPEN $2 FOR SELECT * FROM table_2; RETURN NEXT $2; END; $$ LANGUAGE plpgsql; -- need to be in a transaction to use cursors. BEGIN; SELECT * FROM myfunc('a', 'b'); FETCH ALL FROM a; FETCH ALL FROM b; COMMIT;
There is a variant of the FOR
statement that allows
iterating through the rows returned by a cursor. The syntax is:
[ <<label
>> ] FORrecordvar
INbound_cursorvar
[ ( [argument_name
:= ]argument_value
[, ...] ) ] LOOPstatements
END LOOP [label
];
The cursor variable must have been bound to some query when it was
declared, and it cannot be open already. The
FOR
statement automatically opens the cursor, and it closes
the cursor again when the loop exits. A list of actual argument value
expressions must appear if and only if the cursor was declared to take
arguments. These values will be substituted in the query, in just
the same way as during an OPEN
(see Section 41.7.2.3).
The variable recordvar
is automatically
defined as type record
and exists only inside the loop (any
existing definition of the variable name is ignored within the loop).
Each row returned by the cursor is successively assigned to this
record variable and the loop body is executed.