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35.8. Cursors

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.

35.8.1. Declaring Cursor Variables

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 CURSOR [ ( arguments ) ] FOR query ;

(FOR may be replaced by IS for Oracle compatibility.) arguments, if specified, is a comma-separated list of pairs name datatype 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.

Some examples:

DECLARE
    curs1 refcursor;
    curs2 CURSOR FOR SELECT * FROM tenk1;
    curs3 CURSOR (key integer) IS SELECT * FROM tenk1 WHERE unique1 = key;

All three of these variables have the data type refcursor, but the first may 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.

35.8.2. Opening Cursors

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.

35.8.2.1. OPEN FOR SELECT

OPEN unbound_cursor FOR SELECT ...;

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 (that is, as a simple refcursor variable). The SELECT query is treated in the same way as other SELECT statements in PL/pgSQL: PL/pgSQL variable names are substituted, and the query plan is cached for possible reuse.

An example:

OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;

35.8.2.2. OPEN FOR EXECUTE

OPEN unbound_cursor FOR EXECUTE query_string;

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 (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 can vary from one run to the next.

An example:

OPEN curs1 FOR EXECUTE 'SELECT * FROM ' || quote_ident($1);

35.8.2.3. Opening a Bound Cursor

OPEN bound_cursor [ ( argument_values ) ];

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.

Examples:

OPEN curs2;
OPEN curs3(42);

35.8.3. Using Cursors

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.

35.8.3.1. FETCH

FETCH cursor INTO target;

FETCH retrieves the next row from the cursor into a target, which may be a row variable, a record variable, or a comma-separated list of simple variables, just like SELECT INTO. As with SELECT INTO, the special variable FOUND may be checked to see whether a row was obtained or not.

An example:

FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;

35.8.3.2. 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;

35.8.3.3. Returning Cursors

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.

Note: 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;

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 $$
BEGIN
    OPEN $1 FOR SELECT * FROM table_1;
    RETURN NEXT $1;
    OPEN $2 FOR SELECT * FROM table_2;
    RETURN NEXT $2;
    RETURN;
END;
$$ 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;

Comments


Jan. 19, 2005, 4:12 p.m.

It is a good idea to always close the cursor as soon as you are done with it. Failing to do that can get you in trouble, particularly if you are writing recursive plpgsql functions with refcursors in them. If the recursive routine opens a cursor and does not close it, you can get an error about unnamed cursor x already being open.


May 5, 2005, 1:31 p.m.

Simple cursor example:

CREATE or replace FUNCTION mean_example(var_table varchar, var_col varchar) RETURNS real AS $$
DECLARE
curs1 refcursor;
a real;
b real;
c real;
sum real:=0;
BEGIN
open curs1 FOR EXECUTE 'SELECT ' || quote_ident(var_col) || ' FROM ' || quote_ident(var_table);
b:=0;

LOOP
-- some computations
FETCH curs1 INTO a;

IF NOT FOUND THEN
EXIT; -- exit loop
END IF;
b:=b+1;
c:=a;
sum:= sum + c;
END LOOP;

CLOSE curs1;
RETURN sum / b;

END;
$$ LANGUAGE plpgsql;

select mean_example(tablename, columnName);

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