Посмотрите в доках 'Server Programming' => 'Extending SQL' => 'Query Language (SQL) Functions' => 'SQL Functions Returning Sets'.

CREATE TABLE foo (fooid int, foosubid int, fooname text);
INSERT INTO foo VALUES (1, 1, 'Joe');
INSERT INTO foo VALUES (1, 2, 'Ed');
INSERT INTO foo VALUES (2, 1, 'Mary');

CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS '
SELECT * FROM foo WHERE fooid = $1;
' LANGUAGE SQL;

SELECT * FROM getfoo(1) AS t1;

fooid | foosubid | fooname
-------+----------+---------
1 | 1 | Joe
1 | 2 | Ed
(2 rows)