A simpler way has been around since PostgreSQL 8.3:
CREATE FUNCTION test()
RETURNS SETOF first_table AS
$func$
BEGIN
RETURN QUERY
SELECT * FROM first_table;
RETURN QUERY
SELECT * FROM second_table; -- has to return same rowtype as first_table!
END
$func$ LANGUAGE plpgsql;
Call:
SELECT * FROM test();
Both result sets are appended to a single set returned from the function.
See the manual for RETURN QUERY
.