CREATE OR REPLACE FUNCTION my_function(int4)
  RETURNS SETOF int8 AS
$BODY$
SELECT
CASE WHEN (some_condition)
     THEN (
         SELECT ... -- some arbitrary select statement (returning row(s) of int8 values)
     )
     ELSE (
         SELECT ... -- some arbitrary select statement (returning row(s) of int8 values)
     )
END
$BODY$
LANGUAGE 'sql' VOLATILE;