function volatility

How to declare function that return set of data from tables? for
example :

CREATE TYPE "public"."cst_test" AS (
  "code" VARCHAR(10),
  "name" VARCHAR(50),
  "quantity" NUMERIC(10,2) );

CREATE OR REPLACE FUNCTION public."sf_test"(id_code VARCHAR)
  RETURNS SETOF "public"."cst_test" AS
$body$
SELECT
  t1.code,
  t1.name,
  t2.quantity
FROM "public"."test_1" t1
  INNER JOIN "public"."test_2" t2 ON t1.id_test1 = t2.id_test1
WHERE t1.code = $1
$body$
LANGUAGE 'sql' ;

It's STABLE, IMMUTABLE OR VOLATILE?