Potsgresql Immutable

gistfile1.txt

CREATE OR REPLACE FUNCTION test_immutable( a int )
 RETURNS varchar
 LANGUAGE plpgsql
 IMMUTABLE 
AS $function$
BEGIN
  raise notice 'plouf';
  return 'toto';
END
$function$
;

CREATE TABLE test (a varchar);
INSERT INTO test values ('a') ;
INSERT INTO test values ('a') ;
INSERT INTO test values ('a') ;
INSERT INTO test values ('a') ;
PREPARE test (int) as select * from test 
where a = test_immutable($1);
 EXECUTE test(1);