Thank you all for the valuable replies. I've also got suggestions to use IS NOT DISTINCT FROM or STRICT at http://stackoverflow.com/questions/35742865/how-to-ensure-that-a-stored-function-always-returns-true-or-false
but the former has the edge case of NULL=NULL returning TRUE and with the latter I would have to be careful with the way I call my function - and I am worried I might forget it later and this is a security related... So I will probably use this function: CREATE OR REPLACE FUNCTION check_user(in_social integer, in_sid varchar(255), in_auth varchar(32)) RETURNS boolean AS $func$ SELECT CASE WHEN in_social IS NULL THEN FALSE WHEN in_sid IS NULL THEN FALSE WHEN in_auth IS NULL THEN FALSE ELSE (MD5('secret word' || in_social || in_sid) = in_auth) END; $func$ LANGUAGE sql IMMUTABLE; Regards Alex