CREATE FUNCTION asset_id_ins_upd () RETURNS OPAQUE AS '
 DECLARE
     found INTEGER;
 BEGIN
     IF TG_OP = ''UPDATE'' THEN
         -- Return OK for unchanging asset_id on
         -- update operations
         IF NEW.asset_id = OLD.asset_id THEN
             RETURN NEW;
         END IF;
     END IF;

     SELECT 1 INTO found
         FROM  assets
         WHERE assets.asset_id = NEW.asset_id;
     IF FOUND THEN
         RAISE EXCEPTION 
     END IF;
 END;
 ' LANGUAGE 'plpgsql' WITH (isstrict);
 
 CREATE TRIGGER assets_asset_id_ins_upd BEFORE INSERT OR UPDATE ON assets
     FOR EACH ROW EXECUTE PROCEDURE asset_id_ins_upd();