检测更新不会更改行的触发器

我写了以下触发器:

CREATE FUNCTION trig_func() RETURNS trigger AS $$
  BEGIN
    IF NEW = OLD
    THEN -- update would do nothing, doing something...
    END IF;
    RETURN NEW;
  END;
$$ LANGUAGE plpgsql;
CREATE TRIGGER trig BEFORE UPDATE ON some_table
  FOR EACH ROW EXECUTE PROCEDURE trig_func();

它清楚地表明了我想要达到的目标,但是什么才是合适的 NEW = OLD ?