CREATE TABLE _t
(
  id integer NOT NULL,
  center point,
  CONSTRAINT _t_pkey PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);


CREATE OR REPLACE FUNCTION t_to_queue()
  RETURNS trigger AS
$BODY$
BEGIN	
              
        IF (tg_op='UPDATE') THEN
                IF (old<>new) THEN
			RAISE NOTICE '!';
                end if;
	END IF;
	RETURN NEW;
end;
$BODY$
 LANGUAGE plpgsql ;



CREATE TRIGGER t__t_a100_event
  AFTER  UPDATE
  ON _t
  FOR EACH ROW
  EXECUTE PROCEDURE t_to_queue();

INSERT INTO _t (id,center) VALUES (1,'(61.379879,55.164235)');

---------------
--на этом запросе будет ошибка
UPDATE  _t
set id=id;
---------------
--подчищаем
DROP table _t;
DROP FUNCTION t_to_queue();