DROP TABLE mdt;

CREATE TABLE mdt (
	id		int4,
	idesc		text,
	moddate timestamp DEFAULT CURRENT_TIMESTAMP NOT NULL
);

CREATE TRIGGER mdt_moddatetime
	BEFORE UPDATE ON mdt
	FOR EACH ROW 
	EXECUTE PROCEDURE moddatetime (moddate);

INSERT INTO mdt VALUES (1, 'first');
INSERT INTO mdt VALUES (2, 'second');
INSERT INTO mdt VALUES (3, 'third');

SELECT * FROM mdt;

UPDATE mdt SET id = 4
	WHERE id = 1;
UPDATE mdt SET id = 5
	WHERE id = 2;
UPDATE mdt SET id = 6
	WHERE id = 3;

SELECT * FROM mdt;