A PostgreSQL trigger function to updated a timestamp column whenever a row is updated.

trigger_update.sql

CREATE OR REPLACE FUNCTION trigger_updated RETURNS TRIGGER LANGUAGE plpgsql AS $$
BEGIN
	NEW.updated = NOW();
	RETURN NEW;
END;
$$

CREATE TRIGGER updated BEFORE UPDATE ON payment 
OR EACH ROW WHEN (OLD.* IS DISTINCT FROM NEW.*) 
EXECUTE PROCEDURE trigger_updated();