CREATE TABLE tbl_employees (
    id serial NOT NULL,
    first_name character varying NOT NULL,
    last_name character varying NOT NULL
)

CREATE TABLE tbl_employees (
    id serial NOT NULL,
    employee_id NOT NULL,
    first_name character varying NOT NULL,
    last_name character varying NOT NULL,
    changed_on timestamp NOT NULL
)

CREATE OR REPLACE FUNCTION log_employee_changes() RETURNS trigger as
$BODY$
BEGIN
     IF (NEW.last_name <> OLD.last_name) or (NEW.first_name <> OLD.first_name) THEN
     INSERT INTO tbl_employee_history(employee_id,last_name, first_name,changed_on)
     VALUES(OLD.id,OLD.last_name, OLD.first_name,now());
     END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;

CREATE TRIGGER add_log
BEFORE UPDATE ON tbl_employees
FOR EACH ROW
EXECUTE PROCEDURE log_employee_changes();

CREATE OR REPLACE FUNCTION log_employee_changes() RETURNS trigger as
$BODY$
BEGIN
     IF ?????there is one change on whatever column????? THEN
     INSERT INTO tbl_employee_history(employee_id,last_name, first_name,changed_on)
     VALUES(OLD.id,OLD.last_name, OLD.first_name,now());
     END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;

if new <> old then ...

if new is distinct from old then ...

CREATE OR REPLACE FUNCTION log_employee_changes() RETURNS trigger as
$BODY$
BEGIN
     IF OLD!=NEW THEN
     INSERT INTO tbl_employee_history(employee_id,last_name, first_name,changed_on)
     VALUES(OLD.id,OLD.last_name, OLD.first_name,now());
     END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;