createlang -U postgres -P <password> plpgsql <dbname>

##</code><code>##

CREATE FUNCTION timestamp_row () RETURNS OPAQUE AS '
    BEGIN
        NEW.timestamp := timeofday();
        RETURN NEW;
    END;
' LANGUAGE 'plpgsql';

##</code><code>##

CREATE TRIGGER timestamp_session BEFORE INSERT OR UPDATE ON sessions
  FOR EACH ROW EXECUTE PROCEDURE timestamp_row();

##</code><code>##

ALTER TABLE sessions ADD COLUMN timestamp timestamp;
ALTER TABLE sessions ALTER COLUMN timestamp SET DEFAULT CURRENT_TIMESTAMP;