Selective Auto-Timestamp [using triggers?]

Rajit Singh writes:
> The thing is, I also want to be able to update the timestamp field
> if I want to...

Here's my solution:

CREATE FUNCTION updated_stamp () RETURNS OPAQUE AS
' BEGIN
IF NEW.updated ISNULL THEN
NEW.updated := ''now'';
END IF;
RETURN NEW;
END;
' LANGUAGE 'plpgsql';