Record Log Trigger

Hi all,
I am building a database in postgresql and I made a function that returns the
system time and the current user... like this:

CREATE OR REPLACE FUNCTION generate_idx() returns text as
$$
select to_char(now(),'YYYYMMDDHHMISSUSTZ')||CURRENT_USER;
$$ language 'SQL';

CREATE OR REPLACE FUNCTION TG_idxm() RETURNS trigger AS
$$
BEGIN
NEW.idxm = generate_idx();
RETURN NEW;
END;
$$ LANGUAGE plpgsql;

And my all tables have the "idxm" field, its something like a log for the
record, to know Who and When the record have changed. I.e:

CREATE TABLE products(
id serial primary key,
description varchar(50),
...
idxm varchar(100)
);
CREATE TRIGGER TG_products_idxm BEFORE INSERT or UPDATE on products FOR EACH ROW
EXECUTE PROCEDURE TG_idxm();

Okay, it runs fine... but my question is:
Is it right??? In the future (when the database will be bigger with many of
millions records) this functions for each table will depreceate my database
performance???
Is there any other way to build it???

Thank you