CREATE RULE table_update AS ON UPDATE TO table 
   DO INSERT INTO table_log ( 
      id, 
      titile, 
      contents, 
      pguser, 
      date_modified 
    ) 
    VALUES ( 
       OLD.id, 
       OLD.title, 
       OLD.description, 
       getpgusername(), 
       'now'::text 
   );