ction rule systems
     are conceptually simple, but there are many subtle points
     involved in actually using them.</P
><P
>     Some other database systems define active database rules, which
     are usually stored procedures and triggers.  In
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, these can be implemented
     using functions and triggers as well.</P
><P
>     The rule system (more precisely speaking, the query rewrite rule
     system) is totally different from stored procedures and tri