<Chapter>
<Title>The <ProductName>Postgres</ProductName> Rule System</Title>

<Para>
     Production  rule  systems  are conceptually simple, but
     there are many subtle points involved in actually using
     them.  Consequently, we will not attempt to explain the
     actual syntax and operation of the <ProductName>Postgres</ProductName> rule system
     here.  Instead, you should read 
<XRef LinkEnd="STON90b" EndTerm="[STON90b]"> to understand
     some of these points and the theoretical foundations of
     the  <ProductName>Postgres</ProductName>  rule  system before trying to use rules.
     The discussion in this section is intended  to  provide
     an  overview  of the <ProductName>Postgres</ProductName> rule system and point the
     user at helpful references and examples.

     The "query rewrite" rule  system  modifies  queries  to
     take rules into consideration, and then passes the modified 
     query to the query optimizer for  execution.   It
     is  very powerful, and can be used for many things such
     as query language procedures, views, and versions.  The
     power  of  this  rule system is discussed in 
<XRef LinkEnd="ONG90" EndTerm="[ONG90]">
 as well as
<XRef LinkEnd="STON90b" EndTerm="[STON90b]">.
</Para>

</Chapter>
