individual row(s) modified by the statement.  But an <literal>AFTER
    STATEMENT</literal> trigger can request that <firstterm>transition tables</firstterm>
    be created to make the sets of affected rows available to the trigger.
    <literal>AFTER ROW</literal> triggers can also request transition tables, so
    that they can see the total changes in the table as well as the change in
    the individual row they are currently being fired for.  The method for
    examining the transition tables again depends on the programming language
    that is being used, but the typical approach is to make the transition
    tables act like read-only temporary tables that can be accessed by SQL
    commands issued within the trigger function.
   </para>

  </sect1>

  <sect1 id="trigger-datachanges">
   <title>Visibility of Data Changes</title>

   <para>
    If you execute SQL commands in your trigger function, and these
    commands access the table that the trigger is for, then
    you need to be aware of the data visibility rules, because they determine
    whether these SQL commands will see the data change that the trigger
    is fired for.  Briefly:

    <itemizedlist>

     <listitem>
      <para>
       Statement-level triggers follow simple visibility rules: none of
       the changes made by a statement are visible to statement-level
       <literal>BEFORE</literal> triggers, whereas all
       modifications are visible to statement-level <literal>AFTER</literal>
       triggers.
      </para>
     </listitem>

     <listitem>
      <para>
       The data change (insertion, update, or deletion) causing the
       trigger to fire is naturally <emphasis>not</emphasis> visible
       to SQL commands executed in a row-level <literal>BEFORE</literal> trigger,
       because it hasn't happened yet.
      </para>
     </listitem>

     <listitem>
      <para>
       However, SQL commands executed in a row-level <literal>BEFORE</literal>
       trigger <emphasis>will</emphasis> see the effects of data
       changes for rows previously processed in the same outer
       command.  This requires caution, since the ordering of these
       change events is not in general predictable; an SQL command that
       affects multiple rows can visit the rows in any order.
      </para>
     </listitem>

     <listitem>
      <para>
       Similarly, a row-level <literal>INSTEAD OF</literal> trigger will see the
       effects of data changes made by previous firings of <literal>INSTEAD
       OF</literal> triggers in the same outer command.
      </para>
     </listitem>

     <listitem>
      <para>
       When a row-level <literal>AFTER</literal> trigger is fired, all data
       changes made
       by the outer command are already complete, and are visible to
       the invoked trigger function.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    If your trigger function is written in any of the standard procedural
    languages, then the above statements apply only if the function is
    declared <literal>VOLATILE</literal>.  Functions that are declared
    <literal>STABLE</literal> or <literal>IMMUTABLE</literal> will not see changes made by
    the calling command in any case.
   </para>

   <para>
    Further information about data visibility rules can be found in
    <xref linkend="spi-visibility"/>.  The example in <xref
    linkend="trigger-example"/> contains a demonstration of these rules.
   </para>
  </sect1>

  <sect1 id="trigger-interface">
   <title>Writing Trigger Functions in C</title>

   <indexterm zone="trigger-interface">
    <primary>trigger</primary>
    <secondary>in C</secondary>
   </indexterm>

   <indexterm>
    <primary>transition tables</primary>
    <secondary>referencing from C trigger</secondary>
   </indexterm>

   <para>
    This section describes the low-level details of the interface to a
    trigger function.  This information is only needed when writing
    trigger functions in