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 C.  If you are using a higher-level language then
    these details are handled for you.  In most cases you should consider
    using a procedural language before writing your triggers in C.  The
    documentation of each procedural language explains how to write a
    trigger in that language.
   </para>

   <para>
    Trigger functions must use the <quote>version 1</quote> function manager
    interface.
   </para>

   <para>
    When a function is called by the trigger manager, it is not passed
    any normal arguments, but it is passed a <quote>context</quote>
    pointer pointing to a <structname>TriggerData</structname> structure.  C
    functions can check whether they were called from the trigger
    manager or not by executing the macro:
<programlisting>
CALLED_AS_TRIGGER(fcinfo)
</programlisting>
    which expands to:
<programlisting>
((fcinfo)-&gt;context != NULL &amp;&amp; IsA((fcinfo)-&gt;context, TriggerData))
</programlisting>
    If this returns true, then it is safe to cast
    <literal>fcinfo-&gt;context</literal> to type <literal>TriggerData
    *</literal> and make use of the pointed-to
    <structname>TriggerData</structname> structure.  The function must
    <emphasis>not</emphasis> alter the <structname>TriggerData</structname>
    structure or any of the data it points to.
   </para>

   <para>
    <structname>struct TriggerData</structname> is defined in
    <filename>commands/trigger.h</filename>:

<programlisting>
typedef struct TriggerData
{
    NodeTag          type;
    TriggerEvent     tg_event;
    Relation         tg_relation;
    HeapTuple        tg_trigtuple;
    HeapTuple        tg_newtuple;
    Trigger         *tg_trigger;
    TupleTableSlot  *tg_trigslot;
    TupleTableSlot  *tg_newslot;
    Tuplestorestate *tg_oldtable;
    Tuplestorestate *tg_newtable;
    const Bitmapset *tg_updatedcols;
} TriggerData;
</programlisting>

    where the members are defined as follows:

    <variablelist>
     <varlistentry>
      <term><structfield>type</structfield></term>
      <listitem>
       <para>
        Always <literal>T_TriggerData</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_event</structfield></term>
      <listitem>
       <para>
        Describes the event for which the function is called. You can use the
        following macros to examine <literal>tg_event</literal>:

        <variablelist>
         <varlistentry>
          <term><literal>TRIGGER_FIRED_BEFORE(tg_event)</literal></term>
          <listitem>
           <para>
            Returns true if the trigger fired before the operation.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>TRIGGER_FIRED_AFTER(tg_event)</literal></term>
          <listitem>
           <para>
            Returns true if the trigger fired after the operation.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>