EXECUTE FUNCTION process_emp_audit();
CREATE TRIGGER emp_audit_upd
    AFTER UPDATE ON emp
    REFERENCING OLD TABLE AS old_table NEW TABLE AS new_table
    FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
CREATE TRIGGER emp_audit_del
    AFTER DELETE ON emp
    REFERENCING OLD TABLE AS old_table
    FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
</programlisting>
   </example>

</sect2>

  <sect2 id="plpgsql-event-trigger">
   <title>Triggers on Events</title>

   <para>
    <application>PL/pgSQL</application> can be used to define
    <link linkend="event-triggers">event triggers</link>.
    <productname>PostgreSQL</productname> requires that a function that
    is to be called as an event trigger must be declared as a function with
    no arguments and a return type of <literal>event_trigger</literal>.
   </para>

   <para>
    When a <application>PL/pgSQL</application> function is called as an
    event trigger, several special variables are created automatically
    in the top-level block. They are:

   <variablelist>
    <varlistentry id="plpgsql-event-trigger-tg-event">
     <term><varname>TG_EVENT</varname> <type>text</type></term>
     <listitem>
      <para>
       event the trigger is fired for.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="plpgsql-event-trigger-tg-tag">
     <term><varname>TG_TAG</varname> <type>text</type></term>
     <listitem>
      <para>
       command tag for which the trigger is fired.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

   <para>
    <xref linkend="plpgsql-event-trigger-example"/> shows an example of an
    event trigger function in <application>PL/pgSQL</application>.
   </para>

   <example id="plpgsql-event-trigger-example">
    <title>A <application>PL/pgSQL</application> Event Trigger Function</title>

    <para>
     This example trigger simply raises a <literal>NOTICE</literal> message
     each time a supported command is executed.
    </para>

<programlisting>
CREATE OR REPLACE FUNCTION snitch() RETURNS event_trigger AS $$
BEGIN
    RAISE NOTICE 'snitch: % %', tg_event, tg_tag;
END;
$$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER snitch ON ddl_command_start EXECUTE FUNCTION snitch();
</programlisting>
   </example>
  </sect2>

  </sect1>

  <sect1 id="plpgsql-implementation">
   <title><application>PL/pgSQL</application> under the Hood</title>

   <para>
    This section discusses some implementation details that are
    frequently important for <application>PL/pgSQL</application> users to know.
   </para>

  <sect2 id="plpgsql-var-subst">
   <title>Variable Substitution</title>

   <para>
    SQL statements and expressions within a <application>PL/pgSQL</application> function
    can refer to variables and parameters of the function.  Behind the scenes,
    <application>PL/pgSQL</application> substitutes query parameters for such references.
    Query parameters will only be substituted in places where they are
    syntactically permissible.  As an extreme case, consider
    this example of poor programming style:
<programlisting>
INSERT INTO foo (foo) VALUES (foo(foo));
</programlisting>
    The first occurrence of <literal>foo</literal> must syntactically be a table
    name, so it will not be substituted, even if the function has a variable
    named <literal>foo</literal>.  The second occurrence must be the name of a
    column of that table, so it will not be substituted either.  Likewise
    the third occurrence must be a function name, so it also will not be
    substituted for.  Only the last occurrence is a candidate to be a
    reference to a variable of the <application>PL/pgSQL</application>
    function.
   </para>

   <para>
    Another way to understand this is that variable substitution can only
    insert data values into an SQL command; it cannot dynamically change which
    database objects are referenced by the command.  (If you want to do
    that, you must build a command string