<!-- doc/src/sgml/event-trigger.sgml -->

 <chapter id="event-triggers">
  <title>Event Triggers</title>

  <indexterm zone="event-triggers">
   <primary>event trigger</primary>
  </indexterm>

  <para>
   To supplement the trigger mechanism discussed in <xref linkend="triggers">,
   <productname>PostgreSQL</> also provides event triggers.  Unlike regular
   triggers, which are attached to a single table and capture only DML events,
   event triggers are global to a particular database and are capable of
   capturing DDL events.
  </para>

  <para>
   Like regular triggers, event triggers can be written in any procedural
   language that includes event trigger support, or in C, but not in plain
   SQL.
  </para>

  <sect1 id="event-trigger-definition">
   <title>Overview of Event Trigger Behavior</title>

   <para>
     An event trigger fires whenever the event with which it is associated
     occurs in the database in which it is defined. Currently, the only
     supported events are
     <literal>ddl_command_start</>,
     <literal>ddl_command_end</>
     and <literal>sql_drop</>.
     Support for additional events may be added in future releases.
   </para>

   <para>
     The <literal>ddl_command_start</> event occurs just before the
     execution of a <literal>CREATE</>, <literal>ALTER</>, or <literal>DROP</>
     command.  No check whether the affected object exists or doesn't exist is
     performed before the event trigger fires.
     As an exception, however, this event does not occur for
     DDL commands targeting shared objects &mdash; databases, roles, and tablespaces
     &mdash; or for commands targeting event triggers themselves.  The event trigger
     mechanism does not support these object types.
     <literal>ddl_command_start</> also occurs just before the execution of a
     <literal>SELECT INTO</literal> command, since this is equivalent to
     <literal>CREATE TABLE AS</literal>.
   </para>

   <para>
    The <literal>ddl_command_end</> event occurs just after the execution of
    this same set of commands.
   </para>

   <para>
    The <literal>sql_drop</> event occurs just before the
    <literal>ddl_command_end</> event trigger for any operation that drops
    database objects.  To list the objects that have been dropped, use the
    set-returning function <literal>pg_event_trigger_dropped_objects()</> from the
    <literal>sql_drop</> event trigger code (see
    <xref linkend="functions-event-triggers">). Note that
    the trigger is executed after the objects have been deleted from the
    system catalogs, so it's not possible to look them up anymore.
   </para>

   <para>
     Event triggers (like other functions) cannot be executed in an aborted
     transaction.  Thus, if a DDL command fails with an error, any associated
     <literal>ddl_command_end</> triggers will not be executed.  Conversely,
     if a <literal>ddl_command_start</> trigger fails with an error, no
     further event triggers will fire, and no attempt will be made to execute
     the command itself.  Similarly, if a <literal>ddl_command_end</> trigger
     fails with an error, the effects of the DDL statement will be rolled
     back, just as they would be in any other case where the containing
     transaction aborts.
   </para>

   <para>
     For a complete list of commands supported by the event trigger mechanism,
     see <xref linkend="event-trigger-matrix">.
   </para>

   <para>
     Event triggers are created using the command <xref linkend="sql-createeventtrigger">.
     In order to create an event trigger, you must first create a function with
     the special return type <literal>event_trigger</literal>.  This function
     need not (and may not) return a value; the return type serves merely as
     a signal that the function is to be invoked as an event trigger.
   </para>

   <para>
     If more than one event trigger is defined for a particular event, they will
     fire in alphabetical order by trigger name.
   </para>

   <para>
     A trigger definition can also specify a <literal>WHEN</literal>
     condition so that, for example, a <literal>ddl_command_start</literal>
     trigger can be fired only for particular commands which the user wishes
     to intercept. A common use of such triggers is to restrict the range of
     DDL operations which users may perform.
   </para>
  </sect1>

  <sect1 id="event-trigger-matrix">
   <title>Event Trigger Firing Matrix</title>

   <para>
     <xref linkend="event-trigger-by-command-tag"> lists all commands
     for which event triggers are supported.
   </para>

   <table id="event-trigger-by-command-tag">
     <title>Event Trigger Support by Command Tag</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>command tag</entry>
        <entry><literal>ddl_command_start</literal></entry>
        <entry><literal>ddl_command_end</literal></entry>
        <entry><literal>sql_drop</literal></entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry align="left"><literal>ALTER AGGREGATE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER COLLATION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER CONVERSION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER DOMAIN</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER EXTENSION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER FOREIGN DATA WRAPPER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER FOREIGN TABLE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER FUNCTION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER LANGUAGE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER OPERATOR</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER OPERATOR CLASS</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER OPERATOR FAMILY</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER SCHEMA</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER SEQUENCE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER SERVER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TABLE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TEXT SEARCH CONFIGURATION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TEXT SEARCH DICTIONARY</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TEXT SEARCH PARSER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TEXT SEARCH TEMPLATE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TRIGGER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER TYPE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER USER MAPPING</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>ALTER VIEW</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE AGGREGATE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE CAST</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE COLLATION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE CONVERSION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE DOMAIN</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE EXTENSION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE FOREIGN DATA WRAPPER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE FOREIGN TABLE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE FUNCTION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE INDEX</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE LANGUAGE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE OPERATOR</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE OPERATOR CLASS</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE OPERATOR FAMILY</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE RULE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE SCHEMA</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE SEQUENCE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE SERVER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TABLE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TABLE AS</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TEXT SEARCH CONFIGURATION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TEXT SEARCH DICTIONARY</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TEXT SEARCH PARSER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TEXT SEARCH TEMPLATE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TRIGGER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE TYPE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE USER MAPPING</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>CREATE VIEW</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP AGGREGATE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP CAST</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP COLLATION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP CONVERSION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP DOMAIN</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP EXTENSION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP FOREIGN DATA WRAPPER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP FOREIGN TABLE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP FUNCTION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP INDEX</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP LANGUAGE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP OPERATOR</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP OPERATOR CLASS</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP OPERATOR FAMILY</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP OWNED</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP RULE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP SCHEMA</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP SEQUENCE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP SERVER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TABLE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TEXT SEARCH CONFIGURATION</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TEXT SEARCH DICTIONARY</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TEXT SEARCH PARSER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TEXT SEARCH TEMPLATE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TRIGGER</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP TYPE</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP USER MAPPING</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>DROP VIEW</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
       </row>
       <row>
        <entry align="left"><literal>SELECT INTO</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>X</literal></entry>
        <entry align="center"><literal>-</literal></entry>
       </row>
      </tbody>
     </tgroup>
   </table>
  </sect1>

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

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

   <para>
    This section describes the low-level details of the interface to an
    event trigger function. This information is only needed when writing
    event 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 event triggers
    in C. The documentation of each procedural language explains how to
    write an event trigger in that language.
   </para>

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

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

   <para>
    <structname>struct EventTriggerData</structname> is defined in
    <filename>commands/event_trigger.h</filename>:

<programlisting>
typedef struct EventTriggerData
{
    NodeTag     type;
    const char *event;      /* event name */
    Node       *parsetree;  /* parse tree */
    const char *tag;        /* command tag */
} EventTriggerData;
</programlisting>

    where the members are defined as follows:

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

     <varlistentry>
      <term><structfield>tg_event</></term>
      <listitem>
       <para>
        Describes the event for which the function is called, one of
        <literal>"ddl_command_start"</literal>, <literal>"ddl_command_end"</literal>,
        <literal>"sql_drop"</literal>.
        See <xref linkend="event-trigger-definition"> for the meaning of these
        events.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>parsetree</></term>
      <listitem>
       <para>
        A pointer to the parse tree of the command.  Check the PostgreSQL
        source code for details.  The parse tree structure is subject to change
        without notice.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tag</></term>
      <listitem>
       <para>
        The command tag associated with the event for which the event trigger
        is run, for example <literal>"CREATE FUNCTION"</literal>.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    An event trigger function must return a <symbol>NULL</> pointer
    (<emphasis>not</> an SQL null value, that is, do not
    set <parameter>isNull</parameter> true).
   </para>
  </sect1>

  <sect1 id="event-trigger-example">
   <title>A Complete Event Trigger Example</title>

   <para>
    Here is a very simple example of an event trigger function written in C.
    (Examples of triggers written in procedural languages can be found in
    the documentation of the procedural languages.)
   </para>

   <para>
    The function <function>noddl</> raises an exception each time it is called.
    The event trigger definition associated the function with
    the <literal>ddl_command_start</literal> event.  The effect is that all DDL
    commands (with the exceptions mentioned
    in <xref linkend="event-trigger-definition">) are prevented from running.
   </para>

   <para>
    This is the source code of the trigger function:
<programlisting><![CDATA[
#include "postgres.h"
#include "commands/event_trigger.h"


PG_MODULE_MAGIC;

Datum noddl(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(noddl);

Datum
noddl(PG_FUNCTION_ARGS)
{
    EventTriggerData *trigdata;

    if (!CALLED_AS_EVENT_TRIGGER(fcinfo))  /* internal error */
        elog(ERROR, "not fired by event trigger manager");

    trigdata = (EventTriggerData *) fcinfo->context;

    ereport(ERROR,
        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("command \"%s\" denied", trigdata->tag)));

    PG_RETURN_NULL();
}
]]></programlisting>
   </para>

   <para>
    After you have compiled the source code (see <xref linkend="dfunc">),
    declare the function and the triggers:
<programlisting>
CREATE FUNCTION noddl() RETURNS event_trigger
    AS 'noddl' LANGUAGE C;

CREATE EVENT TRIGGER noddl ON ddl_command_start
    EXECUTE PROCEDURE noddl();
</programlisting>
   </para>

   <para>
    Now you can test the operation of the trigger:
<screen>
=# \dy
                     List of event triggers
 Name  |       Event       | Owner | Enabled | Procedure | Tags 
-------+-------------------+-------+---------+-----------+------
 noddl | ddl_command_start | dim   | enabled | noddl     | 
(1 row)

=# CREATE TABLE foo(id serial);
ERROR:  command "CREATE TABLE" denied
</screen>
   </para>

   <para>
    In this situation, in order to be able to run some DDL commands when you
    need to do so, you have to either drop the event trigger or disable it.  It
    can be convenient to disable the trigger for only the duration of a
    transaction:
<programlisting>
BEGIN;
ALTER EVENT TRIGGER noddl DISABLE;
CREATE TABLE foo (id serial);
ALTER EVENT TRIGGER noddl ENABLE;
COMMIT;
</programlisting>
    (Recall that DDL commands on event triggers themselves are not affected by
    event triggers.)
   </para>
  </sect1>
</chapter>
