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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>39.1. Overview of Event Trigger Behavior</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="event-triggers.html" title="Chapter 39. Event Triggers" /><link rel="next" href="event-trigger-matrix.html" title="39.2. Event Trigger Firing Matrix" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">39.1. Overview of Event Trigger Behavior</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="event-triggers.html" title="Chapter 39. Event Triggers">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="event-triggers.html" title="Chapter 39. Event Triggers">Up</a></td><th width="60%" align="center">Chapter 39. Event Triggers</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="event-trigger-matrix.html" title="39.2. Event Trigger Firing Matrix">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="EVENT-TRIGGER-DEFINITION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">39.1. Overview of Event Trigger Behavior</h2></div></div></div><p>

  3.      An event trigger fires whenever the event with which it is associated

  4.      occurs in the database in which it is defined. Currently, the only

  5.      supported events are

  6.      <code class="literal">ddl_command_start</code>,

  7.      <code class="literal">ddl_command_end</code>,

  8.      <code class="literal">table_rewrite</code>

  9.      and <code class="literal">sql_drop</code>.

  10.      Support for additional events may be added in future releases.

  11.    </p><p>

  12.      The <code class="literal">ddl_command_start</code> event occurs just before the

  13.      execution of a <code class="literal">CREATE</code>, <code class="literal">ALTER</code>, <code class="literal">DROP</code>,

  14.      <code class="literal">SECURITY LABEL</code>,

  15.      <code class="literal">COMMENT</code>, <code class="literal">GRANT</code> or <code class="literal">REVOKE</code>

  16.      command.  No check whether the affected object exists or doesn't exist is

  17.      performed before the event trigger fires.

  18.      As an exception, however, this event does not occur for

  19.      DDL commands targeting shared objects — databases, roles, and tablespaces

  20.      — or for commands targeting event triggers themselves.  The event trigger

  21.      mechanism does not support these object types.

  22.      <code class="literal">ddl_command_start</code> also occurs just before the execution of a

  23.      <code class="literal">SELECT INTO</code> command, since this is equivalent to

  24.      <code class="literal">CREATE TABLE AS</code>.

  25.    </p><p>

  26.     The <code class="literal">ddl_command_end</code> event occurs just after the execution of

  27.     this same set of commands.  To obtain more details on the <acronym class="acronym">DDL</acronym>

  28.     operations that took place, use the set-returning function

  29.     <code class="literal">pg_event_trigger_ddl_commands()</code> from the

  30.     <code class="literal">ddl_command_end</code> event trigger code (see

  31.     <a class="xref" href="functions-event-triggers.html" title="9.28. Event Trigger Functions">Section 9.28</a>).  Note that the trigger fires

  32.     after the actions have taken place (but before the transaction commits),

  33.     and thus the system catalogs can be read as already changed.

  34.    </p><p>

  35.     The <code class="literal">sql_drop</code> event occurs just before the

  36.     <code class="literal">ddl_command_end</code> event trigger for any operation that drops

  37.     database objects.  To list the objects that have been dropped, use the

  38.     set-returning function <code class="literal">pg_event_trigger_dropped_objects()</code> from the

  39.     <code class="literal">sql_drop</code> event trigger code (see

  40.     <a class="xref" href="functions-event-triggers.html" title="9.28. Event Trigger Functions">Section 9.28</a>). Note that

  41.     the trigger is executed after the objects have been deleted from the

  42.     system catalogs, so it's not possible to look them up anymore.

  43.    </p><p>

  44.     The <code class="literal">table_rewrite</code> event occurs just before a table is

  45.     rewritten by some actions of the commands <code class="literal">ALTER TABLE</code> and

  46.     <code class="literal">ALTER TYPE</code>.  While other

  47.     control statements are available to rewrite a table,

  48.     like <code class="literal">CLUSTER</code> and <code class="literal">VACUUM</code>,

  49.     the <code class="literal">table_rewrite</code> event is not triggered by them.

  50.    </p><p>

  51.      Event triggers (like other functions) cannot be executed in an aborted

  52.      transaction.  Thus, if a DDL command fails with an error, any associated

  53.      <code class="literal">ddl_command_end</code> triggers will not be executed.  Conversely,

  54.      if a <code class="literal">ddl_command_start</code> trigger fails with an error, no

  55.      further event triggers will fire, and no attempt will be made to execute

  56.      the command itself.  Similarly, if a <code class="literal">ddl_command_end</code> trigger

  57.      fails with an error, the effects of the DDL statement will be rolled

  58.      back, just as they would be in any other case where the containing

  59.      transaction aborts.

  60.    </p><p>

  61.      For a complete list of commands supported by the event trigger mechanism,

  62.      see <a class="xref" href="event-trigger-matrix.html" title="39.2. Event Trigger Firing Matrix">Section 39.2</a>.

  63.    </p><p>

  64.      Event triggers are created using the command <a class="xref" href="sql-createeventtrigger.html" title="CREATE EVENT TRIGGER"><span class="refentrytitle">CREATE EVENT TRIGGER</span></a>.

  65.      In order to create an event trigger, you must first create a function with

  66.      the special return type <code class="literal">event_trigger</code>.  This function

  67.      need not (and may not) return a value; the return type serves merely as

  68.      a signal that the function is to be invoked as an event trigger.

  69.    </p><p>

  70.      If more than one event trigger is defined for a particular event, they will

  71.      fire in alphabetical order by trigger name.

  72.    </p><p>

  73.      A trigger definition can also specify a <code class="literal">WHEN</code>

  74.      condition so that, for example, a <code class="literal">ddl_command_start</code>

  75.      trigger can be fired only for particular commands which the user wishes

  76.      to intercept. A common use of such triggers is to restrict the range of

  77.      DDL operations which users may perform.

  78.    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="event-triggers.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="event-triggers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="event-trigger-matrix.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 39. Event Triggers </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 39.2. Event Trigger Firing Matrix</td></tr></table></div></body></html>
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