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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
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+<!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>40.1. 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 Vsnapshot" /><link rel="prev" href="event-triggers.html" title="Chapter 40. Event Triggers" /><link rel="next" href="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">40.1. Overview of Event Trigger Behavior</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="event-triggers.html" title="Chapter 40. Event Triggers">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="event-triggers.html" title="Chapter 40. Event Triggers">Up</a></td><th width="60%" align="center">Chapter 40. Event Triggers</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix">Next</a></td></tr></table><hr /></div><div class="sect1" id="EVENT-TRIGGER-DEFINITION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">40.1. Overview of Event Trigger Behavior</h2></div></div></div><p>
+ 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
+ <code class="literal">ddl_command_start</code>,
+ <code class="literal">ddl_command_end</code>,
+ <code class="literal">table_rewrite</code>
+ and <code class="literal">sql_drop</code>.
+ Support for additional events may be added in future releases.
+ </p><p>
+ The <code class="literal">ddl_command_start</code> event occurs just before the
+ execution of a <code class="literal">CREATE</code>, <code class="literal">ALTER</code>, <code class="literal">DROP</code>,
+ <code class="literal">SECURITY LABEL</code>,
+ <code class="literal">COMMENT</code>, <code class="literal">GRANT</code> or <code class="literal">REVOKE</code>
+ 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 — databases, roles, and tablespaces
+ — or for commands targeting event triggers themselves. The event trigger
+ mechanism does not support these object types.
+ <code class="literal">ddl_command_start</code> also occurs just before the execution of a
+ <code class="literal">SELECT INTO</code> command, since this is equivalent to
+ <code class="literal">CREATE TABLE AS</code>.
+ </p><p>
+ The <code class="literal">ddl_command_end</code> event occurs just after the execution of
+ this same set of commands. To obtain more details on the <acronym class="acronym">DDL</acronym>
+ operations that took place, use the set-returning function
+ <code class="literal">pg_event_trigger_ddl_commands()</code> from the
+ <code class="literal">ddl_command_end</code> event trigger code (see
+ <a class="xref" href="functions-event-triggers.html" title="9.29. Event Trigger Functions">Section 9.29</a>). Note that the trigger fires
+ after the actions have taken place (but before the transaction commits),
+ and thus the system catalogs can be read as already changed.
+ </p><p>
+ The <code class="literal">sql_drop</code> event occurs just before the
+ <code class="literal">ddl_command_end</code> event trigger for any operation that drops
+ database objects. To list the objects that have been dropped, use the
+ set-returning function <code class="literal">pg_event_trigger_dropped_objects()</code> from the
+ <code class="literal">sql_drop</code> event trigger code (see
+ <a class="xref" href="functions-event-triggers.html" title="9.29. Event Trigger Functions">Section 9.29</a>). 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.
+ </p><p>
+ The <code class="literal">table_rewrite</code> event occurs just before a table is
+ rewritten by some actions of the commands <code class="literal">ALTER TABLE</code> and
+ <code class="literal">ALTER TYPE</code>. While other
+ control statements are available to rewrite a table,
+ like <code class="literal">CLUSTER</code> and <code class="literal">VACUUM</code>,
+ the <code class="literal">table_rewrite</code> event is not triggered by them.
+ </p><p>
+ Event triggers (like other functions) cannot be executed in an aborted
+ transaction. Thus, if a DDL command fails with an error, any associated
+ <code class="literal">ddl_command_end</code> triggers will not be executed. Conversely,
+ if a <code class="literal">ddl_command_start</code> 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 <code class="literal">ddl_command_end</code> 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.
+ </p><p>
+ For a complete list of commands supported by the event trigger mechanism,
+ see <a class="xref" href="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix">Section 40.2</a>.
+ </p><p>
+ 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>.
+ In order to create an event trigger, you must first create a function with
+ the special return type <code class="literal">event_trigger</code>. 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.
+ </p><p>
+ If more than one event trigger is defined for a particular event, they will
+ fire in alphabetical order by trigger name.
+ </p><p>
+ A trigger definition can also specify a <code class="literal">WHEN</code>
+ condition so that, for example, a <code class="literal">ddl_command_start</code>
+ 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.
+ </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" title="Chapter 40. Event Triggers">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="event-triggers.html" title="Chapter 40. Event Triggers">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="event-trigger-matrix.html" title="40.2. Event Trigger Firing Matrix">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 40. Event Triggers </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 40.2. Event Trigger Firing Matrix</td></tr></table></div></body></html> \ No newline at end of file