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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
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+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!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>40.4. A Complete Event Trigger Example</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-trigger-interface.html" title="40.3. Writing Event Trigger Functions in C" /><link rel="next" href="event-trigger-table-rewrite-example.html" title="40.5. A Table Rewrite Event Trigger Example" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">40.4. A Complete Event Trigger Example</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="event-trigger-interface.html" title="40.3. Writing Event Trigger Functions in C">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-table-rewrite-example.html" title="40.5. A Table Rewrite Event Trigger Example">Next</a></td></tr></table><hr /></div><div class="sect1" id="EVENT-TRIGGER-EXAMPLE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">40.4. A Complete Event Trigger Example</h2></div></div></div><p>
+ 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.)
+ </p><p>
+ The function <code class="function">noddl</code> raises an exception each time it is called.
+ The event trigger definition associated the function with
+ the <code class="literal">ddl_command_start</code> event. The effect is that all DDL
+ commands (with the exceptions mentioned
+ in <a class="xref" href="event-trigger-definition.html" title="40.1. Overview of Event Trigger Behavior">Section 40.1</a>) are prevented from running.
+ </p><p>
+ This is the source code of the trigger function:
+</p><pre class="programlisting">
+#include "postgres.h"
+#include "commands/event_trigger.h"
+
+
+PG_MODULE_MAGIC;
+
+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-&gt;context;
+
+ ereport(ERROR,
+ (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+ errmsg("command \"%s\" denied",
+ GetCommandTagName(trigdata-&gt;tag))));
+
+ PG_RETURN_NULL();
+}
+</pre><p>
+ </p><p>
+ After you have compiled the source code (see <a class="xref" href="xfunc-c.html#DFUNC" title="38.10.5. Compiling and Linking Dynamically-Loaded Functions">Section 38.10.5</a>),
+ declare the function and the triggers:
+</p><pre class="programlisting">
+CREATE FUNCTION noddl() RETURNS event_trigger
+ AS 'noddl' LANGUAGE C;
+
+CREATE EVENT TRIGGER noddl ON ddl_command_start
+ EXECUTE FUNCTION noddl();
+</pre><p>
+ </p><p>
+ Now you can test the operation of the trigger:
+</p><pre class="screen">
+=# \dy
+ List of event triggers
+ Name | Event | Owner | Enabled | Function | Tags
+-------+-------------------+-------+---------+----------+------
+ noddl | ddl_command_start | dim | enabled | noddl |
+(1 row)
+
+=# CREATE TABLE foo(id serial);
+ERROR: command "CREATE TABLE" denied
+</pre><p>
+ </p><p>
+ 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:
+</p><pre class="programlisting">
+BEGIN;
+ALTER EVENT TRIGGER noddl DISABLE;
+CREATE TABLE foo (id serial);
+ALTER EVENT TRIGGER noddl ENABLE;
+COMMIT;
+</pre><p>
+ (Recall that DDL commands on event triggers themselves are not affected by
+ event triggers.)
+ </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="event-trigger-interface.html" title="40.3. Writing Event Trigger Functions in C">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-table-rewrite-example.html" title="40.5. A Table Rewrite Event Trigger Example">Next</a></td></tr><tr><td width="40%" align="left" valign="top">40.3. Writing Event Trigger Functions in C </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"> 40.5. A Table Rewrite Event Trigger Example</td></tr></table></div></body></html> \ No newline at end of file