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