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doc: Add event trigger C API documentation

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From: Dimitri Fontaine <dimitri@2ndQuadrant.fr>
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Peter Eisentraut 2013-07-03 21:06:20 -04:00
parent 82b0102650
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doc/src/sgml/event-trigger.sgml
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@ -37,21 +37,27 @@
<para>
The <literal>ddl_command_start</> event occurs just before the
execution of a <literal>CREATE</>, <literal>ALTER</>, or <literal>DROP</>
command. As an exception, however, this event does not occur for
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 command targeting event triggers themselves. The event trigger
&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>. The <literal>ddl_command_end</>
event occurs just after the execution of this same set of commands.
<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 your
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
@ -76,6 +82,7 @@
</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
@ -607,4 +614,209 @@
</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>