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>37.4. A Complete Event Trigger Example</A
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><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 <TT
CLASS="LITERAL"
>ddl_command_start</TT
> event.  The effect is that all DDL
    commands (with the exceptions mentioned
    in <A
HREF="event-trigger-definition.html"
>Section 37.1</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;

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-&#62;context;

    ereport(ERROR,
        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("command \"%s\" denied", trigdata-&#62;tag)));

    PG_RETURN_NULL();
}</PRE
><P>
   </P
><P
>    After you have compiled the source code (see <A
HREF="xfunc-c.html#DFUNC"
>Section 35.9.6</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 PROCEDURE 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 | Procedure | 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
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