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.)

The function noddl raises an exception each time it is called. The event trigger definition associated the function with the ddl_command_start event. The effect is that all DDL commands (with the exceptions mentioned in Section 39.1) are prevented from running.

This is the source code of the trigger function:

#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();
}

After you have compiled the source code (see Section 37.9.5), declare the function and the triggers:

CREATE FUNCTION noddl() RETURNS event_trigger
    AS 'noddl' LANGUAGE C;

CREATE EVENT TRIGGER noddl ON ddl_command_start
    EXECUTE PROCEDURE noddl();

Now you can test the operation of the trigger:

=# \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

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:

BEGIN;
ALTER EVENT TRIGGER noddl DISABLE;
CREATE TABLE foo (id serial);
ALTER EVENT TRIGGER noddl ENABLE;
COMMIT;

(Recall that DDL commands on event triggers themselves are not affected by event triggers.)