Overview of Event Trigger Behavior
An event trigger fires whenever the event with which it is associated occurs in the database in which it is defined. Currently, the only supported events are ddl_command_start, ddl_command_end and sql_drop. Support for additional events may be added in future releases.
The ddl_command_start event occurs just before the execution of a CREATE, ALTER, or DROP 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 — databases, roles, and tablespaces — or for commands targeting event triggers themselves. The event trigger mechanism does not support these object types. ddl_command_start also occurs just before the execution of a SELECT INTO command, since this is equivalent to CREATE TABLE AS.
The ddl_command_end event occurs just after the execution of this same set of commands.
The sql_drop event occurs just before the 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 pg_event_trigger_dropped_objects() from the sql_drop event trigger code (see Section 9.28). Note that the trigger is executed after the objects have been deleted from the system catalogs, so it's not possible to look them up anymore.
Event triggers (like other functions) cannot be executed in an aborted transaction. Thus, if a DDL command fails with an error, any associated ddl_command_end triggers will not be executed. Conversely, if a ddl_command_start trigger fails with an error, no further event triggers will fire, and no attempt will be made to execute the command itself. Similarly, if a ddl_command_end trigger fails with an error, the effects of the DDL statement will be rolled back, just as they would be in any other case where the containing transaction aborts.
For a complete list of commands supported by the event trigger mechanism, see Section 37.2.
Event triggers are created using the command CREATE EVENT TRIGGER. In order to create an event trigger, you must first create a function with the special return type event_trigger. This function need not (and may not) return a value; the return type serves merely as a signal that the function is to be invoked as an event trigger.
If more than one event trigger is defined for a particular event, they will fire in alphabetical order by trigger name.
A trigger definition can also specify a WHEN condition so that, for example, a ddl_command_start trigger can be fired only for particular commands which the user wishes to intercept. A common use of such triggers is to restrict the range of DDL operations which users may perform.