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
sql_drop. Support for additional events may be
added in future releases.
occurs just before the execution of a
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
INTO command, since this is equivalent to
CREATE TABLE AS.
ddl_command_end event occurs
just after the execution of this same set of commands. To obtain
more details on the DDL
operations that took place, use the set-returning function
ddl_command_end event trigger
code (see Section 9.28). Note
that the trigger fires after the actions have taken place (but
before the transaction commits), and thus the system catalogs can
be read as already changed.
sql_drop event occurs just
trigger for any operation that drops database objects. To list
the objects that have been dropped, use the set-returning
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
table_rewrite event occurs
just before a table is rewritten by some actions of the commands
ALTER TABLE and
ALTER TYPE. While other control statements are
available to rewrite a table, like
table_rewrite event is not
triggered by them.
Event triggers (like other functions) cannot be executed in an
aborted transaction. Thus, if a DDL command fails with an error,
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
For a complete list of commands supported by the event trigger mechanism, see Section 40.2.
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.
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