Event Triggers

Redrock Postgres provides event triggers. Unlike regular triggers, which are attached to a single table and capture only DML events, event triggers are global to a particular database and are capable of capturing DDL events.

Like regular triggers, event triggers can be written in any procedural language that includes event trigger support, but not in plain SQL.

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, table_rewrite 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, DROP, SECURITY LABEL, COMMENT, GRANT or REVOKE 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 — 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. To obtain more details on the DDL operations that took place, use the set-returning function pg_event_trigger_ddl_commands() from the ddl_command_end event trigger code (see Event Trigger Functions). 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.

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 Event Trigger Functions). 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.

The 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 CLUSTER and VACUUM, the 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, 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 Event Trigger Firing Matrix.

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.

Table 1 lists all commands for which event triggers are supported.

Table 1. Event Trigger Support by Command Tag

Event triggers can not capture DDL commands targeting database objects, including: CREATE DATABASE, ALTER DATABASE, DROP DATABASE, COMMENT ON DATABASE, GRANT ON DATABASE, REVOKE ON DATABASE.

Redrock Postgres provides these helper functions to retrieve information from event triggers.

pg_event_trigger_ddl_commands () → setof record

pg_event_trigger_ddl_commands returns a list of DDL commands executed by each user action, when invoked in a function attached to a ddl_command_end event trigger. If called in any other context, an error is raised. pg_event_trigger_ddl_commands returns one row for each base command executed; some commands that are a single SQL sentence may return more than one row. This function returns the following columns:

pg_event_trigger_dropped_objects () → setof record

pg_event_trigger_dropped_objects returns a list of all objects dropped by the command in whose sql_drop event it is called. If called in any other context, an error is raised. This function returns the following columns:

The pg_event_trigger_dropped_objects function can be used in an event trigger like this:

CREATE FUNCTION test_event_trigger_for_drops()
  RETURNS event_trigger AS $$
DECLARE
    obj record;
BEGIN
    FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
    LOOP
        RAISE NOTICE '% dropped object: % %.% %',
                     tg_tag,
                     obj.object_type,
                     obj.schema_name,
                     obj.object_name,
                     obj.object_identity;
    END LOOP;
END;
$$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER test_event_trigger_for_drops
    ON sql_drop
    EXECUTE PROCEDURE test_event_trigger_for_drops();

The functions shown in Table 2 provide information about a table for which a table_rewrite event has just been called. If called in any other context, an error is raised.

Table 2. Table Rewrite Information

The pg_event_trigger_table_rewrite_oid function can be used in an event trigger like this:

CREATE FUNCTION test_event_trigger_table_rewrite_oid()
  RETURNS event_trigger AS $$
BEGIN
  RAISE NOTICE 'rewriting table % for reason %',
                pg_event_trigger_table_rewrite_oid()::regclass,
                pg_event_trigger_table_rewrite_reason();
END;
$$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER test_table_rewrite_oid
    ON table_rewrite
    EXECUTE PROCEDURE test_event_trigger_table_rewrite_oid();

pg_ddl_command_deparse ( pg_ddl_command ) → text

pg_ddl_command_deparse deparses a command from internal format of type pg_ddl_command, and returns a SQL statement in textual representation of type text. The function can be used in an event trigger like this:

CREATE TABLE ddl_history(
  ord             int,
  op_time         timestamp,
  username        text,
  command_tag     text,
  object_type     text,
  schema_name     text,
  object_identity text,
  command         text
);

CREATE OR REPLACE FUNCTION ddl_trigger_func()
  RETURNS event_trigger AS $$
BEGIN
  INSERT INTO ddl_history
    SELECT ordinality, now(), current_user, command_tag,
           object_type, schema_name, object_identity,
           pg_ddl_command_deparse(command)
      FROM pg_event_trigger_ddl_commands() WITH ORDINALITY;
END;
$$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER deparse_event_trigger
    ON ddl_command_end
    EXECUTE PROCEDURE ddl_trigger_func();