# 9.29. Event Trigger Functions

9.29.1. Capturing Changes at Command End

9.29.2. Processing Objects Dropped by a DDL Command

9.29.3. Handling a Table Rewrite Event

PostgreSQL provides these helper functions to retrieve information from event triggers.

For more information about event triggers, see Chapter 40.

# 9.29.1. Capturing Changes at Command End

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:

Name Type Description
classid oid OID of catalog the object belongs in
objid oid OID of the object itself
objsubid integer Sub-object ID (e.g., attribute number for a column)
command_tag text Command tag
object_type text Type of the object
schema_name text Name of the schema the object belongs in, if any; otherwise NULL. No quoting is applied.
object_identity text Text rendering of the object identity, schema-qualified. Each identifier included in the identity is quoted if necessary.
in_extension boolean True if the command is part of an extension script
command pg_ddl_command A complete representation of the command, in internal format. This cannot be output directly, but it can be passed to other functions to obtain different pieces of information about the command.

# 9.29.2. Processing Objects Dropped by a DDL Command

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:

Name Type Description
classid oid OID of catalog the object belonged in
objid oid OID of the object itself
objsubid integer Sub-object ID (e.g., attribute number for a column)
original boolean True if this was one of the root object(s) of the deletion
normal boolean True if there was a normal dependency relationship in the dependency graph leading to this object
is_temporary boolean True if this was a temporary object
object_type text Type of the object
schema_name text Name of the schema the object belonged in, if any; otherwise NULL. No quoting is applied.
object_name text Name of the object, if the combination of schema and name can be used as a unique identifier for the object; otherwise NULL. No quoting is applied, and name is never schema-qualified.
object_identity text Text rendering of the object identity, schema-qualified. Each identifier included in the identity is quoted if necessary.
address_names text[] An array that, together with object_type and address_args, can be used by the pg_get_object_address function to recreate the object address in a remote server containing an identically named object of the same kind.
address_args text[] Complement for address_names

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

# 9.29.3. Handling a Table Rewrite Event

The functions shown in Table 9.100 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 9.100. Table Rewrite Information Functions

Function

Description
pg_event_trigger_table_rewrite_oid () → oid

Returns the OID of the table about to be rewritten.
pg_event_trigger_table_rewrite_reason () → integer

Returns a code explaining the reason(s) for rewriting. The exact meaning of the codes is release dependent.

These functions can be used in an event trigger like this:

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

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