9.28. Event Trigger Functions
PostgreSQL provides these helper functions to retrieve information from event triggers.
For more information about event triggers, see Chapter 39.
9.28.1. Capturing Changes at Command End
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 in the catalog |
objsubid | integer | Object sub-id (e.g. attribute number for columns) |
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 and every identifier present in the identity is quoted if necessary. |
in_extension | bool | whether 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. |
9.28.2. Processing Objects Dropped by a DDL Command
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, pg_event_trigger_dropped_objects raises an error. pg_event_trigger_dropped_objects returns the following columns:
| Name | Type | Description |
|---|---|---|
classid | Oid | OID of catalog the object belonged in |
objid | Oid | OID the object had within the catalog |
objsubid | int32 | Object sub-id (e.g. attribute number for columns) |
original | bool | Flag used to identify the root object(s) of the deletion |
normal | bool | Flag indicating that there's a normal dependency relationship in the dependency graph leading to this object |
is_temporary | bool | Flag indicating that the object 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 and every identifier present 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() 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 above. |
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 PROCEDURE test_event_trigger_for_drops();
9.28.3. Handling a Table Rewrite Event
The functions shown in Table 9.90 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 9.90. 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
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 PROCEDURE test_event_trigger_table_rewrite_oid();