## 9.28. Event Trigger Functions

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

For more information about event triggers, see Chapter 40.

### 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 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` | `bool` | 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.

### 9.28.2. 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 of the object itself
`objsubid` | `integer` | Sub-object ID (e.g. attribute number for a column)
`original` | `bool` | True if this was one of the root object(s) of the deletion
`normal` | `bool` | True if there was a normal dependency relationship in the dependency graph leading to this object
`is_temporary` | `bool` | 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 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

Name | Return Type | Description
--- | --- | ---
``pg_event_trigger_table_rewrite_oid()`` | `Oid` | The OID of the table about to be rewritten.
``pg_event_trigger_table_rewrite_reason()`` | `int` | The reason code(s) explaining the reason for rewriting. The exact meaning of the codes is release dependent.

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();