# 9.29. 事件觸發函式
PostgreSQL provides these helper functions to retrieve information from event triggers.
For more information about event triggers, see [Chapter 39](https://www.postgresql.org/docs/13/event-triggers.html).
## 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 9.98](https://www.postgresql.org/docs/13/functions-event-triggers.html#FUNCTIONS-EVENT-TRIGGER-TABLE-REWRITE) 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 9.98. 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();
```