# 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();
```
---
# Agent Instructions: Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter:
```
GET https://docs.postgresql.tw/16/the-sql-language/functions-and-operators/event-trigger-functions.md?ask=
```
The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.