9.29. Event Trigger Functions
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## 9.29. Event Trigger Functions #

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

For more information about event triggers, see Chapter 42.

### 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 9.104 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 9.104. 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 value is a bitmap built from the following values: `1` (the table has changed its persistence), `2` (default value of a column has changed), `4` (a column has a new data type) and `8` (the table access method has changed).

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();

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