9.28. Event Trigger Functions

9.28. Event Trigger Functions
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9.28.1. Capturing Changes at Command End
9.28.2. Processing Objects Dropped by a DDL Command
9.28.3. Handling a Table Rewrite Event

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 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 FUNCTION test_event_trigger_for_drops();

9.28.3. Handling a Table Rewrite Event

The functions shown in Table 9.96 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 9.96. 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 FUNCTION test_event_trigger_table_rewrite_oid();

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