-------+-------------------+-------+---------+----------+------
 noddl | ddl_command_start | dim   | enabled | noddl    |
(1 row)

=# CREATE TABLE foo(id serial);
ERROR:  command "CREATE TABLE" denied
</screen>
   </para>

   <para>
    In this situation, in order to be able to run some DDL commands when you
    need to do so, you have to either drop the event trigger or disable it.  It
    can be convenient to disable the trigger for only the duration of a
    transaction:
<programlisting>
BEGIN;
ALTER EVENT TRIGGER noddl DISABLE;
CREATE TABLE foo (id serial);
ALTER EVENT TRIGGER noddl ENABLE;
COMMIT;
</programlisting>
    (Recall that DDL commands on event triggers themselves are not affected by
    event triggers.)
   </para>
  </sect1>

  <sect1 id="event-trigger-table-rewrite-example">
   <title>A Table Rewrite Event Trigger Example</title>

   <para>
    Thanks to the <literal>table_rewrite</literal> event, it is possible to implement
    a table rewriting policy only allowing the rewrite in maintenance windows.
   </para>

   <para>
    Here's an example implementing such a policy.
<programlisting>
CREATE OR REPLACE FUNCTION no_rewrite()
 RETURNS event_trigger
 LANGUAGE plpgsql AS
$$
---
--- Implement local Table Rewriting policy:
---   public.foo is not allowed rewriting, ever
---   other tables are only allowed rewriting between 1am and 6am
---   unless they have more than 100 blocks
---
DECLARE
  table_oid oid := pg_event_trigger_table_rewrite_oid();
  current_hour integer := extract('hour' from current_time);
  pages integer;
  max_pages integer := 100;
BEGIN
  IF pg_event_trigger_table_rewrite_oid() = 'public.foo'::regclass
  THEN
        RAISE EXCEPTION 'you''re not allowed to rewrite the table %',
                        table_oid::regclass;
  END IF;

  SELECT INTO pages relpages FROM pg_class WHERE oid = table_oid;
  IF pages > max_pages
  THEN
        RAISE EXCEPTION 'rewrites only allowed for table with less than % pages',
                        max_pages;
  END IF;

  IF current_hour NOT BETWEEN 1 AND 6
  THEN
        RAISE EXCEPTION 'rewrites only allowed between 1am and 6am';
  END IF;
END;
$$;

CREATE EVENT TRIGGER no_rewrite_allowed
                  ON table_rewrite
   EXECUTE FUNCTION no_rewrite();
</programlisting>
   </para>
 </sect1>