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>

  <sect1 id="event-trigger-database-login-example">
    <title>A Database Login Event Trigger Example</title>

    <para>
      The event trigger on the <literal>login</literal> event can be
      useful for logging user logins, for verifying the connection and
      assigning roles according to current circumstances, or for session
      data initialization. It is very important that any event trigger using
      the <literal>login</literal> event checks whether or not the database is
      in recovery before performing any writes. Writing to a standby server
      will make it inaccessible.
    </para>

    <para>
      The following example demonstrates these options.
<programlisting>
-- create test tables and roles
CREATE TABLE user_login_log (
  "user" text,
  "session_start" timestamp with time zone
);
CREATE ROLE day_worker;
CREATE ROLE night_worker;

-- the example trigger function
CREATE OR REPLACE FUNCTION init_session()
  RETURNS event_trigger SECURITY DEFINER
  LANGUAGE plpgsql AS
$$
DECLARE
  hour integer = EXTRACT('hour' FROM current_time at time zone 'utc');
  rec boolean;
BEGIN
-- 1. Forbid logging in between 2AM and 4AM.
IF hour BETWEEN 2 AND 4 THEN
  RAISE EXCEPTION 'Login forbidden';
END IF;

-- The checks below cannot be performed on standby servers so
-- ensure the database is not in recovery before we perform any
-- operations.
SELECT pg_is_in_recovery() INTO rec;
IF rec THEN
  RETURN;
END IF;

-- 2. Assign some roles. At daytime, grant the day_worker role, else the
-- night_worker role.
IF hour BETWEEN 8 AND 20 THEN
  EXECUTE 'REVOKE night_worker FROM ' || quote_ident(session_user);
  EXECUTE 'GRANT day_worker TO ' || quote_ident(session_user);
ELSE
  EXECUTE 'REVOKE day_worker FROM ' || quote_ident(session_user);
  EXECUTE 'GRANT night_worker TO ' || quote_ident(session_user);
END IF;

-- 3. Initialize user session data
CREATE TEMP TABLE session_storage (x float, y integer);
ALTER TABLE session_storage OWNER TO session_user;

-- 4. Log the connection time
INSERT INTO public.user_login_log VALUES (session_user, current_timestamp);

END;
$$;

-- trigger definition
CREATE EVENT TRIGGER init_session
  ON login
  EXECUTE FUNCTION init_session();
ALTER EVENT TRIGGER init_session ENABLE ALWAYS;
</programlisting>
    </para>
  </sect1>
</chapter>