Fixed problem with ISNULL on fields that could not be null

  void update_used_tables()

  {

    if (!args[0]->maybe_null)

    {

      used_tables_cache= 0;			/* is always false */

      cached_value= (longlong) 0;

    }

    else

    {

      args[0]->update_used_tables();

      used_tables_cache=args[0]->used_tables();

    }

    if (!used_tables_cache)

      if (!(used_tables_cache=args[0]->used_tables()))

      {

	/* Remember if the value is always NULL or never NULL */

	args[0]->val();

	cached_value= args[0]->null_value ? (longlong) 1 : (longlong) 0;

      }

    }

  }

  optimize_type select_optimize() const { return OPTIMIZE_NULL; }

};