  inline bool is_mergeable()
  {
    return (next_select() == 0 && group_list.elements == 0 &&
            having == 0 && with_sum_func == 0 &&
            table_list.elements >= 1 && !(options & SELECT_DISTINCT) &&
            select_limit == 0);
  }
  inline bool is_mergeable()
  {
    return (next_select() == 0 && group_list.elements == 0 &&
            having == 0 && with_sum_func == 0 &&
            table_list.elements >= 1 && !(options & SELECT_DISTINCT) &&
            select_limit == 0);
  }