public function select_all_by_keys($table, array $fields = [], array $where = [], array $order = [], int $limit = 0, int $start = 0): ?array
  {
    if ( $rows = $this->rselect_all($table, $fields, $where, $order, $limit, $start) ){
      return x::index_by_first_val($rows);
    }
    return $this->check() ? [] : null;
  }