protected function getFields()/*# : bool */
    {
        if (null === $this->cols) {
            $result = $this->statement->result_metadata();
            if (false === $result) {
                return false;
            }

            $this->cols = [];

            // set column name
            foreach ($result->fetch_fields() as $col) {
                $this->cols[] = $col->name;
            }
        }
        return true;
    }