execute(sql, values, cb) {
    if (typeof values === 'function') {
      cb = values;
      values = [];
    }
    this.getConnection((err, conn) => {
      if (err) {
        return cb(err);
      }
      try {
        conn.execute(sql, values, cb).once('end', () => {
          conn.release();
        });
      } catch (e) {
        conn.release();
        throw e;
      }
    });
  }
pool cluster execute
@param {*} sql
@param {*} values
@param {*} cb