code: func builtinBitLength(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	if args[0].IsNull() {
  		return d, nil
  	}
  
  	str, err := args[0].ToString()
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	d.SetInt64(int64(len(str) * 8))
  	return d, nil
  }
docs: // https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_bit-length