code: func (b *builtinRandomBytesSig) evalString(row chunk.Row) (string, bool, error) {
  	len, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	if len < 1 || len > 1024 {
  		return "", false, types.ErrOverflow.GenWithStackByArgs("length", "random_bytes")
  	}
  	buf := make([]byte, len)
  	if n, err := rand.Read(buf); err != nil {
  		return "", true, err
  	} else if int64(n) != len {
  		return "", false, errors.New("fail to generate random bytes")
  	}
  	return string(buf), false, nil
  }
docs: // evalString evals RANDOM_BYTES(len).
  // See https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_random-bytes