code: func (b *builtinMD5Sig) evalString(row chunk.Row) (string, bool, error) {
  	arg, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	sum := md5.Sum([]byte(arg))
  	hexStr := fmt.Sprintf("%x", sum)
  	return hexStr, false, nil
  }
docs: // evalString evals a builtinMD5Sig.
  // See https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_md5