code: func (b *builtinUnHexSig) evalString(row chunk.Row) (string, bool, error) {
  	var bs []byte
  
  	d, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return d, isNull, err
  	}
  	// Add a '0' to the front, if the length is not the multiple of 2
  	if len(d)%2 != 0 {
  		d = "0" + d
  	}
  	bs, err = hex.DecodeString(d)
  	if err != nil {
  		return "", true, nil
  	}
  	return string(bs), false, nil
  }
docs: // evalString evals a builtinUnHexSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_unhex