code: func (b *builtinRightBinarySig) evalString(row chunk.Row) (string, bool, error) {
  	str, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	right, isNull, err := b.args[1].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	strLength, rightLength := len(str), int(right)
  	if rightLength > strLength {
  		rightLength = strLength
  	} else if rightLength < 0 {
  		rightLength = 0
  	}
  	return str[strLength-rightLength:], false, nil
  }
docs: // evalString evals RIGHT(str,len).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_right