code: func (b *builtinReverseSig) evalString(row chunk.Row) (string, bool, error) {
  	str, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	reversed := reverseRunes([]rune(str))
  	return string(reversed), false, nil
  }
docs: // evalString evals a REVERSE(str).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_reverse