code: func builtinReverse(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	x := args[0]
  	switch x.Kind() {
  	case types.KindNull:
  		return d, nil
  	default:
  		s, err := x.ToString()
  		if err != nil {
  			return d, errors.Trace(err)
  		}
  		d.SetString(stringutil.Reverse(s))
  		return d, nil
  	}
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_reverse