code: func builtinRepeat(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	str, err := args[0].ToString()
  	if err != nil {
  		return d, err
  	}
  	ch := fmt.Sprintf("%v", str)
  	num := 0
  	x := args[1]
  	switch x.Kind() {
  	case types.KindInt64:
  		num = int(x.GetInt64())
  	case types.KindUint64:
  		num = int(x.GetUint64())
  	}
  	if num < 1 {
  		d.SetString("")
  		return d, nil
  	}
  	d.SetString(strings.Repeat(ch, num))
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_repeat