code: func strDatetimeAddDuration(sc *stmtctx.StatementContext, d string, arg1 types.Duration) (string, error) {
  	arg0, err := types.ParseTime(sc, d, mysql.TypeDatetime, types.MaxFsp)
  	if err != nil {
  		return "", err
  	}
  	ret, err := arg0.Add(sc, arg1)
  	if err != nil {
  		return "", err
  	}
  	fsp := types.MaxFsp
  	if ret.Time.Microsecond() == 0 {
  		fsp = types.MinFsp
  	}
  	ret.Fsp = fsp
  	return ret.String(), nil
  }
docs: // strDatetimeAddDuration adds duration to datetime string, returns a string value.