code: func (us *UnionScanExec) Less(i, j int) bool {
  	cmp, err := us.compare(us.addedRows[i], us.addedRows[j])
  	if err != nil {
  		us.sortErr = errors.Trace(err)
  		return true
  	}
  	return cmp < 0
  }
docs: // Less implements sort.Interface interface.