code: func (us *UnionScanExec) shouldPickFirstRow(a, b []types.Datum) (bool, error) {
  	var isFirstRow bool
  	addedCmpSrc, err := us.compare(a, b)
  	if err != nil {
  		return isFirstRow, err
  	}
  	// Compare result will never be 0.
  	if us.desc {
  		if addedCmpSrc > 0 {
  			isFirstRow = true
  		}
  	} else {
  		if addedCmpSrc < 0 {
  			isFirstRow = true
  		}
  	}
  	return isFirstRow, nil
  }
docs: // shouldPickFirstRow picks the suitable row in order.
  // The value returned is used to determine whether to pick the first input row.