@Override
  public boolean next(RecordIdentifier recordIdentifier, OrcStruct prev) throws IOException {
    boolean keysSame = true;
    while (keysSame && primary != null) {

      // The primary's nextRecord is the next value to return
      OrcStruct current = primary.nextRecord;
      recordIdentifier.set(primary.key);

      // Advance the primary reader to the next record
      primary.next(extraValue);

      // Save the current record as the new extraValue for next time so that
      // we minimize allocations
      extraValue = current;

      // now that the primary reader has advanced, we need to see if we
      // continue to read it or move to the secondary.
      if (primary.nextRecord == null || primary.key.compareTo(secondaryKey) > 0) {

        // if the primary isn't done, push it back into the readers
        if (primary.nextRecord != null) {
          readers.put(primary.key, primary);
        }

        // update primary and secondaryKey
        Map.Entry<ReaderKey, ReaderPair> entry = readers.pollFirstEntry();
        if (entry != null) {
          primary = entry.getValue();
          if (readers.isEmpty()) {
            secondaryKey = null;
          } else {
            secondaryKey = readers.firstKey();
          }
        } else {
          primary = null;
        }
      }

      // if this transaction isn't ok, skip over it
      if (!validTxnList.isTxnValid(((ReaderKey) recordIdentifier).getCurrentTransactionId())) {
        continue;
      }

      // if we are collapsing, figure out if this is a new row
      if (collapse) {
        keysSame = prevKey.compareRow(recordIdentifier) == 0;
        if (!keysSame) {
          prevKey.set(recordIdentifier);
        }
      } else {
        keysSame = false;
      }

      // set the output record by fiddling with the pointers so that we can
      // avoid a copy.
      prev.linkFields(current);
    }
    return !keysSame;
  }
 @Override
 public int compareTo(RecordIdentifier other) {
   int sup = compareToInternal(other);
   if (sup == 0) {
     if (other.getClass() == ReaderKey.class) {
       ReaderKey oth = (ReaderKey) other;
       if (currentTransactionId != oth.currentTransactionId) {
         return currentTransactionId < oth.currentTransactionId ? +1 : -1;
       }
     } else {
       return -1;
     }
   }
   return sup;
 }
 @Override
 public void set(RecordIdentifier other) {
   super.set(other);
   currentTransactionId = ((ReaderKey) other).currentTransactionId;
 }