private static Path[] getDeltaDirsFromBucketTracker(BucketTracker bucketTracker) {
      List<Path> resultList = new ArrayList<>(bucketTracker.buckets.size());

      for (int i = 0; i < bucketTracker.buckets.size(); ++i) {
        Path p = bucketTracker.buckets.get(i).getParent();
        if (p.getName().startsWith(AcidUtils.DELTA_PREFIX) ||
            p.getName().startsWith(AcidUtils.DELETE_DELTA_PREFIX)) {
          resultList.add(p);
        }
      }

      return resultList.toArray(new Path[0]);
    }
This input format returns its own input split as a value.  This is because our splits
contain information needed to properly construct the writer.  Crazy, huh?
ql/src/java/org/apache/hadoop/hive/ql/txn/compactor/MRCompactor.java