@Override
    public RecordReader<NullWritable, CompactorInputSplit> getRecordReader(
        InputSplit inputSplit,  JobConf entries, Reporter reporter) throws IOException {
      return new CompactorRecordReader((CompactorInputSplit)inputSplit);
    }
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