@Override
  public float getProgress() throws IOException {
    //this is not likely to do the right thing for Compaction of "original" files when there are copyN files
    return baseReader == null ? 1 : baseReader.getProgress();
  }
This determines the set of {@link ReaderPairAcid} to create for a given delta/.
For unbucketed tables {@code bucket} can be thought of as a write tranche.
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcRawRecordMerger.java