protected @Nullable Collection<Integer> getKeysForCommit(int commit) throws IOException {
    ForwardIndex forwardIndex = myMapReduceIndex.getForwardIndex();
    KeyCollectionForwardIndexAccessor<Integer, T> forwardIndexAccessor =
      ((KeyCollectionForwardIndexAccessor<Integer, T>)myMapReduceIndex.getForwardIndexAccessor());
    if (forwardIndex == null || forwardIndexAccessor == null) return null;
    return forwardIndexAccessor.deserializeData(forwardIndex.get(commit));
  }
protected @Nullable Collection<Integer> getKeysForCommit(int commit)