public void init(Object fileKey, int stripeIx, int rgIx, int columnCount) {
      if (batchKey == null) {
        batchKey = new OrcBatchKey(fileKey, stripeIx, rgIx);
      } else {
        batchKey.set(fileKey, stripeIx, rgIx);
      }
      resetColumnArrays(columnCount);
    }
All the previous streams are data streams, this and the next ones are index streams.
We assume the order will stay the same for backward compat.
ql/src/java/org/apache/hadoop/hive/ql/io/orc/encoded/Reader.java