private void ensureDataReader() throws IOException {
    ensureOrcReader();
    if (stripeReader != null) {
      try {
        stripeReader.close();
      } catch (IOException ex) {
        // Ignore.
      }
    }
    // Reader creation updates HDFS counters, don't do it here.
    DataWrapperForOrc dw = new DataWrapperForOrc();
    stripeReader = orcReader.encodedReader(
        fileKey, dw, dw, useObjectPools ? POOL_FACTORY : null, trace, useCodecPool, cacheTag, isReadCacheOnly);
    stripeReader.setTracing(LlapIoImpl.ORC_LOGGER.isTraceEnabled());
    stripeReader.setStopped(isStopped);
  }
stripeRgs[stripeIx'] => boolean array (could be a bitmask) of rg-s that need to be read.
Contains only stripes that are read, and only columns included. null => read all RGs.
llap-server/src/java/org/apache/hadoop/hive/llap/io/encoded/OrcEncodedDataReader.java