private int getAndVerifyIndex(HashMap<Long, Integer> posMap,
      List<HdfsFileStatusWithId> files, OrcTail[] result, Long fileId) {
    int ix = posMap.get(fileId);
    assert result[ix] == null;
    assert fileId != null && fileId.equals(files.get(ix).getFileId());
    return ix;
  }
Metastore-based footer cache storing serialized footers. Also has a local cache.
ql/src/java/org/apache/hadoop/hive/ql/io/orc/ExternalCache.java