private void ensurePutReader(ReaderKey key, ReaderPair deltaPair) throws IOException {
    ReaderPair oldPair = readers.put(key, deltaPair);
    if (oldPair == null) return;
    String error = "Two readers for " + key + ": new " + deltaPair + ", old " + oldPair;
    LOG.error(error);
    throw new IOException(error);
  }
it's possible that the file for a specific {@link bucket} doesn't exist in any given
delta since since no rows hashed to it (and not configured to create empty buckets)
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcRawRecordMerger.java