static Path[] getDeltaFiles(Path deltaDirectory, int bucket, Options mergerOptions, Integer attemptId) {
    assert (!mergerOptions.isCompacting &&
        deltaDirectory.getName().startsWith(AcidUtils.DELETE_DELTA_PREFIX)
    ) || mergerOptions.isCompacting : "Unexpected delta: " + deltaDirectory +
        "(isCompacting=" + mergerOptions.isCompacting() + ")";
    return new Path[] {AcidUtils.createBucketFile(deltaDirectory, bucket, attemptId)};
  }
This determines the set of {@link ReaderPairAcid} to create for a given delta/.
For unbucketed tables {@code bucket} can be thought of as a write tranche.
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcRawRecordMerger.java