@Explain(displayName = "Alias Bucket Output File Name Mapping", explainLevels = { Level.EXTENDED })
  public Map<String, Integer> getBucketFileNameMapping() {
    return bucketFileNameMapping;
  }
was inner class of MapreLocalWork. context for bucket mapjoin (or smb join)
ql/src/java/org/apache/hadoop/hive/ql/plan/BucketMapJoinContext.java