private Map<String, List<String>> convert(Map<Partition, List<String>> mapping) {
    Map<String, List<String>> converted = new HashMap<String, List<String>>();
    for (Map.Entry<Partition, List<String>> entry : mapping.entrySet()) {
      converted.put(entry.getKey().getName(), entry.getValue());
    }
    return converted;
  }
this transformation does bucket map join optimization.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/AbstractBucketJoinProc.java