private Map<String, String> revert(Map<String, List<String>> mapping) {
    Map<String, String> converted = new HashMap<String, String>();
    for (Map.Entry<String, List<String>> entry : mapping.entrySet()) {
      String partSpec = entry.getKey();
      for (String file : entry.getValue()) {
        converted.put(URI.create(file).getPath(), partSpec);
      }
    }
    return converted;
  }
Given a small table input file, find the mapping
big table input file with the smallest bucket number.
ql/src/java/org/apache/hadoop/hive/ql/plan/BucketMapJoinContext.java