public String createFileId(String inputPath) {
    String bucketNum = String.valueOf(bucketFileNameMapping.get(inputPath));
    if (bigTablePartSpecToFileMapping != null) {
      // partSpecToFileMapping is null if big table is partitioned
      return prependPartSpec(inputPath, bucketNum);
    }
    return bucketNum;
  }
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