public String createFileName(String inputPath, String fileName) {
    if (bigTablePartSpecToFileMapping != null) {
      // partSpecToFileMapping is null if big table is partitioned
      return prependPartSpec(inputPath, fileName);
    }
    return fileName;
  }
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