public static Path setMergeWork(JobConf conf, MergeJoinWork mergeJoinWork, Path mrScratchDir,
      boolean useCache) {
    for (BaseWork baseWork : mergeJoinWork.getBaseWorkList()) {
      setBaseWork(conf, baseWork, mrScratchDir, baseWork.getName() + MERGE_PLAN_NAME, useCache);
      String prefixes = conf.get(DagUtils.TEZ_MERGE_WORK_FILE_PREFIXES);
      if (prefixes == null) {
        prefixes = baseWork.getName();
      } else {
        prefixes = prefixes + "," + baseWork.getName();
      }
      conf.set(DagUtils.TEZ_MERGE_WORK_FILE_PREFIXES, prefixes);
    }

    // nothing to return
    return null;
  }
ReduceField:
KEY: record key
VALUE: record value
ql/src/java/org/apache/hadoop/hive/ql/exec/Utilities.java