private void initializeMultipleSources(ReduceWork redWork, int numTags, ObjectInspector[] ois,
      ReduceRecordSource[] sources) throws Exception {
    for (int tag = 0; tag < redWork.getTagToValueDesc().size(); tag++) {
      if (redWork.getTagToValueDesc().get(tag) == null) {
        continue;
      }
      checkAbortCondition();
      initializeSourceForTag(redWork, tag, ois, sources, redWork.getTagToValueDesc().get(tag),
          redWork.getTagToInput().get(tag));
    }
  }
Process input from tez LogicalInput and write output - for a map plan
Just pump the records through the query plan.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/ReduceRecordProcessor.java