@Explain(displayName = "Reduce Operator Tree", explainLevels = { Level.USER, Level.DEFAULT, Level.EXTENDED },
      vectorization = Vectorization.OPERATOR_PATH)
  public Operator<?> getReducer() {
    return reducer;
  }
If the plan has a reducer and correspondingly a reduce-sink, then store the TableDesc pointing
to keySerializeInfo of the ReduceSink

@param keyDesc
ql/src/java/org/apache/hadoop/hive/ql/plan/ReduceWork.java