@Override
  public Set<Operator<?>> getAllRootOperators() {
    Set<Operator<?>> opSet = new LinkedHashSet<Operator<?>>();
    opSet.add(getReducer());
    return opSet;
  }
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