public void setTagToValueDesc(final List<TableDesc> tagToValueDesc) {
    this.tagToValueDesc = tagToValueDesc;
  }
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