public TableDesc getKeySerializeInfo() {
    return keySerializeInfo;
  }
Returns the number of reducers for the map-reduce job. -1 means to decide
the number of reducers at runtime. This enables Hive to estimate the number
of reducers based on the map-reduce input data size, which is only
available right before we start the map-reduce job.
ql/src/java/org/apache/hadoop/hive/ql/plan/ReduceSinkDesc.java