public void setNumReduceTasks(final Integer numReduceTasks) {
    this.numReduceTasks = numReduceTasks;
  }
If the number of reducers is -1, the runtime will automatically figure it
out by input data size.

The number of reducers will be a positive number only in case the target
table is bucketed into N buckets (through CREATE TABLE). This feature is
not supported yet, so the number of reducers will always be -1 for now.
ql/src/java/org/apache/hadoop/hive/ql/plan/ReduceWork.java