@Explain(vectorization = Vectorization.DETAIL, displayName = "reduceColumnNullOrder", explainLevels = { Level.DEFAULT, Level.EXTENDED })
    public String getReduceColumnNullOrder() {
      if (!getVectorizationExamined()) {
        return null;
      }
      return reduceWork.getVectorReduceColumnNullOrder();
    }
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