private VectorizationCondition[] createReduceExplainVectorizationConditions() {

      boolean enabled = reduceWork.getReduceVectorizationEnabled();

      String engine = reduceWork.getVectorReduceEngine();
      String engineInSupportedCondName =
          HiveConf.ConfVars.HIVE_EXECUTION_ENGINE.varname + " " + engine + " IN " + reduceVectorizableEngines;

      boolean engineInSupported = reduceVectorizableEngines.contains(engine);

      VectorizationCondition[] conditions = new VectorizationCondition[] {
          new VectorizationCondition(
              enabled,
              HiveConf.ConfVars.HIVE_VECTORIZATION_REDUCE_ENABLED.varname),
          new VectorizationCondition(
              engineInSupported,
              engineInSupportedCondName)
      };
      return conditions;
    }
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