private static AggregationDesc buildAggregationDescCountStar(
      VectorizationContext ctx) {
    AggregationDesc agg = new AggregationDesc();
    agg.setGenericUDAFName("count");
    agg.setMode(GenericUDAFEvaluator.Mode.PARTIAL1);
    agg.setParameters(new ArrayList<ExprNodeDesc>());
    agg.setGenericUDAFEvaluator(new GenericUDAFCount.GenericUDAFCountEvaluator());
    return agg;
  }
Unit test for the vectorized GROUP BY operator.
ql/src/test/org/apache/hadoop/hive/ql/exec/vector/TestVectorGroupByOperator.java