@Override
    public void aggregateInputSelection(
      VectorAggregationBufferRow[] aggregationBufferSets,
      int aggregateIndex,
      VectorizedRowBatch batch) throws HiveException {

      int batchSize = batch.size;

      if (batchSize == 0) {
        return;
      }

      // count(*) cares not about NULLs nor selection
      for (int i=0; i < batchSize; ++i) {
        Aggregation myAgg = getCurrentAggregationBuffer(
            aggregationBufferSets, aggregateIndex, i);
        ++myAgg.count;
      }
    }
class for storing the current aggregate value.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/aggregates/VectorUDAFCountStar.java