@Override
    public void initialize(Configuration hconf) throws HiveException {
      streamAggregationBufferRowPool = new VectorUtilBatchObjectPool<VectorAggregationBufferRow>(
          VectorizedRowBatch.DEFAULT_SIZE,
          new VectorUtilBatchObjectPool.IAllocator<VectorAggregationBufferRow>() {

            @Override
            public VectorAggregationBufferRow alloc() throws HiveException {
              return allocateAggregationBuffer();
            }

            @Override
            public void free(VectorAggregationBufferRow t) {
              // Nothing to do
            }
          });
      LOG.info("using unsorted streaming aggregation processing mode");
    }
A pool of VectorAggregationBufferRow to avoid repeated allocations
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/VectorGroupByOperator.java