private KeyWrapper cloneKeyWrapper(VectorHashKeyWrapperBase from) {
      if (reusableKeyWrapperBuffer != null && reusableKeyWrapperBuffer.size() > 0) {
        KeyWrapper keyWrapper = reusableKeyWrapperBuffer.poll();
        from.copyKey(keyWrapper);
        return keyWrapper;
      } else {
        return from.copyKey();
      }
    }
Locates the aggregation buffer sets to use for each key in the current batch.
The keyWrappersBatch must have evaluated the current batch first.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/VectorGroupByOperator.java