[HIVE-20177] Vectorization: Reduce KeyWrapper allocation in GroupBy Streaming mode

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 3.2.0
• Component/s: Vectorization
• Labels: performance
• Assignee: Gopal Vijayaraghavan
• Reporter: Gopal Vijayaraghavan
• Created: 15/Jul/18 09:20
• Updated: 31/Jul/18 21:13
• Resolved: 31/Jul/18 21:13

Description

The streaming mode for VectorGroupBy allocates a large number of arrays due to VectorKeyHashWrapper::duplicateTo()

Since the vectors can't be mutated in-place while a single batch is being processed, this operation can be cut by 1000x by allocating a streaming key at the end of the loop, instead of reallocating within the loop.

      for(int i = 0; i < batch.size; ++i) {
        if (!batchKeys[i].equals(streamingKey)) {
          // We've encountered a new key, must save current one
          // We can't forward yet, the aggregators have not been evaluated
          rowsToFlush[flushMark] = currentStreamingAggregators;
          if (keysToFlush[flushMark] == null) {
            keysToFlush[flushMark] = (VectorHashKeyWrapper) streamingKey.copyKey();
          } else {
            streamingKey.duplicateTo(keysToFlush[flushMark]);
          }

          currentStreamingAggregators = streamAggregationBufferRowPool.getFromPool();
          batchKeys[i].duplicateTo(streamingKey);
          ++flushMark;
        }

The duplicateTo can be pushed out of the loop since there only one to truly keep a copy of is the last unique key in the VRB.

The actual byte[] values within the keys are safely copied out by - VectorHashKeyWrapperBatch.assignRowColumn() which calls setVal() and not setRef().