@Override
  public long getBufferRetainedSizeInBytes() {
    return downStreamChannelList.stream()
        .map(ISink::getBufferRetainedSizeInBytes)
        .reduce(Long::sum)
        .orElse(0L);
  }
SinkHandle may have multiple channels. we need to choose the next channel each time we send a
TsBlock.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/exchange/sink/ShuffleSinkHandle.java