bool Aggregator::executeOnBlock(Columns columns, UInt64 num_rows, AggregatedDataVariants & result,
    ColumnRawPtrs & key_columns, AggregateColumns & aggregate_columns, bool & no_more_keys)
{
  ……
    size_t result_size = result.sizeWithoutOverflowRow();
    Int64 current_memory_usage = 0;
    if (auto * memory_tracker_child = CurrentThread::getMemoryTracker())  // 当前线程的MemoryTracker
        if (auto * memory_tracker = memory_tracker_child->getParent())  // 父是Process级MemoryTracker
            current_memory_usage = memory_tracker->get();

    /// Here all the results in the sum are taken into account, from different threads.
    auto result_size_bytes = current_memory_usage - memory_usage_before_aggregation;