And waits for them. That's pointless.
projection_future_part.assign(std::move(projection_parts));
        projection_future_part.name = projection.name;
        projection_future_part.path = future_part.path + "/" + projection.name + ".proj/";
        projection_future_part.part_info = {"all", 0, 0, 0};
        MergeTreeData::MergingParams projection_merging_params;
        projection_merging_params.mode = MergeTreeData::MergingParams::Ordinary;
        if (projection.type == ProjectionDescription::Type::Aggregate)
            projection_merging_params.mode = MergeTreeData::MergingParams::Aggregating;
        
        auto merged_projection_part = mergePartsToTemporaryPart(
            projection_future_part,
            projection.metadata,
            merge_entry,
            holder,
            time_of_merge,
            context,
            space_reservation,
            deduplicate,
            deduplicate_by_columns,
            projection_merging_params,
            new_data_part.get());
        new_data_part->addProjectionPart(projection.name, std::move(merged_projection_part));
    }
    if (chosen_merge_algorithm != MergeAlgorithm::Vertical)
        to.writeSuffixAndFinalizePart(new_data_part, need_sync);
    else
        to.writeSuffixAndFinalizePart(new_data_part, need_sync, &storage_columns, &checksums_gathered_columns);
    return new_data_part;
}
MergeTreeData::MutableDataPartPtr MergeTreeDataMergerMutator::mutatePartToTemporaryPart(