And waits for them. That's pointless.
"Merge sorted {} rows, containing {} columns ({} merged, {} gathered) in {} sec., {} rows/sec., {}/sec.",
            merge_entry->rows_read,
            all_column_names.size(),
            merging_column_names.size(),
            gathering_column_names.size(),
            elapsed_seconds,
            merge_entry->rows_read / elapsed_seconds,
            ReadableSize(merge_entry->bytes_read_uncompressed / elapsed_seconds));
    }
    for (const auto & projection : metadata_snapshot->getProjections())
    {
        MergeTreeData::DataPartsVector projection_parts;
        for (const auto & part : parts)
        {
            auto it = part->getProjectionParts().find(projection.name);
            if (it != part->getProjectionParts().end())
                projection_parts.push_back(it->second);
        }
        if (projection_parts.size() < parts.size())
        {
            LOG_DEBUG(log, "Projection {} is not merged because some parts don't have it", projection.name);
            continue;
        }
        LOG_DEBUG(
            log,
            "Selected {} projection_parts from {} to {}",
            projection_parts.size(),
            projection_parts.front()->name,
            projection_parts.back()->name);
        FutureMergedMutatedPart projection_future_part;