Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

const MergeTreeData::DataPartsVector & parts,
    ProjectionCandidate *& selected_candidate,
    size_t & min_sum_marks)
{
    MergeTreeData::DataPartsVector projection_parts;
    MergeTreeData::DataPartsVector normal_parts;
    for (const auto & part : parts)
    {
        const auto & projections = part->getProjectionParts();
        auto it = projections.find(candidate.desc->name);
        if (it != projections.end())
            projection_parts.push_back(it->second);
        else
            normal_parts.push_back(part);
    }
    if (projection_parts.empty())
        return;
    candidate.merge_tree_data_select_base_cache = std::make_unique<MergeTreeDataSelectCache>();
    candidate.merge_tree_data_select_projection_cache = std::make_unique<MergeTreeDataSelectCache>();
    reader.readFromParts(
        projection_parts,
        candidate.required_columns,
        metadata_snapshot,
        candidate.desc->metadata,
        query_info, /
        query_context,
        0, /
        settings.max_threads,
        max_added_blocks,
        candidate.merge_tree_data_select_projection_cache.get());