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. - 

}
        /
        if (isPrimaryOrMinMaxKeyColumnPossiblyWrappedInFunctions(left_in_operand, metadata_snapshot))
            return true;
        if (metadata_snapshot->selected_projection
            && metadata_snapshot->selected_projection->isPrimaryKeyColumnPossiblyWrappedInFunctions(left_in_operand))
            return true;
        return false;
    }
    else
    {
        for (const auto & index : metadata_snapshot->getSecondaryIndices())
            if (index_wrapper_factory.get(index)->mayBenefitFromIndexForIn(left_in_operand))
                return true;
        if (metadata_snapshot->selected_projection
            && metadata_snapshot->selected_projection->isPrimaryKeyColumnPossiblyWrappedInFunctions(left_in_operand))
            return true;
        return isPrimaryOrMinMaxKeyColumnPossiblyWrappedInFunctions(left_in_operand, metadata_snapshot);
    }
}
using PartitionIdToMaxBlock = std::unordered_map<String, Int64>;
static void selectBestProjection(
    const MergeTreeDataSelectExecutor & reader,
    const StorageMetadataPtr & metadata_snapshot,
    const SelectQueryInfo & query_info,
    const Names & required_columns,
    ProjectionCandidate & candidate,
    ContextPtr query_context,
    const PartitionIdToMaxBlock * max_added_blocks,
    const Settings & settings,