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. - 

{
        auto lock = lockParts();
        const auto it = column_sizes.find(name);
        return it == std::end(column_sizes) ? 0 : it->second.data_compressed;
    }
    ColumnSizeByName getColumnSizes() const override
    {
        auto lock = lockParts();
        return column_sizes;
    }
    
    String getPartitionIDFromQuery(const ASTPtr & ast, ContextPtr context) const;
    
    
    
    MergeTreeData & checkStructureAndGetMergeTreeData(const StoragePtr & source_table, const StorageMetadataPtr & src_snapshot, const StorageMetadataPtr & my_snapshot) const;
    MergeTreeData & checkStructureAndGetMergeTreeData(IStorage & source_table, const StorageMetadataPtr & src_snapshot, const StorageMetadataPtr & my_snapshot) const;
    MergeTreeData::MutableDataPartPtr cloneAndLoadDataPartOnSameDisk(
        const MergeTreeData::DataPartPtr & src_part, const String & tmp_part_prefix, const MergeTreePartInfo & dst_part_info, const StorageMetadataPtr & metadata_snapshot);
    virtual std::vector<MergeTreeMutationStatus> getMutationsStatus() const = 0;
    
    
    virtual bool canUseAdaptiveGranularity() const