**Describe the bug**

Since we upgraded to 20.10.3.30, one of our clickhouse cluster takes much more time to start. (Up to 50 minutes, and before it took ~3 minutes)
The log file is filled with
>  <Debug> DiskLocal: Reserving 1.00 MiB on disk `default`, having unreserved 16.90 TiB

Additionally, the memory consumption increases, linearly, a lot (up to arround 230 G for us) and then drop back to normal values once clickhouse finished starting up. During that loading phase, a single CPU core is used at 100%.
If that memory is not available for the machine, clickhouse gets OOMed by the kernel.


**How to reproduce**
* Issue appeared after we upgraded to ClickHouse server version 20.10.3.30

* How could I find wich of our tables may be responsible for that ?


**Expected behavior**
A fast startup :)

const MergeTreeDataSelectExecutor & reader,
    const StorageSnapshotPtr & storage_snapshot,
    const SelectQueryInfo & query_info,
    const ActionDAGNodes & added_filter_nodes,
    const Names & required_columns,
    ProjectionCandidate & candidate,
    ContextPtr query_context,
    std::shared_ptr<PartitionIdToMaxBlock> max_added_blocks,
    const Settings & settings,
    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)