**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 :)

}
            else /
            {
                cloned_part = parts_mover.clonePart(moving_part);
                parts_mover.swapClonedPart(cloned_part);
            }
            write_part_log({});
        }
        catch (...)
        {
            write_part_log(ExecutionStatus::fromCurrentException());
            if (cloned_part)
                cloned_part->remove();
            throw;
        }
    }
    return result;
}
bool MergeTreeData::partsContainSameProjections(const DataPartPtr & left, const DataPartPtr & right)
{
    if (left->getProjectionParts().size() != right->getProjectionParts().size())
        return false;
    for (const auto & [name, _] : left->getProjectionParts())
    {
        if (!right->hasProjection(name))
            return false;
    }
    return true;
}
bool MergeTreeData::canUsePolymorphicParts(const MergeTreeSettings & settings, String * out_reason) const