**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 :)

/
    /
    std::unordered_set<String> subdirs;
    for (const String & filename : filenames)
    {
        /
        auto separator_pos = filename.rfind('/');
        if (separator_pos != String::npos)
        {
            String subdir = filename.substr(0, separator_pos);
            if (subdirs.emplace(subdir).second)
                disk->createDirectories(temp_part_dir / subdir);
        }
        /
        if (filename.ends_with(IMergeTreeDataPart::TXN_VERSION_METADATA_FILE_NAME))
            continue;
        auto backup_entry = backup->readFile(part_path_in_backup_fs / filename);
        auto read_buffer = backup_entry->getReadBuffer();
        auto write_buffer = disk->writeFile(temp_part_dir / filename);
        copyData(*read_buffer, *write_buffer);
        reservation->update(reservation->getSize() - backup_entry->getSize());
    }
    auto single_disk_volume = std::make_shared<SingleDiskVolume>(disk->getName(), disk, 0);
    auto data_part_storage = std::make_shared<DataPartStorageOnDisk>(single_disk_volume, temp_part_dir.parent_path(), part_name);
    auto part = createPart(part_name, part_info, data_part_storage);
    part->version.setCreationTID(Tx::PrehistoricTID, nullptr);
    part->loadColumnsChecksumsIndexes(false, true);
    restored_parts_holder->addPart(part);
}
String MergeTreeData::getPartitionIDFromQuery(const ASTPtr & ast, ContextPtr local_context, DataPartsLock * acquired_lock) const