**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 auto & partition_ast = ast->as<ASTPartition &>();
    if (partition_ast.all)
        throw Exception("Only Support DETACH PARTITION ALL currently", ErrorCodes::SUPPORT_IS_DISABLED);
    if (!partition_ast.value)
    {
        MergeTreePartInfo::validatePartitionID(partition_ast.id, format_version);
        return partition_ast.id;
    }
    if (format_version < MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING)
    {
        /
        const auto * partition_lit = partition_ast.value->as<ASTLiteral>();
        if (partition_lit && partition_lit->value.getType() == Field::Types::String)
        {
            String partition_id = partition_lit->value.get<String>();
            MergeTreePartInfo::validatePartitionID(partition_id, format_version);
            return partition_id;
        }
    }
    /
    auto metadata_snapshot = getInMemoryMetadataPtr();
    const Block & key_sample_block = metadata_snapshot->getPartitionKey().sample_block;
    size_t fields_count = key_sample_block.columns();
    if (partition_ast.fields_count != fields_count)
        throw Exception(ErrorCodes::INVALID_PARTITION_VALUE,
                        "Wrong number of fields in the partition expression: {}, must be: {}",
                        partition_ast.fields_count, fields_count);