**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 :)

{
            for (const auto * child : action.node->children)
                columns_alter_type_forbidden.insert(child->result_name);
        }
        
        for (const String & col : partition_key_expr->getRequiredColumns())
            columns_alter_type_check_safe_for_partition.insert(col);
    }
    for (const auto & index : old_metadata.getSecondaryIndices())
    {
        for (const String & col : index.expression->getRequiredColumns())
            columns_alter_type_forbidden.insert(col);
    }
    if (old_metadata.hasSortingKey())
    {
        auto sorting_key_expr = old_metadata.getSortingKey().expression;
        for (const auto & action : sorting_key_expr->getActions())
        {
            for (const auto * child : action.node->children)
                columns_alter_type_forbidden.insert(child->result_name);
        }
        for (const String & col : sorting_key_expr->getRequiredColumns())
            columns_alter_type_metadata_only.insert(col);
        
        
    }
    if (!merging_params.sign_column.empty())
        columns_alter_type_forbidden.insert(merging_params.sign_column);
    
    NameSet columns_in_keys;