Statistics
https://play.clickhouse.com/play?user=play#c2VsZWN0IAp0b1N0YXJ0T2ZIb3VyKGNoZWNrX3N0YXJ0X3RpbWUpIGFzIGQsCmNvdW50KCksICBncm91cFVuaXFBcnJheShwdWxsX3JlcXVlc3RfbnVtYmVyKSwgIGFueShyZXBvcnRfdXJsKQpmcm9tIGNoZWNrcyB3aGVyZSAnMjAyMi0wNi0wMScgPD0gY2hlY2tfc3RhcnRfdGltZSBhbmQgdGVzdF9uYW1lIGxpa2UgJyUwMjQ5MV9wYXJ0X2xvZ19oYXNfdGFibGVfdXVpZCUnIGFuZCB0ZXN0X3N0YXR1cyBpbiAoJ0ZBSUwnLCAnRkxBS1knKSBncm91cCBieSBkIG9yZGVyIGJ5IGQgZGVzYw==

Some times there are not output from requests.
NameSet dropped_columns;
    std::map<String, const IDataType *> old_types;
    for (const auto & column : old_metadata.getColumns().getAllPhysical())
        old_types.emplace(column.name, column.type.get());
    NamesAndTypesList columns_to_check_conversion;
    auto name_deps = getDependentViewsByColumn(local_context);
    for (const AlterCommand & command : commands)
    {
        
        if (command.partition)
        {
            getPartitionIDFromQuery(command.partition, getContext());
        }
        if (command.column_name == merging_params.version_column)
        {
            
            if (command.type == AlterCommand::MODIFY_COLUMN)
            {
                const IDataType * new_type = command.data_type.get();
                const IDataType * old_type = old_types[command.column_name];
                if (new_type)
                    checkVersionColumnTypesConversion(old_type, new_type, command.column_name);
                
                continue;
            }
            else if (command.type == AlterCommand::DROP_COLUMN)
            {
                throw Exception(
                    "Trying to ALTER DROP version " + backQuoteIfNeed(command.column_name) + " column",
                    ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);