And waits for them. That's pointless.
if (command.type == MutationCommand::DROP_COLUMN)
            removed_columns.insert(command.column_name);
        if (command.type == MutationCommand::RENAME_COLUMN)
            renamed_columns_to_from.emplace(command.rename_to, command.column_name);
    }
    Names source_column_names = source_part->getColumns().getNames();
    NameSet source_columns_name_set(source_column_names.begin(), source_column_names.end());
    for (auto it = storage_columns.begin(); it != storage_columns.end();)
    {
        if (updated_header.has(it->name))
        {
            auto updated_type = updated_header.getByName(it->name).type;
            if (updated_type != it->type)
                it->type = updated_type;
            ++it;
        }
        else
        {
            if (!source_columns_name_set.count(it->name))
            {
                
                
                auto renamed_it = renamed_columns_to_from.find(it->name);
                if (renamed_it != renamed_columns_to_from.end()
                    && source_columns_name_set.count(renamed_it->second))
                    ++it;
                else
                    it = storage_columns.erase(it);
            }
            else