And waits for them. That's pointless.
}
        else if (command.type == MutationCommand::Type::DROP_COLUMN)
        {
            ISerialization::StreamCallback callback = [&](const ISerialization::SubstreamPath & substream_path)
            {
                String stream_name = ISerialization::getFileNameForStream({command.column_name, command.data_type}, substream_path);
                
                if (--stream_counts[stream_name] == 0)
                {
                    rename_vector.emplace_back(stream_name + ".bin", "");
                    rename_vector.emplace_back(stream_name + mrk_extension, "");
                }
            };
            auto column = source_part->getColumns().tryGetByName(command.column_name);
            if (column)
            {
                auto serialization = source_part->getSerializationForColumn(*column);
                serialization->enumerateStreams(callback);
            }
        }
        else if (command.type == MutationCommand::Type::RENAME_COLUMN)
        {
            String escaped_name_from = escapeForFileName(command.column_name);
            String escaped_name_to = escapeForFileName(command.rename_to);
            ISerialization::StreamCallback callback = [&](const ISerialization::SubstreamPath & substream_path)
            {
                String stream_from = ISerialization::getFileNameForStream({command.column_name, command.data_type}, substream_path);
                String stream_to = boost::replace_first_copy(stream_from, escaped_name_from, escaped_name_to);
                if (stream_from != stream_to)
                {