And waits for them. That's pointless.
size_t column_elems_written = 0;
            column_to.writePrefix();
            while (!merges_blocker.isCancelled() && (block = column_gathered_stream.read()))
            {
                column_elems_written += block.rows();
                column_to.write(block);
            }
            if (merges_blocker.isCancelled())
                throw Exception("Cancelled merging parts", ErrorCodes::ABORTED);
            column_gathered_stream.readSuffix();
            auto changed_checksums = column_to.writeSuffixAndGetChecksums(new_data_part, checksums_gathered_columns, need_sync);
            checksums_gathered_columns.add(std::move(changed_checksums));
            if (rows_written != column_elems_written)
            {
                throw Exception("Written " + toString(column_elems_written) + " elements of column " + column_name +
                                ", but " + toString(rows_written) + " rows of PK columns", ErrorCodes::LOGICAL_ERROR);
            }
            
            merge_entry->columns_written += 1;
            merge_entry->bytes_written_uncompressed += column_gathered_stream.getProfileInfo().bytes;
            merge_entry->progress.store(progress_before + column_sizes->columnWeight(column_name), std::memory_order_relaxed);
        }
        tmp_disk->removeFile(rows_sources_file_path);
    }
    for (const auto & part : parts)
        new_data_part->minmax_idx.merge(part->minmax_idx);
    
    {
        double elapsed_seconds = merge_entry->watch.elapsedSeconds();
        LOG_DEBUG(log,