And waits for them. That's pointless.
{
            const String & column_name = it_name_and_type->name;
            Names column_names{column_name};
            Float64 progress_before = merge_entry->progress.load(std::memory_order_relaxed);
            MergeStageProgress column_progress(progress_before, column_sizes->columnWeight(column_name));
            for (size_t part_num = 0; part_num < parts.size(); ++part_num)
            {
                auto column_part_source = std::make_shared<MergeTreeSequentialSource>(
                    data, metadata_snapshot, parts[part_num], column_names, read_with_direct_io, true);
                column_part_source->setProgressCallback(
                    MergeProgressCallback(merge_entry, watch_prev_elapsed, column_progress));
                QueryPipeline column_part_pipeline;
                column_part_pipeline.init(Pipe(std::move(column_part_source)));
                column_part_pipeline.setMaxThreads(1);
                column_part_streams[part_num] =
                        std::make_shared<PipelineExecutingBlockInputStream>(std::move(column_part_pipeline));
            }
            rows_sources_read_buf.seek(0, 0);
            ColumnGathererStream column_gathered_stream(column_name, column_part_streams, rows_sources_read_buf);
            MergedColumnOnlyOutputStream column_to(
                new_data_part,
                metadata_snapshot,
                column_gathered_stream.getHeader(),
                compression_codec,
                
                
                
                std::vector<MergeTreeIndexPtr>{},
                &written_offset_columns,
                to.getIndexGranularity());