And waits for them. That's pointless.
index_factory.getMany(metadata_snapshot->getSecondaryIndices()),
        compression_codec,
        blocks_are_granules_size};
    merged_stream->readPrefix();
    to.writePrefix();
    size_t rows_written = 0;
    const size_t initial_reservation = space_reservation ? space_reservation->getSize() : 0;
    auto is_cancelled = [&]() { return merges_blocker.isCancelled()
        || (need_remove_expired_values && ttl_merges_blocker.isCancelled()); };
    Block block;
    while (!is_cancelled() && (block = merged_stream->read()))
    {
        rows_written += block.rows();
        to.write(block);
        merge_entry->rows_written = merged_stream->getProfileInfo().rows;
        merge_entry->bytes_written_uncompressed = merged_stream->getProfileInfo().bytes;
        
        if (space_reservation && sum_input_rows_upper_bound)
        {
            
            
            Float64 progress = (chosen_merge_algorithm == MergeAlgorithm::Horizontal)
                ? std::min(1., 1. * rows_written / sum_input_rows_upper_bound)
                : std::min(1., merge_entry->progress.load(std::memory_order_relaxed));
            space_reservation->update(static_cast<size_t>((1. - progress) * initial_reservation));
        }
    }
    merged_stream->readSuffix();
    merged_stream.reset();
    if (merges_blocker.isCancelled())