And waits for them. That's pointless.
for (size_t i = 0, size = projections_to_build.size(); i < size; ++i)
        {
            const auto & projection = projections_to_build[i]->projection;
            auto in = InterpreterSelectQuery(
                          projection.query_ast,
                          context,
                          Pipe(std::make_shared<SourceFromSingleChunk>(block, Chunk(block.getColumns(), block.rows()))),
                          SelectQueryOptions{
                              projection.type == ProjectionDescription::Type::Normal ? QueryProcessingStage::FetchColumns : QueryProcessingStage::WithMergeableState})
                          .execute()
                          .getInputStream();
            in = std::make_shared<SquashingBlockInputStream>(in, block.rows(), std::numeric_limits<UInt64>::max());
            in->readPrefix();
            auto & projection_squash = projection_squashes[i];
            auto projection_block = projection_squash.add(in->read());
            if (in->read())
                throw Exception("Projection cannot increase the number of rows in a block", ErrorCodes::LOGICAL_ERROR);
            in->readSuffix();
            if (projection_block)
            {
                projection_parts[projection.name].emplace_back(
                    MergeTreeDataWriter::writeTempProjectionPart(data, log, projection_block, projection, new_data_part.get(), ++block_num));
            }
        }
        merge_entry->rows_written += block.rows();
        merge_entry->bytes_written_uncompressed += block.bytes();
    }
    
    for (size_t i = 0, size = projections_to_build.size(); i < size; ++i)
    {