Method generate

src/Storages/StorageFile.cpp:358–467

    }

    Chunk generate() override
    {
        while (!finished_generate)
        {
            /// Open file lazily on first read. This is needed to avoid too many open files from different streams.
            if (!reader)
            {
                if (!storage->use_table_fd)
                {
                    auto current_file = files_info->next_file_to_read.fetch_add(1);
                    if (current_file >= files_info->files.size())
                        return {};

                    current_path = files_info->files[current_file];

                    /// Special case for distributed format. Defaults are not needed here.
                    if (storage->format_name == "Distributed")
                    {
                        pipeline = std::make_unique<QueryPipeline>();
                        pipeline->init(Pipe(StorageDistributedDirectoryMonitor::createSourceFromFile(current_path)));
                        reader = std::make_unique<PullingPipelineExecutor>(*pipeline);
                        continue;
                    }
                }

                std::unique_ptr<ReadBuffer> nested_buffer;
                CompressionMethod method;

                if (storage->use_table_fd)
                {
                    nested_buffer = std::make_unique<ReadBufferFromFileDescriptor>(storage->table_fd);
                    method = chooseCompressionMethod("", storage->compression_method);
                }
                else
                {
                    nested_buffer = std::make_unique<ReadBufferFromFile>(current_path, context->getSettingsRef().max_read_buffer_size);
                    method = chooseCompressionMethod(current_path, storage->compression_method);
                }

                /// For clickhouse-local add progress callback to display progress bar.
                if (context->getApplicationType() == Context::ApplicationType::LOCAL)
                {
                    auto & in = static_cast<ReadBufferFromFileDescriptor &>(*nested_buffer);
                    in.setProgressCallback(context);
                }

                read_buf = wrapReadBufferWithCompressionMethod(std::move(nested_buffer), method);

                auto get_block_for_format = [&]() -> Block
                {
                    if (storage->isColumnOriented())
                        return storage_snapshot->getSampleBlockForColumns(columns_description.getNamesOfPhysical());
                    return storage_snapshot->metadata->getSampleBlock();
                };

                auto format = FormatFactory::instance().getInput(
                    storage->format_name, *read_buf, get_block_for_format(), context, max_block_size, storage->format_settings);
                format->setQueryInfo(query_info, context);