Method prepare

src/Processors/Transforms/WindowTransform.cpp:1366–1476

}

IProcessor::Status WindowTransform::prepare()
{
    if (output.isFinished() || isCancelled())
    {
        // The consumer asked us not to continue (or we decided it ourselves),
        // so we abort. Not sure what the difference between the two conditions
        // is, but it seemed that output.isFinished() is not enough to cancel on
        // Ctrl+C. Test manually if you change it.
        input.close();
        return Status::Finished;
    }

    if (output_data.exception)
    {
        // An exception occurred during processing.
        output.pushData(std::move(output_data));
        output.finish();
        input.close();
        return Status::Finished;
    }

    chassert(first_not_ready_row.block >= first_block_number);
    // The first_not_ready_row might be past-the-end if we have already
    // calculated the window functions for all input rows. That's why the
    // equality is also valid here.
    chassert(first_not_ready_row.block <= first_block_number + blocks.size());
    chassert(next_output_block_number >= first_block_number);

    // Output the ready data prepared by work().
    // We inspect the calculation state and create the output chunk right here,
    // because this is pretty lightweight.
    if (next_output_block_number < first_not_ready_row.block)
    {
        if (output.canPush())
        {
            // Output the ready block.
            const auto i = next_output_block_number - first_block_number;
            auto & block = blocks[i];
            auto columns = block.original_input_columns;
            for (auto & res : block.output_columns)
            {
                columns.push_back(ColumnPtr(std::move(res)));
            }
            output_data.chunk.setColumns(columns, block.rows);

            ++next_output_block_number;

            output.pushData(std::move(output_data));
        }

        // We don't need input.setNotNeeded() here, because we already pull with
        // the set_not_needed flag.
        return Status::PortFull;
    }

    if (input_is_finished)
    {
        // The input data ended at the previous prepare() + work() cycle,