ClickHouse/src/Processors/QueryPlan/LimitStep.cpp

#include <Processors/QueryPlan/LimitStep.h>
#include <Processors/QueryPipeline.h>
#include <Processors/LimitTransform.h>
#include <IO/Operators.h>

namespace DB
{

static ITransformingStep::DataStreamTraits getTraits()
{
    return ITransformingStep::DataStreamTraits
    {
            .preserves_distinct_columns = true,
            .returns_single_stream = false,
            .preserves_number_of_streams = true,
            .preserves_number_of_rows = false,
            .preserves_sorting = true,
    };
}

LimitStep::LimitStep(
    const DataStream & input_stream_,
    size_t limit_, size_t offset_,
    bool always_read_till_end_,
    bool with_ties_,
    SortDescription description_)
    : ITransformingStep(input_stream_, input_stream_.header, getTraits())
    , limit(limit_), offset(offset_)
    , always_read_till_end(always_read_till_end_)
    , with_ties(with_ties_), description(std::move(description_))
{
}

void LimitStep::transformPipeline(QueryPipeline & pipeline)
{
    auto transform = std::make_shared<LimitTransform>(
        pipeline.getHeader(), limit, offset, pipeline.getNumStreams(), always_read_till_end, with_ties, description);

    pipeline.addPipe({std::move(transform)});
}

void LimitStep::describeActions(FormatSettings & settings) const
{
    String prefix(settings.offset, ' ');
    settings.out << prefix << "Limit " << limit << '\n';
    settings.out << prefix << "Offset " << offset << '\n';

    if (with_ties || always_read_till_end)
    {
        settings.out << prefix;

        String str;
        if (with_ties)
            settings.out << "WITH TIES";

        if (always_read_till_end)
        {
            if (!with_ties)
                settings.out << ", ";

            settings.out << "Reads all data";
        }

        settings.out << '\n';
    }
}

}