ClickHouse/src/Storages/System/StorageSystemZeros.cpp

#include <Storages/System/StorageSystemZeros.h>

#include <Processors/Sources/SourceWithProgress.h>
#include <Processors/Pipe.h>

#include <DataTypes/DataTypesNumber.h>
#include <Columns/ColumnsNumber.h>

namespace DB
{

namespace
{

struct ZerosState
{
    std::atomic<UInt64> num_generated_rows = 0;
};

using ZerosStatePtr = std::shared_ptr<ZerosState>;


/// Source which generates zeros.
/// Uses state to share the number of generated rows between threads.
/// If state is nullptr, then limit is ignored.
class ZerosSource : public SourceWithProgress
{
public:
    ZerosSource(UInt64 block_size, UInt64 limit_, ZerosStatePtr state_)
            : SourceWithProgress(createHeader()), limit(limit_), state(std::move(state_))
    {
        column = createColumn(block_size);
    }

    String getName() const override { return "Zeros"; }

protected:
    Chunk generate() override
    {
        auto column_ptr = column;
        size_t column_size = column_ptr->size();

        if (state)
        {
            auto generated_rows = state->num_generated_rows.fetch_add(column_size, std::memory_order_acquire);

            if (generated_rows >= limit)
                return {};

            if (generated_rows + column_size > limit)
            {
                column_size = limit - generated_rows;
                column_ptr = createColumn(column_size);
            }
        }

        progress({column->size(), column->byteSize()});

        return { Columns {std::move(column_ptr)}, column_size };
    }

private:
    UInt64 limit;
    ZerosStatePtr state;
    ColumnPtr column;

    static Block createHeader()
    {
        return { ColumnWithTypeAndName(ColumnUInt8::create(), std::make_shared<DataTypeUInt8>(), "zero") };
    }

    static ColumnPtr createColumn(size_t size)
    {
        auto column_ptr = ColumnUInt8::create();
        /// It is probably the fastest method to create zero column, cause resize_fill uses memset internally.
        column_ptr->getData().resize_fill(size);

        return column_ptr;
    }
};

}

StorageSystemZeros::StorageSystemZeros(const StorageID & table_id_, bool multithreaded_, std::optional<UInt64> limit_)
    : IStorage(table_id_), multithreaded(multithreaded_), limit(limit_)
{
    setColumns(ColumnsDescription({{"zero", std::make_shared<DataTypeUInt8>()}}));
}

Pipes StorageSystemZeros::read(
        const Names & column_names,
        const SelectQueryInfo &,
        const Context & /*context*/,
        QueryProcessingStage::Enum /*processed_stage*/,
        size_t max_block_size,
        unsigned num_streams)
{
    check(column_names);

    bool use_multiple_streams = multithreaded;

    if (limit && *limit < max_block_size)
    {
        max_block_size = static_cast<size_t>(*limit);
        use_multiple_streams = false;
    }

    if (!use_multiple_streams)
        num_streams = 1;

    Pipes res;
    res.reserve(num_streams);

    ZerosStatePtr state;

    if (limit)
        state = std::make_shared<ZerosState>();

    for (size_t i = 0; i < num_streams; ++i)
    {
        auto source = std::make_shared<ZerosSource>(max_block_size, limit ? *limit : 0, state);

        if (limit && i == 0)
            source->addTotalRowsApprox(*limit);

        res.emplace_back(std::move(source));
    }

    return res;
}

}