ClickHouse/src/Processors/Transforms/WindowTransform.h

#pragma once

#include <Interpreters/WindowDescription.h>

#include <Processors/IProcessor.h>

#include <Common/AlignedBuffer.h>

#include <deque>

namespace DB
{

class ExpressionActions;
using ExpressionActionsPtr = std::shared_ptr<ExpressionActions>;

class Arena;

// Runtime data for computing one window function.
struct WindowFunctionWorkspace
{
    AggregateFunctionPtr aggregate_function;

    // This field is set for pure window functions. When set, we ignore the
    // window_function.aggregate_function, and work through this interface
    // instead.
    IWindowFunction * window_function_impl = nullptr;

    std::vector<size_t> argument_column_indices;

    // Will not be initialized for a pure window function.
    AlignedBuffer aggregate_function_state;

    // Argument columns. Be careful, this is a per-block cache.
    std::vector<const IColumn *> argument_columns;
    uint64_t cached_block_number = std::numeric_limits<uint64_t>::max();
};

struct WindowTransformBlock
{
    Columns input_columns;
    MutableColumns output_columns;

    size_t rows = 0;
};

struct RowNumber
{
    uint64_t block = 0;
    uint64_t row = 0;

    bool operator < (const RowNumber & other) const
    {
        return block < other.block
            || (block == other.block && row < other.row);
    }

    bool operator == (const RowNumber & other) const
    {
        return block == other.block && row == other.row;
    }

    bool operator <= (const RowNumber & other) const
    {
        return *this < other || *this == other;
    }
};

/*
 * Computes several window functions that share the same window. The input must
 * be sorted by PARTITION BY (in any order), then by ORDER BY.
 * We need to track the following pointers:
 * 1) boundaries of partition -- rows that compare equal w/PARTITION BY.
 * 2) current row for which we will compute the window functions.
 * 3) boundaries of the frame for this row.
 * Both the peer group and the frame are inside the partition, but can have any
 * position relative to each other.
 * All pointers only move forward. For partition boundaries, this is ensured by
 * the order of input data. This property also trivially holds for the ROWS and
 * GROUPS frames. For the RANGE frame, the proof requires the additional fact
 * that the ranges are specified in terms of (the single) ORDER BY column.
 */
class WindowTransform : public IProcessor /* public ISimpleTransform */
{
public:
    WindowTransform(
            const Block & input_header_,
            const Block & output_header_,
            const WindowDescription & window_description_,
            const std::vector<WindowFunctionDescription> &
                functions);

    ~WindowTransform() override;

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

    static Block transformHeader(Block header, const ExpressionActionsPtr & expression);

    /*
     * (former) Implementation of ISimpleTransform.
     */
    void appendChunk(Chunk & chunk) /*override*/;

    /*
     * Implementation of IProcessor;
     */
    Status prepare() override;
    void work() override;

private:
    void advancePartitionEnd();

    bool arePeers(const RowNumber & x, const RowNumber & y) const;

    void advanceFrameStartRowsOffset();
    void advanceFrameStartRangeOffset();
    void advanceFrameStart();

    void advanceFrameEndRowsOffset();
    void advanceFrameEndCurrentRow();
    void advanceFrameEndUnbounded();
    void advanceFrameEnd();
    void advanceFrameEndRangeOffset();

    void updateAggregationState();
    void writeOutCurrentRow();

    Columns & inputAt(const RowNumber & x)
    {
        assert(x.block >= first_block_number);
        assert(x.block - first_block_number < blocks.size());
        return blocks[x.block - first_block_number].input_columns;
    }

    const Columns & inputAt(const RowNumber & x) const
    { return const_cast<WindowTransform *>(this)->inputAt(x); }

    auto & blockAt(const uint64_t block_number)
    {
        assert(block_number >= first_block_number);
        assert(block_number - first_block_number < blocks.size());
        return blocks[block_number - first_block_number];
    }

    const auto & blockAt(const uint64_t block_number) const
    { return const_cast<WindowTransform *>(this)->blockAt(block_number); }

    auto & blockAt(const RowNumber & x)
    { return blockAt(x.block); }

    const auto & blockAt(const RowNumber & x) const
    { return const_cast<WindowTransform *>(this)->blockAt(x); }

    size_t blockRowsNumber(const RowNumber & x) const
    {
        return blockAt(x).rows;
    }

    MutableColumns & outputAt(const RowNumber & x)
    {
        assert(x.block >= first_block_number);
        assert(x.block - first_block_number < blocks.size());
        return blocks[x.block - first_block_number].output_columns;
    }

    void advanceRowNumber(RowNumber & x) const
    {
        assert(x.block >= first_block_number);
        assert(x.block - first_block_number < blocks.size());

        const auto block_rows = blockAt(x).rows;
        assert(x.row < block_rows);

        x.row++;
        if (x.row < block_rows)
        {
            return;
        }

        x.row = 0;
        ++x.block;
    }

    void retreatRowNumber(RowNumber & x) const
    {
        if (x.row > 0)
        {
            --x.row;
            return;
        }

        --x.block;
        assert(x.block >= first_block_number);
        assert(x.block < first_block_number + blocks.size());
        assert(blockAt(x).rows > 0);
        x.row = blockAt(x).rows - 1;

#ifndef NDEBUG
        auto xx = x;
        advanceRowNumber(xx);
        assert(xx == x);
#endif
    }

    auto moveRowNumber(const RowNumber & _x, int offset) const;
    auto moveRowNumberNoCheck(const RowNumber & _x, int offset) const;

    void assertValid(const RowNumber & x) const
    {
        assert(x.block >= first_block_number);
        if (x.block == first_block_number + blocks.size())
        {
            assert(x.row == 0);
        }
        else
        {
            assert(x.row < blockRowsNumber(x));
        }
    }

    RowNumber blocksEnd() const
    { return RowNumber{first_block_number + blocks.size(), 0}; }

    RowNumber blocksBegin() const
    { return RowNumber{first_block_number, 0}; }

public:
    /*
     * Data (formerly) inherited from ISimpleTransform, needed for the
     * implementation of the IProcessor interface.
     */
    InputPort & input;
    OutputPort & output;

    bool has_input = false;
    bool input_is_finished = false;
    Port::Data input_data;
    bool has_output = false;
    Port::Data output_data;

    /*
     * Data for window transform itself.
     */
    Block input_header;

    WindowDescription window_description;

    // Indices of the PARTITION BY columns in block.
    std::vector<size_t> partition_by_indices;
    // Indices of the ORDER BY columns in block;
    std::vector<size_t> order_by_indices;

    // Per-window-function scratch spaces.
    std::vector<WindowFunctionWorkspace> workspaces;

    // FIXME Reset it when the partition changes. We only save the temporary
    // states in it (probably?).
    std::unique_ptr<Arena> arena;

    // A sliding window of blocks we currently need. We add the input blocks as
    // they arrive, and discard the blocks we don't need anymore. The blocks
    // have an always-incrementing index. The index of the first block is in
    // `first_block_number`.
    std::deque<WindowTransformBlock> blocks;
    uint64_t first_block_number = 0;
    // The next block we are going to pass to the consumer.
    uint64_t next_output_block_number = 0;
    // The first row for which we still haven't calculated the window functions.
    // Used to determine which resulting blocks we can pass to the consumer.
    RowNumber first_not_ready_row;

    // Boundaries of the current partition.
    // partition_start doesn't point to a valid block, because we want to drop
    // the blocks early to save memory. We still have to track it so that we can
    // cut off a PRECEDING frame at the partition start.
    // The `partition_end` is past-the-end, as usual. When
    // partition_ended = false, it still haven't ended, and partition_end is the
    // next row to check.
    RowNumber partition_start;
    RowNumber partition_end;
    bool partition_ended = false;

    // The row for which we are now computing the window functions.
    RowNumber current_row;
    // The start of current peer group, needed for CURRENT ROW frame start.
    // For ROWS frame, always equal to the current row, and for RANGE and GROUP
    // frames may be earlier.
    RowNumber peer_group_start;

    // Row and group numbers in partition for calculating rank() and friends.
    uint64_t current_row_number = 1;
    uint64_t peer_group_start_row_number = 1;
    uint64_t peer_group_number = 1;

    // The frame is [frame_start, frame_end) if frame_ended && frame_started,
    // and unknown otherwise. Note that when we move to the next row, both the
    // frame_start and the frame_end may jump forward by an unknown amount of
    // blocks, e.g. if we use a RANGE frame. This means that sometimes we don't
    // know neither frame_end nor frame_start.
    // We update the states of the window functions after we find the final frame
    // boundaries.
    // After we have found the final boundaries of the frame, we can immediately
    // output the result for the current row, w/o waiting for more data.
    RowNumber frame_start;
    RowNumber frame_end;
    bool frame_ended = false;
    bool frame_started = false;

    // The previous frame boundaries that correspond to the current state of the
    // aggregate function. We use them to determine how to update the aggregation
    // state after we find the new frame.
    RowNumber prev_frame_start;
    RowNumber prev_frame_end;

    // Comparison function for RANGE OFFSET frames. We choose the appropriate
    // overload once, based on the type of the ORDER BY column. Choosing it for
    // each row would be slow.
    int (* compare_values_with_offset) (
        const IColumn * compared_column, size_t compared_row,
        const IColumn * reference_column, size_t reference_row,
        // We can make it a Field later if we need the Decimals. Now we only
        // have ints and datetime, and the underlying Field type for them is
        // uint64_t anyway.
        uint64_t offset,
        bool offset_is_preceding);
};

}

/// See https://fmt.dev/latest/api.html#formatting-user-defined-types
template <>
struct fmt::formatter<DB::RowNumber>
{
    constexpr auto parse(format_parse_context & ctx)
    {
        auto it = ctx.begin();
        auto end = ctx.end();

        /// Only support {}.
        if (it != end && *it != '}')
            throw format_error("invalid format");

        return it;
    }

    template <typename FormatContext>
    auto format(const DB::RowNumber & x, FormatContext & ctx)
    {
        return format_to(ctx.out(), "{}:{}", x.block, x.row);
    }
};