ClickHouse/dbms/src/Storages/MergeTree/MergeTreeRangeReader.h

#pragma once
#include <Core/Block.h>
#include <common/logger_useful.h>

namespace DB
{

class MergeTreeReader;

// Used in MergeTreeReader to allow sequential reading for any number of rows between pairs of marks in the same part
class MergeTreeRangeReader
{
public:
    size_t unreadRows() const { return (last_mark - current_mark) * index_granularity - read_rows_after_current_mark; }
    size_t unreadRowsInCurrentGranule() const { return index_granularity - read_rows_after_current_mark; }

    size_t readRowsInCurrentGranule() const { return read_rows_after_current_mark; }

    // seek to next mark before next reading
    size_t skipToNextMark();
    // returns state will be afrer reading rows_to_read, no reading happens
    MergeTreeRangeReader getFutureState(size_t rows_to_read) const;
    // returns the number of rows was read
    size_t read(Block & res, size_t max_rows_to_read);

    bool isReadingFinished() const { return is_reading_finished; }

    void disableNextSeek() { seek_to_from_mark = false; }
    // return the same state for other MergeTreeReader
    MergeTreeRangeReader copyForReader(MergeTreeReader & reader);

private:
    MergeTreeRangeReader(MergeTreeReader & merge_tree_reader, size_t from_mark, size_t to_mark, size_t index_granularity);

    Poco::Logger * logger;

    std::reference_wrapper<MergeTreeReader> merge_tree_reader;
    size_t current_mark;
    size_t last_mark;
    size_t read_rows_after_current_mark;
    size_t index_granularity;
    bool seek_to_from_mark;
    bool is_reading_finished;

    friend class MergeTreeReader;
};

}