ClickHouse/dbms/src/Interpreters/RowRefs.h

#pragma once

#include <Common/RadixSort.h>
#include <Columns/IColumn.h>

#include <optional>
#include <variant>
#include <list>
#include <mutex>
#include <algorithm>

namespace DB
{

class Block;

/// Reference to the row in block.
struct RowRef
{
    const Block * block = nullptr;
    size_t row_num = 0;

    RowRef() {}
    RowRef(const Block * block_, size_t row_num_) : block(block_), row_num(row_num_) {}
};

/// Single linked list of references to rows. Used for ALL JOINs (non-unique JOINs)
struct RowRefList : RowRef
{
    RowRefList * next = nullptr;

    RowRefList() {}
    RowRefList(const Block * block_, size_t row_num_) : RowRef(block_, row_num_) {}
};

/**
 * This class is intended to push sortable data into.
 * When looking up values the container ensures that it is sorted for log(N) lookup
 * After calling any of the lookup methods, it is no longer allowed to insert more data as this would invalidate the
 * references that can be returned by the lookup methods
 */

template <typename TEntry, typename TKey>
class SortedLookupVector
{
public:
    using Base = std::vector<TEntry>;

    // First stage, insertions into the vector
    template <typename U, typename ... TAllocatorParams>
    void insert(U && x, TAllocatorParams &&... allocator_params)
    {
        assert(!sorted.load(std::memory_order_acquire));
        array.push_back(std::forward<U>(x), std::forward<TAllocatorParams>(allocator_params)...);
    }

    // Transition into second stage, ensures that the vector is sorted
    typename Base::const_iterator upper_bound(const TEntry & k)
    {
        sort();
        return std::upper_bound(array.cbegin(), array.cend(), k);
    }

    // After ensuring that the vector is sorted by calling a lookup these are safe to call
    typename Base::const_iterator cbegin() const { return array.cbegin(); }
    typename Base::const_iterator cend() const { return array.cend(); }

private:
    std::atomic<bool> sorted = false;
    Base array;
    mutable std::mutex lock;

    struct RadixSortTraits : RadixSortNumTraits<TKey>
    {
        using Element = TEntry;
        static TKey & extractKey(Element & elem) { return elem.asof_value; }
    };

    // Double checked locking with SC atomics works in C++
    // https://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/
    // The first thread that calls one of the lookup methods sorts the data
    // After calling the first lookup method it is no longer allowed to insert any data
    // the array becomes immutable
    void sort()
    {
        if (!sorted.load(std::memory_order_acquire))
        {
            std::lock_guard<std::mutex> l(lock);
            if (!sorted.load(std::memory_order_relaxed))
            {
                if (!array.empty())
                {
                    /// TODO: It has been tested only for UInt32 yet. It needs to check UInt64, Float32/64.
                    if constexpr (std::is_same_v<TKey, UInt32>)
                        RadixSort<RadixSortTraits>::execute(&array[0], array.size());
                    else
                        std::sort(array.begin(), array.end());
                }

                sorted.store(true, std::memory_order_release);
            }
        }
    }
};

class AsofRowRefs
{
public:
    template <typename T>
    struct Entry
    {
        using LookupType = SortedLookupVector<Entry<T>, T>;
        using LookupPtr = std::unique_ptr<LookupType>;
        T asof_value;
        RowRef row_ref;

        Entry(T v) : asof_value(v) {}
        Entry(T v, RowRef rr) : asof_value(v), row_ref(rr) {}

        bool operator < (const Entry & o) const
        {
            return asof_value < o.asof_value;
        }
    };

    using Lookups = std::variant<
        Entry<UInt32>::LookupPtr,
        Entry<UInt64>::LookupPtr,
        Entry<Float32>::LookupPtr,
        Entry<Float64>::LookupPtr>;

    enum class Type
    {
        key32,
        key64,
        keyf32,
        keyf64,
    };

    AsofRowRefs() {}
    AsofRowRefs(Type t);

    static std::optional<Type> getTypeSize(const IColumn * asof_column, size_t & type_size);

    // This will be synchronized by the rwlock mutex in Join.h
    void insert(Type type, const IColumn * asof_column, const Block * block, size_t row_num);

    // This will internally synchronize
    const RowRef * findAsof(Type type, const IColumn * asof_column, size_t row_num) const;

private:
    // Lookups can be stored in a HashTable because it is memmovable
    // A std::variant contains a currently active type id (memmovable), together with a union of the types
    // The types are all std::unique_ptr, which contains a single pointer, which is memmovable.
    // Source: https://github.com/yandex/ClickHouse/issues/4906
    Lookups lookups;
};

}