虚函数的额外开销

class IAggregateFunction : public std::enable_shared_from_this<IAggregateFunction>
{
    /** The inner loop that uses the function pointer is better than using the virtual function.
      * The reason is that in the case of virtual functions GCC 5.1.2 generates code,
      *  which, at each iteration of the loop, reloads the function address (the offset value in the virtual function table) from memory to the register.
      * This gives a performance drop on simple queries around 12%.
      * After the appearance of better compilers, the code can be removed.
      */
    using AddFunc = void (*)(const IAggregateFunction *, AggregateDataPtr, const IColumn **, size_t, Arena *);
    virtual AddFunc getAddressOfAddFunction() const = 0;
    // codes ...
}

template <typename Derived>
class IAggregateFunctionHelper : public IAggregateFunction
{
private:
    static void addFree(const IAggregateFunction * that, AggregateDataPtr place, const IColumn ** columns, size_t row_num, Arena * arena)
    {
        static_cast<const Derived &>(*that).add(place, columns, row_num, arena);
    }

public:
    AddFunc getAddressOfAddFunction() const override { return &addFree; }
    // codes ...
}