cursor_result_set.hpp

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#pragma once
 
/// @file userver/storages/sqlite/cursor_result_set.hpp
 
#include <userver/storages/sqlite/result_set.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace storages::sqlite {
 
/// @brief A wrapper for read-only cursor.
///
/// You should always retrieve it from `storages::sqlite::Client` for correct
/// behavior.
template <typename T>
class CursorResultSet final {
public:
    explicit CursorResultSet(ResultSet&& result_set, size_t batch_size);
    ~CursorResultSet();
 
    CursorResultSet(const CursorResultSet& other) = delete;
    CursorResultSet(CursorResultSet&& other) noexcept;
 
    /// @brief Fetches all the rows from cursor and for each new row executes
    /// row_callback.
    ///
    /// Usable when the result set is expected to be big enough to put too
    /// much memory pressure if fetched as a whole.
    template <typename RowCallback>
    void ForEach(RowCallback&& row_callback) &&;
 
private:
    ResultSet result_set_;
    size_t batch_size_;
};
 
template <typename T>
CursorResultSet<T>::CursorResultSet(ResultSet&& result_set, size_t batch_size)
    : result_set_{std::move(result_set)},
      batch_size_{batch_size}
{}
 
template <typename T>
CursorResultSet<T>::CursorResultSet(CursorResultSet<T>&& other) noexcept = default;
 
template <typename T>
CursorResultSet<T>::~CursorResultSet() = default;
 
template <typename T>
template <typename RowCallback>
void CursorResultSet<T>::ForEach(RowCallback&& row_callback) && {
    using IntermediateStorage = std::vector<T>;
 
    bool keep_going = true;
    impl::TypedExtractor<T, RowTag> extractor{*result_set_.pimpl_};
 
    while (keep_going) {
        keep_going = result_set_.FetchResult(extractor, batch_size_);
 
        IntermediateStorage data{extractor.ExtractData()};
        for (auto&& row : data) {
            row_callback(std::move(row));
        }
    }
}
 
}  // namespace storages::sqlite
 
USERVER_NAMESPACE_END