cursor_result_set.hpp

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#pragma once
 
/// @file userver/storages/mysql/cursor_result_set.hpp
 
#include <userver/storages/mysql/statement_result_set.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace storages::mysql {
 
/// @brief A wrapper for read-only cursor.
///
/// Although this class can be constructed from StatementResultSet, you should
/// always retrieve it from `storages::mysql::Cluster` for correct behavior.
template <typename T>
class CursorResultSet final {
public:
    explicit CursorResultSet(StatementResultSet&& result_set);
    ~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.
    // TODO : deadline?
    template <typename RowCallback>
    void ForEach(RowCallback&& row_callback, engine::Deadline deadline) &&;
 
private:
    StatementResultSet result_set_;
};
 
template <typename T>
CursorResultSet<T>::CursorResultSet(StatementResultSet&& result_set)
    : result_set_{std::move(result_set)}
{}
 
template <typename T>
CursorResultSet<T>::~CursorResultSet() = default;
 
template <typename T>
CursorResultSet<T>::CursorResultSet(CursorResultSet<T>&& other) noexcept = default;
 
template <typename T>
template <typename RowCallback>
void CursorResultSet<T>::ForEach(
    RowCallback&& row_callback,
    // TODO : think about separate deadline here
    [[maybe_unused]] engine::Deadline deadline
) && {
    using IntermediateStorage = std::vector<T>;
 
    bool keep_going = true;
    auto extractor = impl::io::TypedExtractor<IntermediateStorage, T, RowTag>{};
 
    while (keep_going) {
        tracing::ScopeTime fetch{impl::tracing::kFetchScope};
        keep_going = result_set_.FetchResult(extractor);
 
        fetch.Reset(impl::tracing::kForEachScope);
        IntermediateStorage data{extractor.ExtractData()};
        for (auto&& row : data) {
            row_callback(std::move(row));
        }
    }
}
 
}  // namespace storages::mysql
 
USERVER_NAMESPACE_END