วิธีแก้ปัญหา N+1 ใน Spring Data JPA ปี 2026: Fetch Join และ EntityGraph

@Entity
@Table(name = "orders")
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String orderNumber;

    private LocalDateTime createdAt;

    // ManyToOne relationship is lazy by default since JPA 2.0
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "customer_id")
    private Customer customer;

    // OneToMany relationship lazy by default
    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderItem> items = new ArrayList<>();

    // Getters and setters omitted
}

@Entity
@Table(name = "customers")
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    private String email;

    // Getters and setters omitted
}

@Service
@RequiredArgsConstructor
public class OrderService {

    private final OrderRepository orderRepository;

    public List<OrderDto> getAllOrders() {
        // 1 query: SELECT * FROM orders
        List<Order> orders = orderRepository.findAll();

        // For each order, accessing customer triggers a query
        return orders.stream()
            .map(order -> new OrderDto(
                order.getId(),
                order.getOrderNumber(),
                // N queries: SELECT * FROM customers WHERE id = ?
                order.getCustomer().getName()
            ))
            .toList();
    }
}

-- Query 1: fetch orders
SELECT o.id, o.order_number, o.created_at, o.customer_id FROM orders o

-- Queries 2-101: fetch each customer
SELECT c.id, c.name, c.email FROM customers c WHERE c.id = 1
SELECT c.id, c.name, c.email FROM customers c WHERE c.id = 2
SELECT c.id, c.name, c.email FROM customers c WHERE c.id = 3
-- ... 97 more identical queries

# application.yml
spring:
  jpa:
    show-sql: true
    properties:
      hibernate:
        # Format SQL for better readability
        format_sql: true
        # Display session statistics (queries, time)
        generate_statistics: true

logging:
  level:
    # Detailed SQL query logging
    org.hibernate.SQL: DEBUG
    # Display prepared statement parameters
    org.hibernate.orm.jdbc.bind: TRACE

Session Metrics {
    23421 nanoseconds spent acquiring 1 JDBC connection;
    0 nanoseconds spent releasing 0 JDBC connections;
    1254789 nanoseconds spent preparing 101 JDBC statements;
    15478963 nanoseconds spent executing 101 JDBC statements;
    0 nanoseconds spent executing 0 JDBC batches;
}

public interface OrderRepository extends JpaRepository<Order, Long> {

    // Explicit fetch join to load customers
    @Query("SELECT o FROM Order o JOIN FETCH o.customer")
    List<Order> findAllWithCustomer();

    // Multiple fetch join for several associations
    @Query("SELECT o FROM Order o " +
           "JOIN FETCH o.customer c " +
           "JOIN FETCH o.items i")
    List<Order> findAllWithCustomerAndItems();

    // Fetch join with WHERE condition
    @Query("SELECT o FROM Order o " +
           "JOIN FETCH o.customer c " +
           "WHERE o.createdAt > :since")
    List<Order> findRecentOrdersWithCustomer(
        @Param("since") LocalDateTime since
    );
}

-- Single query with join
SELECT o.id, o.order_number, o.created_at, o.customer_id,
       c.id, c.name, c.email
FROM orders o
JOIN customers c ON o.customer_id = c.id

@Service
@RequiredArgsConstructor
public class OrderService {

    private final OrderRepository orderRepository;

    public List<OrderDto> getAllOrders() {
        // Single query with join
        List<Order> orders = orderRepository.findAllWithCustomer();

        // No additional queries
        return orders.stream()
            .map(order -> new OrderDto(
                order.getId(),
                order.getOrderNumber(),
                order.getCustomer().getName() // Already loaded
            ))
            .toList();
    }
}

public interface OrderRepository extends JpaRepository<Order, Long> {

    // Inline EntityGraph with attributePaths
    @EntityGraph(attributePaths = {"customer"})
    List<Order> findAll();

    // EntityGraph with multiple attributes
    @EntityGraph(attributePaths = {"customer", "items"})
    List<Order> findByCreatedAtAfter(LocalDateTime since);

    // Named EntityGraph referencing entity definition
    @EntityGraph(value = "Order.withCustomerAndItems")
    List<Order> findByCustomerId(Long customerId);

    // Combination with custom query
    @EntityGraph(attributePaths = {"customer"})
    @Query("SELECT o FROM Order o WHERE o.orderNumber LIKE :prefix%")
    List<Order> findByOrderNumberPrefix(@Param("prefix") String prefix);
}

@Entity
@Table(name = "orders")
@NamedEntityGraph(
    name = "Order.withCustomer",
    attributeNodes = @NamedAttributeNode("customer")
)
@NamedEntityGraph(
    name = "Order.withCustomerAndItems",
    attributeNodes = {
        @NamedAttributeNode("customer"),
        @NamedAttributeNode("items")
    }
)
@NamedEntityGraph(
    name = "Order.full",
    attributeNodes = {
        @NamedAttributeNode("customer"),
        @NamedAttributeNode(value = "items", subgraph = "items-product")
    },
    subgraphs = @NamedSubgraph(
        name = "items-product",
        attributeNodes = @NamedAttributeNode("product")
    )
)
public class Order {
    // Fields unchanged
}

@Entity
@Table(name = "orders")
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String orderNumber;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "customer_id")
    private Customer customer;

    // Batch fetching on the collection
    @OneToMany(mappedBy = "order")
    @BatchSize(size = 25)
    private List<OrderItem> items = new ArrayList<>();
}

-- Without batch fetching: 100 queries
SELECT * FROM order_items WHERE order_id = 1
SELECT * FROM order_items WHERE order_id = 2
-- ... 98 more queries

-- With @BatchSize(size = 25): 4 queries
SELECT * FROM order_items WHERE order_id IN (1, 2, 3, ..., 25)
SELECT * FROM order_items WHERE order_id IN (26, 27, 28, ..., 50)
SELECT * FROM order_items WHERE order_id IN (51, 52, 53, ..., 75)
SELECT * FROM order_items WHERE order_id IN (76, 77, 78, ..., 100)

# application.yml
spring:
  jpa:
    properties:
      hibernate:
        # Global default batch size
        default_batch_fetch_size: 25

@OneToMany(mappedBy = "order")
@Fetch(FetchMode.SUBSELECT)
private List<OrderItem> items = new ArrayList<>();

-- Generated subselect query
SELECT * FROM order_items
WHERE order_id IN (SELECT id FROM orders WHERE created_at > ?)

public record OrderSummaryDto(
    Long orderId,
    String orderNumber,
    String customerName,
    String customerEmail
) {}

public interface OrderRepository extends JpaRepository<Order, Long> {

    // DTO projection with constructor
    @Query("SELECT new com.example.dto.OrderSummaryDto(" +
           "o.id, o.orderNumber, c.name, c.email) " +
           "FROM Order o JOIN o.customer c")
    List<OrderSummaryDto> findAllOrderSummaries();

    // DTO projection with condition
    @Query("SELECT new com.example.dto.OrderSummaryDto(" +
           "o.id, o.orderNumber, c.name, c.email) " +
           "FROM Order o JOIN o.customer c " +
           "WHERE o.createdAt > :since")
    List<OrderSummaryDto> findRecentOrderSummaries(
        @Param("since") LocalDateTime since
    );
}

SELECT o.id, o.order_number, c.name, c.email
FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE o.created_at > ?

public interface OrderRepository extends JpaRepository<Order, Long> {

    // Dynamic EntityGraph with Specification
    @EntityGraph(attributePaths = {"customer"})
    List<Order> findAll(Specification<Order> spec);

    // Pagination with EntityGraph
    @EntityGraph(attributePaths = {"customer"})
    Page<Order> findByCustomerNameContaining(
        String name,
        Pageable pageable
    );

    // Slice for efficient pagination
    @EntityGraph(attributePaths = {"customer"})
    Slice<Order> findByCreatedAtBefore(
        LocalDateTime date,
        Pageable pageable
    );
}

@Service
@RequiredArgsConstructor
public class OrderService {

    private final OrderRepository orderRepository;
    private final EntityManager entityManager;

    public List<Order> getOrdersWithGraph(String graphName) {
        // Dynamic EntityGraph retrieval
        EntityGraph<?> graph = entityManager
            .getEntityGraph(graphName);

        return entityManager
            .createQuery("SELECT o FROM Order o", Order.class)
            .setHint("jakarta.persistence.loadgraph", graph)
            .getResultList();
    }

    public List<Order> getOrdersForListing() {
        // Minimal graph for listing
        return getOrdersWithGraph("Order.withCustomer");
    }

    public List<Order> getOrdersForDetail() {
        // Full graph for detail view
        return getOrdersWithGraph("Order.full");
    }
}

@DataJpaTest
@AutoConfigureTestDatabase(replace = Replace.NONE)
class OrderRepositoryPerformanceTest {

    @Autowired
    private OrderRepository orderRepository;

    @Autowired
    private EntityManager entityManager;

    @PersistenceContext
    private EntityManager em;

    private Statistics statistics;

    @BeforeEach
    void setUp() {
        // Enable Hibernate statistics
        Session session = entityManager.unwrap(Session.class);
        SessionFactory factory = session.getSessionFactory();
        statistics = factory.getStatistics();
        statistics.setStatisticsEnabled(true);
        statistics.clear();
    }

    @Test
    void findAllWithCustomer_shouldExecuteSingleQuery() {
        // Given: 50 orders in database
        createTestOrders(50);
        entityManager.clear();
        statistics.clear();

        // When: fetch with fetch join
        List<Order> orders = orderRepository.findAllWithCustomer();

        // Then: single query executed
        assertThat(orders).hasSize(50);
        assertThat(statistics.getQueryExecutionCount())
            .as("Should execute only 1 query with fetch join")
            .isEqualTo(1);
    }

    @Test
    void findAll_withoutOptimization_triggersNPlus1() {
        // Given: 50 orders in database
        createTestOrders(50);
        entityManager.clear();
        statistics.clear();

        // When: fetch without optimization
        List<Order> orders = orderRepository.findAll();

        // Access customers to trigger lazy loading
        orders.forEach(o -> o.getCustomer().getName());

        // Then: N+1 queries (51 instead of 1)
        assertThat(statistics.getQueryExecutionCount())
            .as("Should detect N+1 problem")
            .isGreaterThan(1);
    }

    private void createTestOrders(int count) {
        for (int i = 0; i < count; i++) {
            Customer customer = new Customer();
            customer.setName("Customer " + i);
            customer.setEmail("customer" + i + "@test.com");
            entityManager.persist(customer);

            Order order = new Order();
            order.setOrderNumber("ORD-" + i);
            order.setCustomer(customer);
            order.setCreatedAt(LocalDateTime.now());
            entityManager.persist(order);
        }
        entityManager.flush();
    }
}