Hibernate N+1 query problem with @OneToMany collections

Loading a list of 100 Order entities and then accessing each order's items collection triggers 1 query for the orders plus 100 individual queries for the items — one per order. This N+1 pattern silently degrades performance at scale.

Use JOIN FETCH in JPQL, EntityGraph, or Hibernate's @BatchSize:

// Option 1: JOIN FETCH in JPQL
@Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.userId = :userId")
List<Order> findWithItemsByUserId(@Param("userId") Long userId);

// Option 2: EntityGraph
@EntityGraph(attributePaths = {"items", "items.product"})
List<Order> findByUserId(Long userId);

// Option 3: @BatchSize (reduces N+1 to ceil(N/batchSize)+1 queries)
@OneToMany(mappedBy = "order")
@BatchSize(size = 25)
private List<OrderItem> items;

Detect N+1 in tests with hibernate-statistics or datasource-proxy to count SQL queries.

Hibernate defaults to LAZY loading for collections. Accessing the collection outside the original query triggers a new SELECT per entity to hydrate the collection. This is correct behavior by design but must be anticipated for list endpoints.