Have you worked with Spring Data and Hibernate? What performance issues did you solve?

Yes, I have worked with Spring Data and Hibernate. The main performance issues I had to address included:

• N+1 queries: A common problem where loading a collection of entities results in many separate database queries. This is resolved using fetch join or configuring @EntityGraph for eager loading.

• Caching: Using Hibernate's second-level cache to reduce database access.

• Query optimization: Rewriting complex queries in JPQL or native SQL for better efficiency.

• Batch processing: For bulk insert/update operations, I used batching to reduce the number of individual queries.

• Lazy Loading vs Eager Loading: Choosing the right loading strategy to minimize load.

Example of solving the N+1 problem:

// Instead of
List<Order> orders = orderRepository.findAll();
// which would trigger N+1 queries when accessing order.getItems()

// Use fetch join
@Query("select o from Order o join fetch o.items")
List<Order> findAllWithItems();

Thus, the key to solving performance issues is analyzing SQL queries, profiling, and effectively utilizing ORM capabilities.