Explain what the N+1 problem is in using Hibernate and how it affects database performance.

The N+1 problem in Hibernate occurs when loading a collection of related entities results in one query for the main entity (1), and then a separate query for each related entity (N), leading to a large number of database queries.

For example, if you have a list of 10 users and need to load related orders for each, Hibernate might execute 1 query for users and 10 separate queries for orders of each user — totaling 11 queries.

This negatively impacts performance due to the high number of database accesses and increased response time.

To avoid the N+1 problem, you can use:

• Fetch Join — combining queries with JOIN FETCH in JPQL/HQL to load related entities in a single query.
• Batch Fetching — configuring Hibernate for batch loading of related entities.
• Entity Graphs — defining a load graph to specify which associations to load immediately.

Example with Fetch Join:

String hql = "SELECT u FROM User u JOIN FETCH u.orders WHERE u.id = :userId";
User user = session.createQuery(hql, User.class)
                   .setParameter("userId", 1L)
                   .uniqueResult();

Thus, the N+1 problem is excessive queries when loading related data, and its solution significantly improves database operation efficiency in Hibernate.