Explain the N+1 problem that can occur when using Hibernate, and how to avoid it.

The N+1 problem in Hibernate occurs when, during the loading of a collection of related entities, one query is executed for the main entity (1), and then a separate query for each of the N related entities. This leads to a large number of database queries and reduced performance.

For example, when loading a list of users and their roles, if roles are loaded lazily, Hibernate will execute 1 query for users and N queries for each user's roles.

To avoid the N+1 problem, the following are used:

• Eager fetching with JOIN FETCH in HQL or the @Fetch(FetchMode.JOIN) annotation.
• Entity Graphs to specify which associations to load immediately.
• Batch fetching — loading related entities in batches.

Example with JOIN FETCH:

String hql = "FROM User u JOIN FETCH u.roles";
List<User> users = session.createQuery(hql, User.class).list();

This allows loading users and their roles with a single query, eliminating the N+1 problem.