What is the N+1 problem in JPA/Hibernate? How was it solved?

The N+1 problem in JPA/Hibernate occurs when one query is executed to load the main entity (1), and then a separate query for each related entity (N), leading 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.

Solutions:

• Use JOIN FETCH in JPQL/HQL for eager loading of related entities in a single query:

  SELECT u FROM User u JOIN FETCH u.roles
  

• Configure fetch = FetchType.EAGER for necessary relations, but this is less flexible.
• Use Entity Graphs to specify which relations to load immediately.
• Use batch fetching (@BatchSize) for group loading of related entities.

Thus, the main idea is to reduce the number of queries by loading related data in a single query or in batches.