How to Solve the N 1 Query Problem in JPA and Hibernate?

What is the Solution for the N 1 Issue in JPA and Hibernate?

The N 1 Query Problem

The N 1 query issue arises when a query retrieves N records and subsequent queries are required to fetch related entities, resulting in N 1 queries.

How to Resolve the N 1 Issue in Hibernate

To resolve this issue, it is crucial to enable proper SQL logging to detect the problem.

Using JOIN FETCH

The primary solution is to use Hibernate's JOIN FETCH to eagerly load the related entities in the initial query. This can be achieved by modifying the query as follows:

List<PostComment> comments = entityManager.createQuery(
"select pc from PostComment pc join fetch pc.post p where pc.review = :review",
PostComment.class)
.setParameter("review", review)
.getResultList();

Additional Considerations

If multiple child associations need to be fetched, consider fetching one collection in the initial query and the additional collections with separate queries.

Automatic Detection with JUnit

Integration tests can be employed to automatically detect the N 1 query issue. Utilizing a framework like db-util can assist in validating the expected count of generated SQL statements.

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