Addressing the N 1 Query Issue in JPA and Hibernate
The N 1 problem arises when a query retrieves N records and subsequent queries fetch related records, leading to additional N queries. This can occur in Hibernate when an association is not fetched during the initial query.
Cause and Effect
To illustrate, consider this JPA query that retrieves PostComment entities:
List<PostComment> comments = entityManager.createQuery("select pc from PostComment pc where pc.review = :review", PostComment.class) .setParameter("review", review) .getResultList();
If we subsequently iterate over the comments and access the post association, Hibernate will issue the following SQL statements:
SELECT pc.id AS id1_1_, pc.post_id AS post_id3_1_, pc.review AS review2_1_ FROM post_comment pc WHERE pc.review = 'Excellent!' INFO - Loaded 3 comments SELECT pc.id AS id1_0_0_, pc.title AS title2_0_0_ FROM post pc WHERE pc.id = 1 INFO - The post title is 'Post nr. 1' SELECT pc.id AS id1_0_0_, pc.title AS title2_0_0_ FROM post pc WHERE pc.id = 2 INFO - The post title is 'Post nr. 2' SELECT pc.id AS id1_0_0_, pc.title AS title2_0_0_ FROM post pc WHERE pc.id = 3 INFO - The post title is 'Post nr. 3'
Here, three additional queries are executed to fetch the Post entities for each Comment. This is the N 1 query issue.
Resolution
To solve this, we can eagerly fetch the required associations using the JOIN FETCH keyword:
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();
For multiple child associations, it's recommended to fetch one collection in the initial query and load the rest through secondary queries.
Automated Detection
To detect the N 1 issue, it's beneficial to implement integration tests with SQL logging and asserts to verify the expected number of generated SQL statements. Tools like db-util can assist in this process.
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