How Can We Address the N 1 Query Issue in JPA and Hibernate?

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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