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

Tackling the N 1 Problem in JPA and Hibernate

The N 1 issue arises when excessive queries are executed to retrieve relational data. For instance, in Hibernate, an initial query fetches N records, and N additional queries are needed to retrieve associated records for each.

To resolve this problem, utilize JOIN FETCH:

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

This JOIN FETCH eager-fetches the post association, eliminating the N 1 queries.

If multiple child associations need fetching, fetch one collection in the initial query and the others with subsequent queries.

Automatic Detection

Integration testing is ideal for detecting the N 1 issue. The db-util project provides an automatic JUnit assert to validate the expected count of SQL statements generated:

@Test
public void shouldNotHaveANPlusOneQuery() {
    // code to configure and execute db-util

    Assert.assertCountOfSqlStatementsPerformed(1);
}

The above is the detailed content of How to Solve the N 1 Problem in JPA and Hibernate?. For more information, please follow other related articles on the PHP Chinese website!