How to get rid of N+1 with JPA Criteria API in Hibernate

Dmitry Senkovich :

I'm migrating our DAO from using Hibernate Criteria API to JPA Criteria API. I've got a class with several @ManyToOne there:

@Entity
@Table(name = "A_TABLE", schema="SCHEMA_NAME")
public class A {
    @ManyToOne
    @JoinFormula("(SELECT * FROM (SELECT B.B_ID FROM SCHEMA_NAME.B_TABLE B WHERE B.A_ID = B_ID AND (B.B_CODE = '1' OR B.B_CODE = '2') ORDER BY B.B_CREATED_TIMESTAMP DESC) WHERE ROWNUM = 1)")
    private B b1;
    @ManyToOne
    @JoinFormula("(SELECT * FROM (SELECT B.B_ID FROM SCHEMA_NAME.B_TABLE B WHERE B.A_ID = B_ID AND (B.B_CODE = '3' OR B.B_CODE = '4') ORDER BY B.B_CREATED_TIMESTAMP DESC) WHERE ROWNUM = 1)")
    private B b2;
    ...
}

@Entity
@Table(name = "B_TABLE", schema="SCHEMA_NAME")
public class B {

}

In a query I'm using JoinType.LEFT in order to get rid of generated by default CROSS JOIN's:

if (LEFT_OUTER_JOIN_ENTITIES.contains(field)) {
    path = ((From) path).join(field, JoinType.LEFT);
} else {
    path = path.get(field);
}

I'm getting right results, all the A and B records are retrieved correctly. However, after the migration I'm getting n+1 problem: all the B records are retrieved one-by-one despite using LEFT OUTER JOIN's in generated queries. Previously (when using Hibernate Criteria API) Hibernate was able to satisfy the query without n+1 having the same joins in generated SQL.

Thank you for any ideas and help!

UPDATE As an example of using the if-else above for search by "b1.fieldName" I would get the following:

criteriaBuilder.equal(root.join("b1", JoinType.LEFT).get("someFiled"), 42)

Vlad Mihalcea :

The N+1 issue comes from the default EAGER fetching strategy of the @ManyToOne associations.

So, you need to switch to FetchType.LAZY, like this:

@ManyToOne(fetch = FetchType.LAZY)
@JoinFormula("(SELECT * FROM (SELECT B.B_ID FROM SCHEMA_NAME.B_TABLE B WHERE B.A_ID = B_ID AND (B.B_CODE = '1' OR B.B_CODE = '2') ORDER BY B.B_CREATED_TIMESTAMP DESC) WHERE ROWNUM = 1)")
private B b1;

@ManyToOne(fetch = FetchType.LAZY)
@JoinFormula("(SELECT * FROM (SELECT B.B_ID FROM SCHEMA_NAME.B_TABLE B WHERE B.A_ID = B_ID AND (B.B_CODE = '3' OR B.B_CODE = '4') ORDER BY B.B_CREATED_TIMESTAMP DESC) WHERE ROWNUM = 1)")
private B b2;

If you want to automatically detect N+1 issues automatically that might affect other parts of your application, check out this article.

If you need to fetch the association eagerly with Criteria API, then you should use fetch instead of join.

if (LEFT_OUTER_JOIN_ENTITIES.contains(field)) {
    path = ((From) path).fetch(field, JoinType.LEFT);
} else {
    path = path.get(field);
}