休眠：实体图的n + 1和性能降低

Question

We did an upgrade to Hibernate 5, after which we began to experience performance issues.

We have several entities with associations like this:

@Entity
@Table(name = "EVENT")
public class Event {
    @Id
    @Column(name = "ID")
    private Long id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "LOCATION", referencedColumnName = "ID")
    private Location location;
}

@Entity
@Table(name = "LOCATION")
public class Location {
    @Id
    @Column(name = "ID")
    private Long id;
}

We are using the Criteria API to fetch the data from the database.

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Event> query = cb.createQuery(Event.class);
Root<Event> from = query.from(Event.class);
query.select(from).where(from.get("id").in(1, 2, 3));
TypedQuery<Event> tQuery = entityManager.createQuery(query);
tQuery.setMaxResults(1000);
tQuery.getResultList();

Previously (version 4, old Criteria API), Hibernate generated one select with a join statement that fetched all the data, just based on the FetchType.EAGER, but with Hibernate 5, it creates multiple additional queries for fetching the 'location' data - the N+1 problem.

Now, we have tried JPA Entity Graph, with mixed results. We were able to reduce the number of queries (no N+1 now), but on the other hand, the performance of the system is even slower.

My questions are:

1. what other ways are there to remove N+1 queries problem?
2. under what circumstances could Entity Graphs have negative performance impact?

(We use SQL Server, Tomcat, Hibernate 5.2.10, Java 8.)

Answer 1

To get the earlier behaviour of fetching location data along with join in a single query can be achieved by adding the fetch

from.fetch("location", javax.persistence.criteria.JoinType.LEFT);

So your code would look like:

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Event> query = cb.createQuery(Event.class);
Root<Event> from = query.from(Event.class);
from.fetch("location", javax.persistence.criteria.JoinType.LEFT);
query.select(from).where(from.get("id").in(1, 2, 3));
TypedQuery<Event> tQuery = entityManager.createQuery(query);
tQuery.setMaxResults(1000);
tQuery.getResultList();