JPA & Hibernate: Eager loading performing subsequent queries to fetch all data, instead of doing it in just one query

Question

I have the following doubt. I would like to know why when using JPA and Hibernate, when performing an Eager loading in a ManyToOne or OneToMany relationship, it calls the DB in order to obtain the Entity information but additionally, produces subsequent queries to fetch every child.

On the other side, when using a query with JOIN FETCH it performs the query as I would expect it to be, taking the information all at once, since the fetchType is denoted as "EAGER".

Here it is a simple example:

I have the Class Student which has ManyToOne relationship with the Class Classroom.

@Entity
@Table(name = "STUDENT")
public class Student {

@ManyToOne(optional = true, fetch = FetchType.EAGER)
        @JoinColumn(name = "ClassroomID")
        private Classroom mainClass;

In the other side there is the class named Classroom as follows:

@Entity
public class Classroom {

@OneToMany(cascade = CascadeType.ALL, mappedBy = "mainClass", fetch = FetchType.EAGER)
private List<Student> studentsList;

When obtaining the Classroom object, it performs one query to obtain the information from itself and subsequent queries to obtain the information of each student contained in the studentsList for every classRoom object.

First query:

Hibernate: 
/* SELECT
         r 
     FROM
         Classroom r 
     LEFT JOIN
         r.classStudents */ 

     select
         classroom0_.id as id1_0_,
         classroom0_.number as number2_0_ 
     from
         Classroom classroom0_ 
     left outer join
         STUDENT classstude1_ 
             on classroom0_.id=classstude1_.ClassroomID

And then it performs the next query as many times as Students assigned to each classroom.

  Hibernate: 
      /* load one-to-many com.hw.access.Classroom.classStudents */ 
      select
          classstude0_.ClassroomID as Classroo4_0_1_,
          classstude0_.id as id1_1_1_,
          classstude0_.id as id1_1_0_,
          classstude0_.FIRST_NAME as FIRST_NA2_1_0_,
          classstude0_.LAST_NAME as LAST_NAM3_1_0_,
          classstude0_.ClassroomID as Classroo4_1_0_ 
      from
          STUDENT classstude0_ 
      where
          classstude0_.ClassroomID=?

The question is: Why doesn't it takes the Information all at once? Why doesn't it takes the information in just one query? As it is already performing the Join clause there.

Why just when adding Fetch explicitly in the query, it does what it is requested?

For example:

SELECT
         r 
     FROM
         Classroom r 
     LEFT JOIN FETCH
         r.classStudents */ 

And then, the output query does takes all the information in just one query:

Hibernate: 

        select
              classroom0_.id as id1_0_0_,
              classstude1_.id as id1_1_1_,
              classroom0_.number as number2_0_0_,
              classstude1_.FIRST_NAME as FIRST_NA2_1_1_,
              classstude1_.LAST_NAME as LAST_NAM3_1_1_,
              classstude1_.ClassroomID as Classroo4_1_1_,
              classstude1_.ClassroomID as Classroo4_0_0__,
              classstude1_.id as id1_1_0__ 
          from
              Classroom classroom0_ 
          left outer join
              STUDENT classstude1_ 
                  on classroom0_.id=classstude1_.ClassroomID

Eveevection

Answer 1

I found it described in https://vladmihalcea.com/eager-fetching-is-a-code-smell/ under EAGER fetching inconsistencies:

Both JPQL and Criteria queries default to select fetching, therefore issuing a secondary select for each individual EAGER association. The larger the associations’ number, the more additional individual SELECTS, the more it will affect our application performance.

Also, note that Hibernate similarly ignoreg fetching annotations for HQL queries: https://developer.jboss.org/wiki/HibernateFAQ-AdvancedProblems#jive_content_id_Hibernate_ignores_my_outerjointrue_or_fetchjoin_setting_and_fetches_an_association_lazily_using_n1_selects

Hibernate ignores my outer-join="true" or fetch="join" setting and fetches an association lazily, using n+1 selects!

HQL queries always ignore the setting for outer-join or fetch="join" defined in mapping metadata. This setting applies only to associations fetched using get() or load(), Criteria queries, and graph navigation. If you need to enable eager fetching for a HQL query, use an explicit LEFT JOIN FETCH.

Perrotta

Answer 2

As you have a OneToMany relationship from Classroom to Student, using a single query would cause the Classroom fields to be repeated for each line. Now imagine you have a second OneToMany relationship from Classroom to, say Course; if, for a given Classroom you have N Students and M Courses, you would have a query returning N+M rows, each containing the same fields of class Classroom.

Caudill

Answer 3

By default fetchtype is lazy, that mean if you dont ask for the List in your request Hibernate will not collect it.

In first request you ask for all attribute of Classroom r including the Student list so Hibernate will load them lazily (after finding out that you need them).

But when fetchtype is set to eager hibernate collect it even if you dont ask it.

Ovida