Java – JPA eager fetch does not join

hibernatejavajoinjpa

What exactly does JPA's fetch strategy control? I can't detect any difference between eager and lazy. In both cases JPA/Hibernate does not automatically join many-to-one relationships.

Example: Person has a single address. An address can belong to many people. The JPA annotated entity classes look like:

@Entity
public class Person {
    @Id
    public Integer id;

    public String name;

    @ManyToOne(fetch=FetchType.LAZY or EAGER)
    public Address address;
}

@Entity
public class Address {
    @Id
    public Integer id;

    public String name;
}

If I use the JPA query:

select p from Person p where ...

JPA/Hibernate generates one SQL query to select from Person table, and then a distinct address query for each person:

select ... from Person where ...
select ... from Address where id=1
select ... from Address where id=2
select ... from Address where id=3

This is very bad for large result sets. If there are 1000 people it generates 1001 queries (1 from Person and 1000 distinct from Address). I know this because I'm looking at MySQL's query log. It was my understanding that setting address's fetch type to eager will cause JPA/Hibernate to automatically query with a join. However, regardless of the fetch type, it still generates distinct queries for relationships.

Only when I explicitly tell it to join does it actually join:

select p, a from Person p left join p.address a where ...

Am I missing something here? I now have to hand code every query so that it left joins the many-to-one relationships. I'm using Hibernate's JPA implementation with MySQL.

Edit: It appears (see Hibernate FAQ here and here) that FetchType does not impact JPA queries. So in my case I have explicitly tell it to join.

Best Solution

JPA doesn't provide any specification on mapping annotations to select fetch strategy. In general, related entities can be fetched in any one of the ways given below

  • SELECT => one query for root entities + one query for related mapped entity/collection of each root entity = (n+1) queries
  • SUBSELECT => one query for root entities + second query for related mapped entity/collection of all root entities retrieved in first query = 2 queries
  • JOIN => one query to fetch both root entities and all of their mapped entity/collection = 1 query

So SELECT and JOIN are two extremes and SUBSELECT falls in between. One can choose suitable strategy based on her/his domain model.

By default SELECT is used by both JPA/EclipseLink and Hibernate. This can be overridden by using:

@Fetch(FetchMode.JOIN) 
@Fetch(FetchMode.SUBSELECT)

in Hibernate. It also allows to set SELECT mode explicitly using @Fetch(FetchMode.SELECT) which can be tuned by using batch size e.g. @BatchSize(size=10).

Corresponding annotations in EclipseLink are:

@JoinFetch
@BatchFetch
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