Details
Major Description
OPENJPA-241 has eliminated some extra SQLs for 2 entities having a LAZY OneToMany, and inverse EAGER ManyToOne relations.
There are still extra SQLs that can be avoided in the cases where 2 entities are related by an EAGER OneToMany, and inverse LAZY ManyToOne relations.
Consider the following 2 entities:
Publisher (1) <---> (M) Magazine
@Entity
public class Publisher implements Serializable {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name="id")
private int id;
@OneToMany(mappedBy="idPublisher", fetch=FetchType.EAGER)
private Set<Magazine> magazineCollection;
...
}
@Entity
public class Magazine implements Serializable {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name="id")
private int id;
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="id_publisher")
private Publisher idPublisher;
...
}
Query = em.createQuery("SELECT p from Publisher p);
Since Publisher has a OneToMany Eager relation (magazines), We generate following SQLs
(1) SELECT t0.id, t0.name FROM Publisher t0
(2) SELECT t0.id, t1.id, t1.date_published, t1.id_publisher, t1.name FROM Publisher t0 INNER JOIN Magazine t1 ON t0.id = t1.id_publisher ORDER BY t0.id ASC
However, the following extra SQLs are also generated (making additional database trips)
(3) SELECT t0.name, t1.id_publisher, t1.id, t1.date_published, t1.name FROM Publisher t0 LEFT OUTER JOIN Magazine t1 ON t0.id = t1.id_publisher WHERE t0.id = ? [params=(int) 2]
... more depending on how many publishers there are in the database.
The fact that SQL(2) has already returned all magazines of all publishers, SQL (3) is unnecessary.
The inverse ManyToOne relation should be established from the SQL (2) result.