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Re: [eclipselink-users] Lazy loading performance problem
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But fetch join on OneToMany/ManyToMany relationship has problems with
pagination.
The problem seems to be called N+1 select problem. I searched hard but
could found a solution.
It's a problem everyone faces, right? How do you solve it?
Regards,
Warren Tang <http://blog.tangcs.com>
On Friday, November 04, 2011 5:43:32 PM, Leon Derks wrote:
Hello.
I think you can do this with a fetch join like:
SELECT pub FROM Publisher pub JOIN pub.magazines mag
Leon
On Nov 4, 2011, at 10:36 AM, Warren Tang wrote:
Hi, everybody
I have an ExamPaper entity which has many ChoiceQuestions. The
questions are lazy loaded by default (fetch=LAZY).
Now I want to get all the quesitions that belong to a specific
ExamPaper, so I use the following JPQL:
"select p.questions from ExamPaper p where p.id=:paperId";
The problem is that questions are "lazy" loaded, so there is one
"SELECT" executed for *every single* question. But I want to select
them all in one go for performance reasons. What should I do?
------------------ Code listing -----------------------------------
@Entity
public class ExamPaper {
public static final String SELECT_QUESTION_BY_PAPER = "select
p.questions from ExamPaper p where p.id=:paperId";
@Id @GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(fetch = FetchType.LAZY, orphanRemoval = true, cascade =
{CascadeType.ALL})
List<ChoiceQuestion> questions;
... ...
}
query = em.createQuery(ExamPaper.SELECT_QUESTION_BY_PAPER,
ChoiceQuestion.class);
query.setParameter("paperId", examPaperId);
query.setFirstResult(startPosition);
query.setMaxResults(maxResult);
list = query.getResultList();
--
Regards,
Warren Tang <http://blog.tangcs.com/>
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