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Re: [eclipselink-users] Performance problem with hierarchical tree structure (JPA) 
Hi Rune,
What is your goal? Do you want the whole tree read in in one SQL statement? I'd be careful about this because the number of joins required to do this gets exponentially big. For that reason, there is not really a way to specify a recursive fetch join in EclipseLink.
Have you considered making your Node.childNodes collection LAZY? That would certainly reduce the amount of SQL you see on the initial read and reduce the SQL on subsequent reads as well.
In your example with the NPE, you are likely seeing the issue because you need to fetch join t.top before you can fetch join t.top.childNodes. 

e.g.

<named-query name="treeQuery">
  <query>select t
         from Tree t
         where t.name = ?1
  </query>
  <hint name="eclipselink.join-fetch" value="t.top"/>
  <hint name="eclipselink.join-fetch" value="t.top.childNodes"/>
</named-query>

-Tom

Rune Kvamme wrote:

Hi,
I have a performance issue that I hope to receive some advice on. I know that one of the major benefits with EclipseLink is its ability to tune the generated selects, and I need some help with the following case. 


Iv created a small test case to illustrate the problem.
I have two classes which represents a hierarchical tree structure. The "Tree" class contains one top Node and each node may contain zero or more child nodes. 


@Entity
public class Tree {
@Id 
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
@Version 
    private Long version;
@Basic 
    private String name;
@OneToOne(cascade = CascadeType.ALL) 
    private Node top;
... 
    ...
} 


@Entity
@Table(name = "NODE")
public class Node extends Tree {
@Id 
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
@Version 
    private Long version;
@Basic 
    private String name;
@Basic 
    private String xyz;
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER) 
    private List<Node> childNodes;
... 
    ...
    ...
}



I have the following JPQL query;



    <query>select t
           from Tree t
           where t.name = ?1
    </query>
When issuing the query EclipseLink generates the following queries (assuming that nothing is in the cache). 


1. On query to get the Tree object.
2. On query to get the Top Node object.
3. Then one query for each Node in the tree structure to get the nodes children.
I have tried to use different types of hints, e.g. eclipselink.join-fetch, but without any luck. 
The below hint just caused a null pointer exception.


  <named-query name="treeQuery">
    <query>select t
           from Tree t
           where t.name = ?1
    </query>
    <hint name="eclipselink.join-fetch" value="t.top.childNodes"/>


 </named-query>
What am i doing wrong. Is there a way to achieve this without eclipslink generating so many queries? 



Please help:-)



Best regards,
Rune


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