Databases
StackOverflow2013
Postsdbo
POAvoiding n+1 eager fetching of child collection element association
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POAvoiding n+1 eager fetching of child collection element association
    primarykey
    data
    text
    <p>I have the following classes:</p> <pre><code>@Entity @Table(name = "base") @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name = "DISCRIMINATOR", discriminatorType = DiscriminatorType.STRING) @ForceDiscriminator public class Base { // ... } @Entity @DiscriminatorValue("foo") public class Foo extends Base { @OneToMany( mappedBy = "foo", cascade=CascadeType.ALL ) private List&lt;Bar&gt; bars = new ArrayList&lt;Bar&gt;(); // ... } @Entity public class Bar { @ManyToOne (optional = false) @JoinColumn(name = "foo_id" ) private Foo foo; @OneToOne @JoinColumn(name = "baz_id", nullable = false) private Baz baz; //... } @Entity public class Baz { // ... } </code></pre> <p>Now I basically want to load all <code>Base</code>, but eager load bars when applicable, so I use the following query:</p> <pre><code>SELECT b FROM Base b LEFT JOIN FETCH b.bars </code></pre> <p>While this works, it seems to generate a SELECT N+1 problem for the Bar entities:</p> <pre><code>Hibernate: /* SELECT b FROM Base b LEFT JOIN FETCH b.bars */ SELECT ... Hibernate: /* load com.company.domain.Baz */ SELECT ... Hibernate: /* load com.company.domain.Baz */ SELECT ... </code></pre> <p>Is it possible to tell hibernate to eagerly load an association for each element in the child collection without resorting to N+1 SELECTs?</p> <p>I tried something along the lines of the following query, which obviously does not work since its a collection:</p> <pre><code>SELECT b FROM Base b LEFT JOIN FETCH b.bars LEFT JOIN FETCH b.bars.baz //Results in: illegal attempt to dereference collection [Foo.id.bars] with element property reference [baz] </code></pre> <p>I also tried using <code>IN(b.bars) bars</code>, and while this allows me to reference the child collection, it does not seem to eagerly load the bars collection which is my goal.</p> <p>An explanation of why this happens would also be nice, since I cannot seem to figure it out.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USMorten Jacobsen
    1. USMorten Jacobsen
    plurals
    1. This table or related slice is empty.
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POAvoiding n+1 eager fetching of child collection element association
      1. USSashi
      1. VTFavorite
    2. VO
      singulars
      1. POAvoiding n+1 eager fetching of child collection element association
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POAvoiding n+1 eager fetching of child collection element association
      1. This table or related slice is empty.
      1. VTUpMod
    1. COAre you having 'n+1 selects' issue for Baz or Bar ? In the stacktrace trace that you provided, it looks like the n+1 selects is for 'Baz'.
      singulars
      1. POAvoiding n+1 eager fetching of child collection element association
      1. USSashi
    2. COThat is correct. The collection of Bar entities is eagerly loaded correctly, as expected, but this eager load apparently forces N selects of Baz elements.
      singulars
      1. POAvoiding n+1 eager fetching of child collection element association
      1. USMorten Jacobsen
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload