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Eager fetch n+1 problem

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mschipperheyn

Post subject: Eager fetch n+1 problem

Posted: Fri Apr 02, 2010 5:48 am

Pro

Joined: Wed Nov 05, 2003 7:22 pm
Posts: 211

Hi,

I have two classes
Offer and Visibility. There is a 1-1 relation between Offer and visibility

Code:

Offer
        @OneToOne(optional = false, cascade = CascadeType.ALL, fetch = FetchType.EAGER)
   @JoinColumn(name = "FK_VisibilityID",nullable=false,updatable=false)
   @IndexedEmbedded
   public Visibility getVisibility() {
      return visibility;
   }
Visibility
@OneToOne(mappedBy="visibility")
   public Offer getOffer() {
      return offer;
   }

When I make a selection of offers, Hibernate always executes #records queries against the visibility table. Even when I use fetchmode join in the query. This is not great for performance and I just want Hibernate to retrieve the Offer-visibility pair in one go. I think it actually does but then redundantly retrieves the visibility separately

This are examples of a query with the problem:

Code:

   public List<Offer> getOffers(Long[] ids) {
      DetachedCriteria dc = DetachedCriteria.forClass(Offer.class);
      dc.setFetchMode("visibility", FetchMode.JOIN).add(
            Restrictions.in("id", ids));
      return getHibernateTemplate().findByCriteria(dc);
   }

a HQL one

        from Offer o inner join fetch o.visibility 

What I will see on these queries is the query being executed and then #records separate queries against the visibility table based on visibilityId. This all happens in the context of the execution of the query

Any ideas?

Kind regards,

Marc

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mschipperheyn

Post subject: Re: Eager fetch n+1 problem

Posted: Sun Apr 04, 2010 6:08 pm

Pro

Joined: Wed Nov 05, 2003 7:22 pm
Posts: 211

Ok, the reason is

Code:

Visibility
@OneToOne(mappedBy="visibility")
   public Offer getOffer() {
      return offer;
   }

If I map it like so

Code:

Visibility
@Transient
   public Offer getOffer() {
      return offer;
   }

or remove it entirely, I don't have the problem anymore. The underlying reason appears to be that if there is a OneToOne mapping on this, Hibernate needs to know whether it is null or not and executes the extra query. Seems to me that Hibernate should be able to deduce the complete state of Visibility from the eagerly fetched visibility relation from the Offer side, but there you have it.

Kind regards,

Marc

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