Hi all,

While Hibernate supports fetching strategies like batch fetching and subselect fetching which allow to more efficiently load a specific collection for a whole set of root objects, I wanted to ask whether it is also possible to load a collection (or other property) for a user-defined set of root objects.

I'm asking since I'm trying to optimize a performance critical part of an application where I'm loading multiple collections of a whole set of root objects and the batch/subselect fetching strategies do not always optimally load the collections on the specific root objects I want.

More generally, I was wondering what would be the most efficient way to prefetch multiple collections on multiple root objects in Hibernate?

I could think of having a method like:



This would allow to load the given property from a set of objects of the given entity type. Is something like this possible (maybe in some other way?)?

I hope my explanation is clear enough since I'm rather new to Hibernate.

Thanks in advance for any help!

May be you can use a HQL like:

You could add fetch="subselect" in the collection mapping.

If you retrieve your root objects via HQL and access the collection on the first object, all collections for all root objects will be loaded automatically.

Hi Litty,

first of all, thanks for your response!


This is indeed what I want to achieve but if I execute this query and subsequently I access the loaded property on any of the root objects specified in the query, Hibernate nevertheless triggers another query in order to load the same collection.

Somehow, it seems that Hibernate does not associate the loaded collections with their respective root objects. Or am I doing something wrong? I'm using version 3.2.6.GA of Hibernate.

What actually seems to work is a HQL query like the following:

However, the problem here is that the query always returns the root object as well (for every entry of the collection) which I want to avoid since I already have the root objects loaded and I want to load multiple collections for all the root objects so this way I would retrieve lots of redundant data from the database.

as i wrote, use fetch=subselect in the mapping and just access the first collection, it's probably exactly what you want

Hi pkleindl,

I have played around a bit with the subselect fetching strategy and it would actually do what I want. What I need to check is whether it would work fine in our application where we typically have the following pattern:

1. load some root objects
2. load a set of collection properties of the root objects
3. edit the loaded collections

Typically, we do the above several times where sometimes we load the same root objects/collections and sometimes different ones. Thereby, in point 1., we only load root objects which are not in the cache yet so I'm not sure whether this would "confuse" the subselect fetching since the query we issue might not contain all the root objects on which we want to load the collections in point 2.

In any case, I must say that your hint really helped and I will have a closer look at subselect fetching to see whether it fits our needs.

Still, I was wondering whether it would be useful if Hibernate provided a way to explicitly preload some property for a user-defined set of root objects since I think this would be extremely powerful when it comes to optimizing performance critical parts of an application? I just think that the existing fetching strategies do not always do what you want and the fetching strategies can also only be defined once globally for a given property.

If you have an association from propertyName back to EntityClass then you can do a "from PropertyName p where p.entityClass.id in (x,y,z)" and pass a list of ids to load the propertyName object for a dynamically defined list of EntityClass objects.

The subselect always takes the query with which the list of root objects was loaded.

I do actually have such a back reference but if I execute the above query, Hibernate seems not to associate the loaded collections to the root objects so if I later on access the collections, additional queries are executed by Hibernate to retrieve the same collections again.