Hibernate lazy collections: What about the collection elements - are they lazy too?

[Darya Akbari]

Hi,

I wonder whether Hibernate creates a Proxy or does a database hit when one try to get an element from a lazy collection.

Any ideas?

Regards,
Darya

[Mark Spritzler]

If you have lazy set, then Hibernate will do a hit to the database to get the data when you hit the Collection. However, you can determine how many times it will hit the database. If you set your fetching strategy to subselect, then it will get the data for the collection in one query. If you have no fetching strategy, then it will hit the database for each element in the database, so if there are 10 elements in the Collection that would be 10 seperate queries.

A good understanding of fetching strategies is very important to make sure you are in control of how much Hibernate hits the database.

Mark

[Darya Akbari]

Originally posted by Mark Spritzler:
If you have lazy set, ...
Mark

I hope it does also work for a lazy list. I still need to check out my log file to see the effect of lazy collections w/o and w/ subselect fetching.

Regards,
Darya

[Darya Akbari]

Originally posted by Mark Spritzler:
If you have no fetching strategy, then it will hit the database for each element in the database, so if there are 10 elements in the Collection that would be 10 seperate queries.

Mark

Hibernate always has a default fetching strategy in place. If I use attribute lazy="true" in my collection I can see (log file) that Hibernate triggers ONE select and NOT multiple selects. That one select provides me a result set of all collection element not in form of real object but kind of proxies, right?

Isn't that what subselect fetching also gives to me ?

Regards,
Darya

[Darya Akbari]

Well I guess I talk about proxies, however I'm not 100% sure . What is the log message that clearly states whether I got a proxy or the real object

[Darya Akbari]

I think when it (log file) says:

Initializing object from ResultSet: [...]

we talk about the real object and not a proxy.

OK, here are my results after adapting my Hibernate Mapping file with subselect and removing again:

When your collection is lazy it doesn't matter whether you have subselect or not. In both cases you have ONE select with a WHERE clause set to the parent's ID. The select is triggered with your first collection.get(0). All other collection.get(1), collection.get(2), ... are initialized w/o doing a select again.

This behavior was the same whether subselect was on or not . Does anyone has an explanation?

Regards,
Darya