Hello,

I've encountered a strange problem recently.

Say I have 10000 students in my db. And I load them by

IList<Student> s = studentDao.FindAll();

now that all the students have been loaded within the session.

The problem is after loading such a nubmer any time I make a query like

Student s = studentDao.FindByName("John");

this could take up to 500 ms which is unacceptable.

The only fix I could figure out was clearing the session.

Could anyone tell me if this is a bug ? Why loading many items degrades the performance of my queries ?

Hi emperon,

When you load entities into the persistence context, NHibernate keeps information about every single object (for transparent persistence, dirty checking, etc.). When you commit/flush etc, the session uses this information to perform the required operations.

If you have 10,000 entities loaded, this process will be slower than it should be because the session will check all 10,000 entities to find those that must be updated. You should help it avoid this waste by evicting unchanged entities. So that's why clearing the session works for you now. And it's probably what should be done, in your scenario.

Hi ,

I understand the issue. The thing is flushing and clearing the session in the middle of a transaction just annoys me. May be I should tell nhibernate to retrieve those items read only. Do you know how to do so ?

There is a way to set ReadOnly for just that session... in Hibernate only... it's not implemented in NHibernate. This is how it would look:

session.CreateCriteria(typeof(Foo)).SetReadOnly(true);

In this case, snapshots for dirty checking are disabled in the persistence context.

This was the case for 1.2 anyway, I don't know if anyone is working on this for NHibernate, it's been mentioned in the forum a few times.

I just followed this up. Support for ReadOnly is not being looked at by anyone. There is a JIRA issue for it, though.

http://jira.nhibernate.org/browse/NH-908