Nhibernate和Unitwork模式的性能非常差

Question

I have one complex object like this

 A
|
|
|                 |->C(As child)---->it has 4 hastomany properties(each B has 10000 C child)
B(has many child)->
                  |->D(As child)---->it has 4 hastomany properties(each B has 1o000 D child)
                  |->B Has many prop also
|
|
|A has many prop also

I, m suffering with performance. All together while retrieving this record i can expect Nh to fire 1000-2000 queries. But Worst performance by NH 10-20K queries it is firing.

Here im doing reading and writing on entity A only. I'm not individually inserting any of A's child nor retrival. I'm firign Get command on entity A and writing back Entity A only.

Cascading is taking care of insertion of A's child->its child too.

here im suffering with Performace very badly, i dont know what to do here.

Answer 1

I would guess that your mappings are eagerly fetching all of those children which I would not do. It's hard to say without looking at your mappings but if you see something like this in your mappings this is your problem:

HasMany(x => x.OrderLines)
            .FetchType.Select();

or

HasMany(x => x.OrderLines)
            .FetchType.Join();

This means when you load the parent object all of these collections will also be loaded which can lead to the infamous "select n+1" problem.

Below is from the nhibernate documentation:

(3) fetch (optional, defaults to join): enables outer-join or sequential select fetching for this association. This is a special case; for full eager fetching (in a single SELECT) of an entity and its many-to-many relationships to other entities, you would enable join fetching not only of the collection itself, but also with this attribute on the nested element.

If you are unable to change your mappings I would recommend creating queries up front to get all the child collections. This way you are only doing 1 query per entity rather than the 1000's nhibernate is doing currently via lazy loading.