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 Post subject: Special Case of n+1 Selects Problem (chapter 7.6.1)
PostPosted: Fri Nov 05, 2004 7:11 am 

Joined: Tue Jan 06, 2004 3:32 pm
Posts: 80
Location: Munich, Germany
If I translate my problem to the example in the book, I'd like to retrieve the 10 most recently created Items together with their bids (in order to display all bids or find out the maximum bid or whatever).

If I implement it like this...


int numberOfResults = 10;

List results = session.createCriteria(Item.class)
  .setFetchMode("bids", FetchMode.EAGER)

...I've got the problem that only 10 non-distinct result sets are returned. If there are 10 Bids for each Item, I'd only get 1 Item after making the results distinct.

Since I don't have any idea how many Bids each Item has, I have to fetch all items. Or am I wrong?

I would wish that Hibernate would somehow do the magic and make the results distinct for me. What's the point of getting non-distinct results anyway? I could formulate statements like "SELECT i, b FROM Item i, Bids b WHERE b.item = i" if I really want a more join-like result.



 Post subject:
PostPosted: Sat Nov 06, 2004 2:51 pm 
Hibernate Team
Hibernate Team

Joined: Mon Aug 25, 2003 9:11 pm
Posts: 4592
Location: Switzerland
The limit is applied to the eager outer join fetch query, so you don't get the "correct" result from an object graph point of view. It is however correct in SQL and also a reason why non-distinct objects are returned. Both views are valid, but we will change this in Hibernate3 (we'll return distinct objects, but then maxresult/firstresult will be even more difficult to understand). Your last statement is a simple inner join, but don't forget that your fetch query is also a join, left outer.

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