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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: Minutes: JDO TCK Conference Call Friday, June 19, 9 am PDT
Date Sat, 20 Jun 2009 14:50:17 GMT
Hi Andy,

On Jun 20, 2009, at 2:52 AM, Andy Jefferson wrote:

>> http://issues.apache.org/jira/browse/JDO-633 Don't know how to do  
>> this
>> since the absolute order is not required to be in the database, but
>> there might be a way to create the database language (e.g. SQL) to do
>> an ordered subselect and choose the "nth" item from the selection?
>> Database-specific SQL? Can we support this in all databases? Is there
>> a use-case?
> Don't understand. A List has an order (whether it is an "indexed"  
> List, or
> an "ordered" List (using JPA-style ordering criteria)). The user  
> puts items
> in the List and persists the List. The user then wants to grab  
> element at
> index 2.

JDO doesn't require that absolute numbers be stored. So the item at  
index 2 might have a row order value of 17. It's only in index  
position 2 because there's also a row with row order value 3.

What I was (am) concerned about is that in order to get to the item at  
index 2 an ordered subquery might be needed, and didn't (don't) know  
the complexity for all the use cases to get the right answer.

If you're happy with implementing this, considering all the possible  
places where get(2) might be used, then I'm happy. ;-)))


> Storing the order in the datastore, in some way, is essential to
> support (indexed) Lists, hence querying should be straightforward.  
> Since JDO
> doesn't define an ordering constraint, it is reasonable to assume  
> that in JDO
> a List is an "indexed" List, hence retains the users ordering.
> As far as testing goes, surely all you need to do is persist an  
> object with a
> List with some elements in some order, and then do a query, assuming  
> the same
> order as they had at the moment of persistence. Where does DB- 
> specific SQL
> come into this ? surely that is the problem for the implementation.
> -- 
> Andy  (DataNucleus - http://www.datanucleus.org)

Craig L Russell
Architect, Sun Java Enterprise System http://db.apache.org/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!

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