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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: [IMPORTANT] Fetch-depth
Date Wed, 25 Jan 2006 19:53:34 GMT

Here's what I'm proposing, in summary.

1. Add a property to FetchPlan: get/setMaxFetchDepth. This property  
limits the depth of the fetched instance graph counting from the root  
of the instances. The roots are defined for queries as the results of  
the query. The roots of retrieve and detachCopy are the parameter  
instances. The default is 1, meaning fetch one level of objects  
reachable from the roots. A value of 0 has no meaning, because if a  
relationship field is in the fetch group, it will be fetched.

2. Add a property to FetchPlan: get/setDetachRoots. The roots for  
DetachAllOnCommit are all the instances in the DetachRoots on  
FetchPlan. The default is all instances in the cache. Or it could be  
all instances retrieved by queries, explicitly made persistent, or  
explicitly retrieved.

3. In order to avoid confusion, remove fetch-depth attribute for the  
field element and replace it with recursion-depth attribute. This has  
two effects: it requires users to change their metadata and think  
about what the fetch-depth was used for; and it better reflects the  
purpose of the attribute to limit recursion. The meaning of recursion- 
depth is the maximum number of links to follow from the target  
instance of this field in case the same class is encountered again.

4. In case of multiple definitions of the same field recursion-depth,  
the largest number for the same field will be used. That is, if the  
same field is declared in multiple fetch groups, a simple MAX of all  
the values is used.

I'll write up a number of examples and send them tomorrow.



On Jan 13, 2006, at 3:10 PM, Craig L Russell wrote:

> Javadogs,
> I've spent some time looking at the semantics of fetch-depth and  
> now agree with the critics of the change that I proposed back in  
> the infamous October 1, 2005 message to the expert group subject:  
> Re: JDO2 ยง12.7.2: fetch-depth only for "recursive fetch group  
> references"?.
> I now believe it's impractical to use fetch-depth to mean the  
> maximum depth of the object graph reachable from the root object(s)  
> field because of several messages sent on the subject by Joerg von  
> Frantzuis, Alexander Bieber, and Marco Schultz.
> Briefly, the argument is that if fetch-depth limits the number  
> absolutely, then it's not possible easily to use the fetch-group to  
> add another field to a fetch plan simply by adding a fetch-group  
> that includes that field. Instead, a new fetch-group that changes  
> the fetch-depth must be used. And each new use-case needs to  
> provide a different fetch-depth number if another level of fetching  
> is desired.
> I believe that the use of fetch-group to determine whether fields  
> (navigating relationships) are fetched should be natural, and that  
> we should therefore use fetch-depth for its original purpose of  
> limiting recursion.
> If you disagree with this position, please reply so we can move  
> forward and define the use of fetch-depth for recursion (as in the  
> original semantics of the attribute).
> Thanks,
> Craig
> Craig Russell
> Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
> 408 276-5638 mailto:Craig.Russell@sun.com
> P.S. A good JDO? O, Gasp!

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

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