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From: Marc Prud'hommeaux <mprudhom@apache.org>
Subject: Re: Proposal: Optimizing empty collection fetch. Meta Column in
 ContainerFieldMappling
Date: Thu, 5 Oct 2006 18:00:34 -0700
To: open-jpa-dev@incubator.apache.org
Sender: Marc Prud'hommeaux <mprudhomapache@gmail.com>

Alex-

You are right about jdbc-container-meta ... I had forgotten about  
that attribute.

> I can certainly file a JIRA request if you'd like me to.

That's the standard mechanism for entering enhancement requests for  
OpenJPA itself, so it is useful to have as a reference point.

> I wonder if there is any chance to get some advice on implementing  
> it in
> Kodo JDO 3.4?

Do you mean in addition to your implementation of it for Kodo 4.0?  
The custom field mapping mechanism is similar to 4.0, so any work  
that you have done for this implementation in 4.0 will probably be  
similar to how you would do it in 3.4.

> Do you think it is reasonable to expect this feature in 4.1 release?

It probably would not make it in in time.

> Is 4.1 coming soon?

Yes, very soon.

> Will it have solid JDO2 implementation?

Yes it will.


On Oct 5, 2006, at 5:55 PM, Roytman, Alex wrote:

> Hello Mark,
>
> There are two mechanisms in Kodo - jdbc-null-indicator for 1-1  
> embedded
> and jdbc-container-meta for collections/maps. In general I was talking
> about jdbc-container-meta one. The docs state:
>
> " 6.2.3.7. jdbc-container-meta
> Container metadata is used to record non-essential information about
> collection and map fields. If this extension is set to true,  
> collections
> and maps will be able to distinguish between the empty state and the
> null state. If this extension is set to false or is unset, then it  
> will
> not be possible for Kodo to differentiate between these two states. In
> this situation, all collections and maps in persistent objects loaded
> from the database will be non-null"
>
> Almost exactly what I want and I was kind of surprised by the limited
> implementation of the feature.
>
> I can certainly file a JIRA request if you'd like me to.
>
> I wonder if there is any chance to get some advice on implementing  
> it in
> Kodo JDO 3.4?
>
> Do you think it is reasonable to expect this feature in 4.1  
> release? Is
> 4.1 coming soon? Will it have solid JDO2 implementation?
>
> Thank you
>
> alex
>
>
>
>
>
> -----Original Message-----
> From: Marc Prud'hommeaux [mailto:mprudhomapache@gmail.com] On  
> Behalf Of
> Marc Prud'hommeaux
> Sent: Thursday, October 05, 2006 8:19 PM
> To: open-jpa-dev@incubator.apache.org
> Subject: Re: Proposal: Optimizing empty collection fetch. Meta  
> Column in
> ContainerFieldMappling
>
> Alex-
>
> That does sound like a good feature to add. Note that I think the
> "null-indicator" attribute is only available for embedded mappings,
> not for container mappings (although I could be wrong about this).
>
> I'd recommend opening a JIRA issue as a reference for the enhancement
> request, and we can build on that.
>
>
>
> On Oct 5, 2006, at 3:52 PM, Roytman, Alex wrote:
>
>> Hello Abe,
>>
>> I would like to present a valid use case and a very useful  
>> performance
>> enhancement.
>>
>> The idea is that, if we know that a collection field is empty  
>> there is
>> no need to fetch it.
>>
>> It can provide a truly dramatic performance improvement when in a
>> large
>> set of instance only some of them have non-empty collection field.
>> Consider a very common case - composite (tree like) data structures.
>> Unlike true composite pattern typical tree structure does not have a
>> special leaf class that is any node of a tree can potentially have
>> sub-nodes. When traversing such a tree as many as 70% of fetches of
>> child nodes will yield empty collection because obviously leaf
>> level is
>> the larges in a tree structure :-)
>>
>> I wrote a prototype custom 1-N mapping which allow to store "empty"
>> flag
>> (whether the collection is empty) on commit and will store empty
>> collection into StateManager on collection field load if the flag
>> is set
>> to true (empty) instead of going to database to fetch it.
>>
>> The results were dramatic - when traversing 800-node tree number of
>> "fetch-sub-nodes" SQL statements was cut from 800 to 130.
>>
>> Non-Tree cases when objects have sparsely populated collection
>> field can
>> be even more dramatic.
>>
>> If concurrency of the collection field is controlled on owned class
>> level (default) I think there is no dander of this flag being out of
>> synch with actual collection content without entering concurrent
>> modification state.
>>
>> I have not had chance to think through transaction commit  
>> implications
>> if any.
>>
>> There is a very nice facility in ContainerFieldMappling for  
>> indicating
>> null container fields. I wonder why it so much hard wired to empty/
>> null
>> and does not allow non-empty/empty/null differentiation and
>> optimization.
>> Any reason it is so restrictive? Any plans to make it a bit more
>> flexible or directly implementing the behavior I outlined above?
>>
>> I would greatly appreciate if you could comment on this and may be
>> suggest the best approach implementing this. Or may be it is already
>> implemented and I am missing it :-)
>>
>> Best Regards
>>
>> Alex Roytman
>> Peace Technology, Inc
>>
>>
>