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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Vertical inheritance
Date Mon, 31 May 2010 13:11:14 GMT
Good thoughts. I really like the adaptive strategy idea, which can  
probably be used for discriminator-free vertical mapping as well.

BTW, semantically "vertical inheritance with discriminator" is  
essentially single-table inheritance with flattened attributes in  
subclasses. Which Cayenne supports already, but without any special  
optimizations for wide|deep hierarchies.

Andrus


On May 31, 2010, at 9:01 AM, Mike Kienenberger wrote:

> I'm not sure how deep our worse-case tree is, but we probably could
> get by with joining all tables.   It's really the width of the tree
> that would prohibit joining everything.  I agree that using an outer
> join on all tables be more performant for certain types of tables with
> small numbers of tables -- this is the case for most of our vertical
> inheritance -- five to ten tables, and I like the idea of being able
> to do so.
>
> We should probably come up with some kind of limit on how many tables
> (or maybe columns) we want to fetch at one time, and then try to pick
> the right strategy based on that.   I'd say that we would try to
> always maximize the amount of data pulled in for each strategy.
>
> So it sounds right to me.   I'd say we'd want to avoid the "without
> joins" query as much as possible.
>
> Do we need to fetch the id/discriminator from the root table as a
> separate query?  I guess we do for the cases where we're not fetching
> a leaf class.   But for a leaf class, we could fetch all records in
> one query.
>
> I suppose we should start with the worse-case scenario and that will
> cover every other type of query as a first pass.
>
> I see that as
>
> 1) fetch the root table to determine the discriminator value.
> (subclass unknown worse case)
> 2) fetch each sub table individually (depth too large worse case)
>
> Then we could consider optimizations:
>
> a) leaf subclass is known -- one query can fetch root and subtables if
> depth is not too deep
> b) depth is small -- one query can fetch all subtables (and possibly
> root if leaf subclass)
> c) total tables involved is small -- can do an outer join of all
> tables and sort things out in memory
>
> I didn't realize how complicated this was going to get :)
>
> I think it's probably a good idea to be able to specify a fetch
> strategy, at least at first -- might be needed to get around
> limitations or bugs in our fetch strategies.   But I think we should
> also be able to derive a best-case fetch strategy based on the model
> as well.
>
> On Mon, May 31, 2010 at 8:29 AM, Andrus Adamchik <andrus@objectstyle.org 
> > wrote:
>> Great. this is the kind of feedback I was looking for :-)
>>
>> On May 31, 2010, at 8:17 AM, Mike Kienenberger wrote:
>>>
>>> What you're suggesting could work, but we have at least one very  
>>> deep
>>> and wide inheritance tree in our current app, and the performance  
>>> hit
>>> of joining more than a hundred tables (so far) unnecessarily would
>>> kill us.
>>
>> So how would you do a fetch based on discriminator column approach?  
>> I can
>> think of a 2 step process:
>>
>> 1. Fetch ID's + discriminator columns from the root table.
>> 2. Fully resolve objects, either doing joins only for the subset of  
>> tables
>> that appeared in result in #1, or doing a separate query (without  
>> joins) for
>> each subclass table matching the set of IDs.
>>
>> Does that sound right?
>>
>> Also I am sort of in favor of my original discriminator-free  
>> approach for
>> smaller hierarchies, so wonder if a fetch strategy can be something
>> specified for a given super-entity and/or query.
>>
>> Andrus
>>
>>
>


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