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From "Alexander Saint Croix" <saintx.opensou...@gmail.com>
Subject Re: Multiple LEFT JOIN FETCH clauses in EJBQL
Date Wed, 16 Jan 2008 23:03:36 GMT
Thank you, Dain.

This is helpful in that it reinforces my intuition to avoid eager loading
for anything that can be replaced by well-written queries.  I'd rather the
extra data were optionally retrieved than enforced across all applications
regardless of circumstance.

I asked one of the University's former EJBQL experts, Rogers George, and his
recommendation regarding using multiple join fetches is "don't."

He continues:

"You can only fetch one multi-valued association at a time in EJBQL.
Hibername 3 QL will let you, but it's almost always a bad idea; the
resultset becomes a cartesian product of the contents of the collections.
Just running the two queries instead is almost certainly faster.

If the two collections are nested, you can usually rearrange your query so
that it selects the "leaves" of your entity graph, and all the join fetch
clauses follow many-to-one relations back towards the root.  Then you can
iterate over the results directly without even triggering collection
fetches, and get at the root object with java accessors."

Big thanks to Rogers George for the above information.  My particular need
involves multiple collection-valued reference, and I'll be investigating
using fetch groups or fetch plans to tune my query for these tests.  Once
it's working, I'll try to post up some code snippets in case others have
this trouble down the line.

Cheers,
--
Alex






On Jan 16, 2008 1:58 PM, Dain Sundstrom <dain@iq80.com> wrote:

> On Jan 16, 2008, at 7:16 AM, Alexander Saint Croix wrote:
>
> > Hey, guys.
> >
> > Is it possible to write an EJBQL query with more than one LEFT JOIN
> > FETCH
> > clause?
>
> Sure.  Just be careful with outer joins.  I always seem to end up
> getting too much data with them, and end up using trial and error to
> get the query right.
>
> > I have a number of entities with more than collection-valued
> > references that are lazy loaded by default.  I'm trying to avoid
> > flagging
> > them as FetchType.EAGER.
>
> FWIU, eager fetch and join fetch, only deal with how much data is
> loaded and cached in the JPA context and not the query result value.
> When it comes to performance of persistent applications, the cache is
> typically the thing that makes or breaks the application.
>
> Most JPA examples I have seen use lots of eager fetching, which makes
> the example perform really well since all data is available, but
> doesn't scale to large data sets.  In the real world, your goal is to
> only load the data you need for your transaction.  If you load too
> little, you end up going back to the database which makes the
> transaction take longer which leaves your locks open longer which
> reduces throughput on the database.  On the other hand, if you load
> too much data, your application with run out of memory and start
> paging, or more likely with the OpenJPA SoftReference chache, your
> extra data gets pushed out of memory and then reloaded.
>
> There is a big trade off here (as with all performance base stuff)
> between memory usage and time spent processing.  There is also a trade
> off between raw performance and complexity of your code since all the
> performance tuning results in more configuration and/or code.  I have
> found that more complex code tends to be slower, simply because no one
> wants to work on it.
>
> > The O'Reilly book on EJB 3 has exactly [--- this
> > ---] much information about JOIN FETCH clauses, and the web's been
> > less than
> > forthcoming.
> >
> > I managed to isolate some patterns in the cascade testing, and am
> > nearly
> > done with the the first module for my project.  At that point I'll be
> > tagging a release and redirecting energy toward articles.
>
> I hope this helped.  I'm not sure what kind of answer you were looking
> for.
>
> -dain
>
>

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