[hibernate-dev] Relations are not loaded when using Fetch Profiles

[Steve Ebersole]

There is a huge difference in join-fetch and what we detail for "EAGER".
Join-fetch is also an eager fetch.  There are 2 pieces of information in
regards to fetching: when, how?

EAGER merely describes when: now.

What you describe as "EAGER" is really a N+1 fetch.  So the relation is
fetched immediately, but as "subsequent select".  A join-fetch is still
EAGER, but now the relation is fetch via join in the initial query.  Very
different behavior.

On Tue, Jun 7, 2016 at 4:38 AM Vladimir Martinek <vm at sykora.cz> wrote:

> HHH-10745, runnable test case attached to that issue
> (org.hibernate.test.fetchprofiles.cycle.tar.gz).
>
> Thank you
>
> Vladimir
>
>
>
> On 06/07/2016 06:00 AM, Gail Badner wrote:
> > Please create a Jira issue and attach a runnable test case.
> > Thanks,
> > Gail
> >
> > On Thu, Jun 2, 2016 at 5:45 AM, Vladimir Martinek <vm at sykora.cz
> > <mailto:vm at sykora.cz>> wrote:
> >
> >     Fetch profiles fail to load certain relations because of invalid
> cycle
> >     detection in MetamodelGraphWalker. Below is an example.
> >
> >     I have compared the behaviour of Fetch Profiles and standard EAGER
> >     relations (EAGER does not suffer from this problem). The
> >     MetamodelGraphWalker graph walker produces the same results, the
> >     SQL is
> >     the same (missing relation).
> >
> >     With EAGER the relation is loaded in second pass, via
> >     SessionImpl.internalLoad(). There it is decided what LoadType is
> >     used -
> >     for eager it uses INTERNAL_LOAD_EAGER, for lazy INTERNAL_LOAD_LAZY.
> It
> >     does not take into account the fetch profiles here!
> >
> >     The last place I can get hold of fetch profiles in in
> >     AbstractLoadPlanBasedEntityLoader line 82. After that the fetch
> >     profile
> >     information is lost, never making it anywhere near to
> >     SessionImpl.internalLoad().
> >
> >     I would like to implement this, but to do that, I need someone to
> >     point
> >     me in the right direction. Most of all I need answers to following
> >     questions:
> >
> >     1) Is it right to assume the fetch profiles should be evaluated in
> >     SessionImpl.internalLoad() and appropriate LoadType used when
> >     detected a
> >     relation affected by a fetch profile?
> >     2) If so, what is the intended way of getting the fetch profile
> >     information to SessionImpl.internalLoad()?
> >
> >     Also, a colleague of mine attempted to implement FetchType.SELECT
> >     fetch
> >     strategy and ended up with precisely the same problem. I believe
> >     solving
> >     my issue would pave way for quick FetchType.SELECT implementation
> >     (which
> >     we could also use on our project).
> >
> >
> >     Thank you
> >
> >     Vladimir Martinek
> >
> >
> >     Example:
> >
> >     Have 5 entities - Start, Via1, Via2, Mid and Finish with following
> >     relations (all LAZY):
> >
> >     Start n:1 Via1 n:1 Mid n:1 Finish
> >     Start n:1 Via2 n:1 Mid n:1 Finish
> >
> >     Now, trying to use Fetch Profiles to load Start entity and all of its
> >     relations. I would expect Hibernate to execute following SQL select:
> >
> >     SELECT * FROM Start s
> >     LEFT OUTER JOIN Via1 v1 (path Start-Via1)
> >     LEFT OUTER JOIN Mid
> >     JOIN Finish
> >
> >     LEFT OUTER JOIN Via2 (path Start-Via2)
> >     LEFT OUTER JOIN Mid
> >     JOIN Finish
> >
> >     Unfortunately, ii ompletely omits the second join from Mid to Finish,
> >     what I am getting is:
> >
> >     SELECT * FROM Start s
> >     LEFT OUTER JOIN Via1 v1 (path Start-Via1)
> >     LEFT OUTER JOIN Mid
> >     JOIN Finish
> >
> >     LEFT OUTER JOIN Via2 (path Start-Via2)
> >     LEFT OUTER JOIN Mid
> >
> >     I dug deeper into this and found cycle detection in
> >     MetamodelGraphWalker, line 144. Basically, when MetamodelGraphWalker
> >     detects a relation that has already been visited, it considers it a
> >     cycle. But in my case it is not a cycle - I just came to the same
> >     relation twice using two different paths.
> >
> >
> >     _______________________________________________
> >     hibernate-dev mailing list
> >     hibernate-dev at lists.jboss.org <mailto:hibernate-dev at lists.jboss.org>
> >     https://lists.jboss.org/mailman/listinfo/hibernate-dev
> >
> >
>
>
> --
>
> *Ing. Vladimír Martinek*
> Programmer
>
> T: +420 723 908 968
> @: vm at sykora.cz
>
> Sykora Data Center s.r.o.
> 28. října 1512/123, 702 00 Ostrava
> www.sykora.cz
>
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