Hello,
actually I would like to learn how the FetchMode.JOIN is supposed to
work in case of cycles (not really cycles but MetamodelGraphWalker
considers them cycles).
Check the test case please - it gets LazyInitializationException on a
relation despite that relation being marked as JOIN FETCH in the fetch
profile.
How is this intended to work? Is it a problem of MetamodelGraphWalker
and its cycle detection? Or is it planned limitation of FetchMode.JOIN?
I can help to fix this, but in order to do that I need to know how this
is expected to work.
Please disregard EAGER. I only mentioned FetchType.EAGER because I
noticed it can cope with the same cyclic relations - and yes, you are
right, those are fetched in subsequent selects. But we can't use EAGER
(performance reasons).
Thank you
Vladimir
On 06/07/2016 03:05 PM, Steve Ebersole wrote:
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(a)sykora.cz
<mailto:vm@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(a)sykora.cz
<mailto:vm@sykora.cz>
> <mailto:vm@sykora.cz <mailto:vm@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.
>
>
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>
--
*Ing. Vladimír Martinek*
Programmer
T: +420 723 908 968
@: vm(a)sykora.cz <mailto:vm@sykora.cz>
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*Ing. Vladimír Martinek*
Programmer
T: +420 723 908 968
@: vm(a)sykora.cz
Sykora Data Center s.r.o.
28. října 1512/123, 702 00 Ostrava