Hi, so I finished my work on the testsuite <https://github.com/beikov/jpa-treat-variations> for the treat operator. I also started a forum topic for this discussion: https://forum.hibernate.org/viewtopic.php?f=1&t=1043553 If anyone is interested, I also started a discussion in the EclipeLink forum about that: https://www.eclipse.org/forums/index.php/mn/msg/1080319/1/on/0/?SQ=5ddc74f601d6750e4794121738a57076
I hope we can all agree on an interpretation of how treat should work and hopefully get a clarification into the spec. Or at least some TCK tests, as it doesn't seem to assert anything useful right now. Regards, Christian Am 09.08.2016 um 13:14 schrieb Christian Beikov: > Hello again, > > I finished the first part of the test cases for root treats and > many-to-one treats. I also tried to explain how I would expect treat > to work and present a reduction/translation strategy to be able to > reduce thinking just about the base cases. > Could you comment on that? > https://github.com/beikov/jpa-treat-variations/blob/master/src/test/java/jpa/test/TreatVariationsTest.java > The tests show various problems with Hibernate as well as EclipseLink. > I will add findings from Datanucleus as soon as the relation mapping > for table per class is supported. > > Here is a collection of JIRAs for treat problems > > * https://hibernate.atlassian.net/browse/HHH-10988 > * https://hibernate.atlassian.net/browse/HHH-9345 > * https://hibernate.atlassian.net/browse/HHH-9862 > * https://hibernate.atlassian.net/browse/HHH-10768 > * https://hibernate.atlassian.net/browse/HHH-9594 > > The definition of the semantics according to JPA spec 4.4.9 are a bit > strange as it does not distinguish between FROM and WHERE clause when > mentioning filtering on the subtype. > It specifically says "Use of the TREAT operator therefore also has the > effect of filtering on the specified type (and its subtypes) as well > as performing the downcast" and I think this is only relevant for > treats in the FROM clause. > Treats in the WHERE clause should not result in filtering. Depending > on the join type which is used for the treat path, the result of the > treat is either the subtype instance, or may be NULL in case of a left > join. > Since any comparison(except NULL aware operations) with NULL will > eventually result in FALSE according to the SQL spec, the requirement > for the treat operator in the WHERE clause to result in FALSE for > instances that are not a subtype is also fulfilled. > The section "... and in the case of a restriction, the associated > predicate is false." didn't consider NULL aware operations so I think > this is an oversight. > > Am 04.08.2016 um 15:52 schrieb Steve Ebersole: >> Hey Christian, >> >> In general terms, one of the items on the docket for SQM is better >> TREAT support; but there is a lot that goes into that statement. One >> aspect is what we support in the parser properly in terms of >> recognition. Another is how this translates into the generates SQL >> query. All of this is being looked at with SQM. >> >> Also, relatedly, better support for switching on/off JPQL-compliance >> checking is a goal. >> >> <background> >> TREAT is an explicit downcast operator. It allows you to down cast a >> type reference in order to reference one of its subclass attributes. >> HQL actually supports implicit downcasting (subclass attribute >> references). In a way you can think of HQL as offering TREAT support >> implicitly. >> </background> >> >> With all that in mind, see my comments inline... >> >> >> On Wed, Aug 3, 2016 at 9:43 PM Christian Beikov >> <christian.bei...@gmail.com <mailto:christian.bei...@gmail.com>> wrote: >> >> >> * Missing support for treated paths like "TREAT(...).property" >> in the >> parser. This is more or less workaroundable for non-root path >> treats. Since root paths can't be treat joined to my >> knowledge, some >> treated paths simple can't be used. Maybe in 90% of the cases >> hibernate will just resolve to the right property without needing >> the treat? >> >> >> I'm not sure what you mean by "root paths can't be treated". You >> mean literally the query root? As in "MyEntity e" in "select e from >> MyEntity e"? If so, yes JPA does not allow for that to be TREAT'ed; >> but really it makes no sense to allow TREAT there, if you step back >> and look at it. A query like "select e from TREAT(MyEntity as MySub) >> e" makes no sense; its ultimately just the same as "select e from >> MySub e". Arguably I guess it *could* make some sense *if* MySub is >> a sub MappedSuperclass since technically JPA also allows only >> entities as query roots (HQL does not have this restriction). >> >> As discussed above, assuming that "someProp" is a persistent property >> defined on MySub, this is a perfectly valid HQL: "select e.someProp >> from MyEntity e". It is not however valid JPQL; in JPQL you'd have >> to say: "select treat( e as MySub).someProp from MyEntity e" >> >> All that said, the inability to dereference a TREAT'ed path (the >> ".property" part) would be a bug, and tbh I am surprised that you say >> this does not work with Hibernate. Which of your tests show this not >> working? Is there a related Jira? >> >> >> * Missing support for a special case of treat join which apparently >> was overseen by the JPA spec which would be >> "TREAT(TREAT(rootPath AS >> Subtype).relation AS RelationSubtype)". Eclipselink not only >> allows >> this syntax but for inner joins also implements the appropriate >> logic. I am thinking that this might even work in hibernate >> without >> the inner treat, but would fail for attributes with same names. >> >> >> Do you mean specifically the nesting of TREAT operators? Again, I am >> surprised that this would not work. Tests? Jira? >> >> * Joins for supertypes and conditions on discriminators are not >> aware >> of the join type actually used in the treated join => left join >> should cascade up the type hierarchy and discriminator checks >> should >> contain OR DTYPE IS NULL for left join semantics. If treats >> are in >> subconditions the DTYPE checks might even be wrong >> >> >> This "restriction" aspect is honestly Hibernate's biggest shortcoming >> in its TREAT support. Hibernate's "SQL generation machinery" simply >> was not built to support this. At all. I hacked together some >> support for TREAT on top of that legacy "SQL generation machinery", >> but it is very limited. >> >> This aspect is specifically what we have discussed in terms of TREAT >> support improvements in SQM. Keeping track of where the TREAT occurs >> so that we can later know how to properly handle it, whether that >> means we can skip parts of the inheritance join tree or maybe need to >> render some extra WHERE-clause restrictions... >> >> * Joins are generated for the whole hierarchy not only the >> types that >> are treated => this might seem to be "just" a performance >> problem, >> but IMO actually is a correctness problem as it makes a >> difference >> if you get e.g. 2 rows or 2 * TYPES rows especially for count >> queries. >> >> >> Generating the joins for the whole hierarchy is important for HQL's >> implicit downcasting support. However, that said, there are attempts >> to restrict that already in place. IIRC this only works for TREATs >> that occur in the FROM-clause specifically. However, I thought that >> other TREAT operations were still handled in terms of adding >> restrictions when dictated. So this might be specific to certain >> circumstances. >> >> Again, which specific tests show this? Jira? >> >> ---- >> >> Some specifics of the plan... >> >> First, internally within the query AST I want to make sure that we >> model *all* downcasts whether they are implicit or explicit. In >> other words, in terms of AST both of these queries would result in >> the same structure: >> >> 1. select e.someProp from MyEntity >> 2. select treat(e as MySub).someProp from MyEntity >> >> Secondly, we need to keep track of various pieces of information >> pertaining to a downcast >> (see >> org.hibernate.sqm.query.from.Downcast/org.hibernate.sqm.query.from.Downcastable). >> >> This includes info like: >> >> 1. The downcast "target" >> 2. The context in which the downcast occurred >> 3. Any/all contexts in which the downcast is used. >> >> The last 2 items there would specifically lead to: >> >> * Which specific inheritance joins are needed - and may indicate >> join type >> * Any extra restrictions we may need to add >> >> > _______________________________________________ hibernate-dev mailing list hibernate-dev@lists.jboss.org https://lists.jboss.org/mailman/listinfo/hibernate-dev
