[hibernate-dev] Hibernate ORM SQL generation

[Steve Ebersole]

I do want to pull ORM in to the hibernate-sqm module as a test dependency
to be able to more easily set up the ModelMetadata stuff based on a
SessionFactory.  That is possibly awkward later when we then use
hibernate-sqm in ORM in terms of having 2 different versions of ORM.  I am
open to alternatives that don't involve *me* developing a real(ish)
ModelMetadata
impl from scratch.

On Thu, Aug 27, 2015 at 8:45 AM Gunnar Morling <gunnar at hibernate.org> wrote:

> 2015-08-26 14:41 GMT+02:00 Steve Ebersole <steve at hibernate.org>:
> > On Wed, Aug 26, 2015 at 2:10 AM Gunnar Morling <gunnar at hibernate.org>
> wrote:
> >>
> >> Hi Steve,
> >>
> >> > The other approach is to use a 3-phase translation (input
> >> > -> semantic-tree -> semantic-SQL-tree(s) -> SQL).  This gives a hint
> to
> >> > one
> >> > of the major problems.  One source "semantic" query will often
> >> > correspond
> >> > to multiple SQL queries; that is hard to manage in the 2-phase
> approach.
> >>
> >> In which situations will this happen? I can see inheritance where a
> >> HQL query targeting a super-type needs to be translated into a SQL
> >> query per sub-type table. What others are there?
> >
> >
> > For ORM the only time this happens today for a SELECT query is in the
> "split
> > query" case I mentioned elsewhere (a query like 'from java.lang.Object').
> > SQM does this much better than we do in ORM today.  in SQM we build a
> > semantic tree that encodes the "unmapped polymorphism" such that we get a
> > tree with 'java.lang.Object' as the root from element. But it is a
> > FromElement with a special type of EntityTypeDescriptor (which comes from
> > the caller remember): PolymorphicEntityTypeDescriptor.  On the ORM side
> then
> > I have a QuerySplitter that takes that query and makes a copy of that
> entire
> > SQM tree, one for each mapped implementor of the specified class.  FWIW,
> ORM
> > does this today, albeit in a different way.  Today we split the query
> based
> > on String manip and then feed it parser.  Here we feed it to the parser
> and
> > use the tree to split it; much less brittle :)
> >
> > Really the cases where this would happen (one "concrete SQM" -> multiple
> > SQL) would be UPDATE and DELETE queries against "multi-table structures"
> > (inheritance, secondary tables).
> >
> >
> >> For the purposes of OGM this phase ideally would not be tied to SQL,
> >> as we phase the same task with non-SQL backends in SQL. I.e. i'd be
> >> beneficial to have input -> semantic-tree ->
> >> semantic-output-query-tree(s) -> (SQL|non-SQL query). There
> >> "semantic-output-query-tree(s)" would be an abstract representation of
> >> the queries to be executed, e.g. referencing the table name(s). But it
> >> would be unaware of SQL specifics.
> >
> >
> > OGM would be doing this.  This SQM is the end result of the shared
> library.
> > WHat each caller does with the SQM is up to that particular caller.  We
> > should consider moving QuerySplitter (its in my PoC, which now acts as
> the
> > PoC for using this in ORM) into the hibernate-sqm module.  Any caller
> > wanting to support those unmapped class references will need to do the
> same
> > thing.
>
> Yes, that'd be good I think. We'd have to apply the same rules for
> splitting as ORM.
>
> >
> > BTW, another cool thing to note is the (still expanding) support for
> "strict
> > JPQL compliance" enforcement.
>