Re: [hibernate-dev] Hibernate ORM SQL generation

2015-08-26 14:41 GMT+02:00 Steve Ebersole <steve(a)hibernate.org&gt;: > On Wed, Aug 26, 2015 at 2:10 AM Gunnar Morling <gunnar(a)hibernate.org&gt; 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.