Re: [hibernate-dev] Entity-joins (HHH-16 and HHH-7321)

On 8 February 2016 at 17:44, Steve Ebersole <steve(a)hibernate.org&gt; wrote: > I am working mainly on HHH-16 which requests adding support for > entity-joins (aka "ad hoc" joins). > > So long-story-short, there is a simple solution with some limitations and > then a more correct solution that unfortunately requires a lot of rework in > the HQL parser. > > The crux of the problem is identifier scoping in the generated SQL and how > Hibernate handles implicit joins currently in HQL. > > As an example, consider a query like: > > select f.id, f.customer.name, f.postDate, u.username > from FinancialRecord f > left join User u on f.lastUpdateBy = u.username > > As I currently process this entity-join ("... join User on ...") I have to > attach it to the end of the FromClause. The reason I have to attach it > there is a bit of a convoluted discussion that gets into the design of the > current HQL AST model and some poor assumptions made there. > > Complicating the fact that I add the entity-join to the end of the > FromClause is the fact that Hibernate currently always handles implicit > joins as theta joins. > > So all told the query above is currently rendered to SQL as: > > select ... > from financial_record f, > customer c > left outer join `user` u > on f.last_updt_by = u.username > where f.customer_id=c.id > > So the problem with scoping is the comma. In SQL terms, that delimits the > start of a new "table reference". This is where a lot of databases diverge > on what is supported in terms of scoping references to aliases between > "table references". H2 for example is fine with this as long as the join > to `user` is an inner join; but it chokes if the join is outer. > > The simply solution would seem to be to have Hibernate render the implicit > join as an ANSI-style join rather than a theta-join. However this is where > the poor design choices that I mentioned in the current parser come into > play. Basically the parser overloads the flag for implicit joins to mean > many, many things. So changing that one value really messes things up. So > that's not realistically an option. It is definitely something we want to > keep in mind for the new parser however! > > Another option is to introduce a concept similar to SQL's "table reference" > into the AST model. This is essentially the same this I do in SQM with > org.hibernate.sqm.query.from.FromElementSpace. However, this is a massive > change in the parser. > > I am inclined for now to simply say that implicit joins and entity-joins > cannot be combined, and to circle back to this later in terms of working > out support for using them in combination. +1 _______________________________________________ hibernate-dev mailing list hibernate-dev(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/hibernate-dev