[hibernate-dev] SQM - alias registry

[Steve Ebersole]

Yes, but generally speaking Java gives you alternative ways to reference to
hidden/shadowed names, sql/hql/jpql do not.



On Wed, Oct 7, 2015 at 9:42 AM Sanne Grinovero <sanne at hibernate.org> wrote:

> On 7 October 2015 at 15:39, Sanne Grinovero <sanne at hibernate.org> wrote:
> > On 7 October 2015 at 15:27, Steve Ebersole <steve at hibernate.org> wrote:
> >>>
> >>> > Here the aliases `c` do infringe.  In the subquery, we don't really
> know
> >>> > which reference the `c` alias should resolve to.  We *could* here
> >>> assuming
> >>> > that the subquery is uncorrelated.  Bu without this rule we really
> would
> >>> > not know that the subquery is correlated
> >>>
> >>> Out of curiosity, Couldn't for this case assume that the second alias
> >>> overrides the first.
> >>> This might cause some hard to spot errors, though.
> >>>
> >>
> >> The issue really is for cases of correlated subqueries (where the
> subquery
> >> refers to the outer query).  So imagine a query such as:
> >>
> >> select ...
> >> from Salesperson s
> >> where exists (
> >>     select count(*)
> >>     from Sale s2
> >>     where s.id = s2.salesperson.id ...
> >>     group by s2.salesperson.id
> >>     having count(*) > :sales
> >> )
> >>
> >> So here the predicate `s.id = s2.salesperson.id` defines a correlation
> >> beween hthe queries.  If we allowed the "alias overriding", it is quite
> >> possible for the user to (mistakenly) write this query as:
> >>
> >> select ...
> >> from Salesperson s
> >> where exists (
> >>     select count(*)
> >>     from Sale s
> >>     where s.id = s.salesperson.id ...
> >>     group by s.salesperson.id
> >>     having count(*) > :sales
> >> )
> >>
> >> Which validates fine, but is not *really* what they meant and would not
> >> return what they are looking for.
> >
> > So the question is about allowing or disallowing variable shadowing.
> >
> > Java allows it, and since Hibernate targets Java developers mostly,
> > being consistent with that has some merits - after all I think people
> > know that using shadowing is a bad idea so I wouldn't stress too much
> > about it.
> >
> > Still if it's not too complex to ban it, that might be nicer: this is
> > not a general purpose language like Java so the improvement could be
> > welcome. I certainly see no problems with preventing mistakes.
>