Re: [hibernate-dev] SQM - alias registry

On 7 October 2015 at 15:39, Sanne Grinovero <sanne(a)hibernate.org&gt; wrote: > On 7 October 2015 at 15:27, Steve Ebersole <steve(a)hibernate.org&gt; 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.