Re: Proposed change to the OWL-2 Direct Semantics entailment regime

Bijan Parsia <bparsia@cs.man.ac.uk> 
> [...]
> I'm still interested in talking with such people regardless of the group 
outcome.

Hi everybody,
Enrico told me that you might want to hear from implementers about the 
distinguished\non-distinguished variables debate.

I've not been able to go through the entire thread and to thoughtfully 
consider the entire question. But I can tell you that we are implementing 
an OWL2-QL+UNA engine here at my center, mainly for data integration 
purposes (please refer to Cangialosi et al. Accessing Data Through 
Ontologies with ONDA, Proc. CASCON 2010 for a preliminary exposition).

Actually, we do support conjunctive queries with non-distinguished 
variables. Also, we did some practical experience with users, including 
Banca d'Italia (the Italian Central Bank) by integrating legacy databases, 
and I can tell you that, in general, customers find conjunctive queries a 
pretty nice way for accessing data through ontologies. 

We don't use SPARQL as a query language (we adopt a datalog-style syntax 
instead) but we might support (some) SPARQL as a front-end in the future, 
as long as it does not misses relevant features. I cannot tell if making 
all variable distinguished would definitely prevent form covering some 
relevant use case. However, I don't see a compelling reason to inhibit 
non-distinguished variables, provided that supporting this feature would 
be up to implementers. After all, not all SPARQL features are going be 
supported by all implementers, I guess.

Hope that helps, and thank you for your great job.

Cordiali Saluti, Best Regards,

Guido Vetere
Manager & Research Coordinator, IBM Center for Advanced Studies Rome
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