On 12.11.2015 16:30, Paul Houle wrote:
This is really great stuff that gets to the heart of the matter.
There is a horrible misconception that SPARQL is complex. People often
show you queries that make your head spin, but it is astonishing how
really simple they are. Here are a set of SPARQL queries that implement
OWL 2 RL:
http://topbraid.org/spin/owlrl-all.html
this shorter and far more human readable than anything has been written
about OWL.
As far as I can tell, this is a SPARQL-based copy of the inference rules
given in the OWL standard [1], which, to be fair, should probably be
counted as something that "has been written about OWL" ;-)
On a more technical note, it should be remarked that the queries do not
really "implement OWL RL". First of all, you would need to run these
queries recursively and add their results to the database in each step,
which is not something you could do through a public SPARQL endpoint.
The queries are actually SPARQL-encoded inference rules that need to be
executed in a way that is not the usual mode of answering a SPARQL query.
Secondly, these rules only implement certain aspects of OWL RL (instance
retrieval). They do not, in particular, correctly implement retrieval of
subclass-of axioms. In fact, it is not possible to implement correct
subclass-of reasoning for OWL RL by using RDF-based rules in this way,
even if they are executed recursively [2]. It is possible to do it with
rules, but these are beyond SPARQL CONSTRUCT, unless your "SPARQL rule
engine" implements some more complex redundancy checking before adding
CONSTRUCT results to the database.
On the other hand, it is entirely possible to implement correct OWL QL
(note: *QL* not *RL*) reasoning in SPARQL without even using "rules"
that need any recursive evaluation [3]. This covers all of RDFS, and
indeed some of the patterns in these queries are quite well-known to
Wikidata users too (e.g., using "subclassOf*" in a query). Depending on
how much of OWL QL you want to support, the SPARQL queries you get in
this case are more or less simple. This work also gives arguments as to
why this style of SPARQL-based implementation does (most likely) not
exist for OWL RL [3].
(I come to bury, not praise OWL -- if there are some OWL
features you like copy them into your rules. If your data gets "big" at
all, you do not want to mindlessly infer facts that aren't part of
expected reasoning chains.)
I am afraid that the rules you cited are actually an example of some
such "mindless" inference. They infer some types of axioms, such as
subclass of, that are not part of their intended output (that is: which
are never inferred completely anyway). There are other inference rule
sets that avoid this useless extra inferences and still obtain all
instance-of relations in OWL RL.
I write this not to praise or "bury" any technology, just to clarify
some apparent misconceptions. (In particular, I completely agree with
the fact that SPARQL is not that hard to grok, and that nicer interfaces
and maybe further tutorial-style introductions would be useful.)
Regards,
Markus
[1]
http://www.w3.org/TR/owl2-profiles/#Reasoning_in_OWL_2_RL_and_RDF_Graphs_using_Rules
[2]
http://korrekt.org/page/The_Not-So-Easy_Task_of_Computing_Class_Subsumptions_in_OWL_RL
[3] http://korrekt.org/page/Schema-Agnostic_Query_Rewriting_in_SPARQL_1.1
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