Inference over linked data SPARQL endpoints - sparql

Question

When querying some linked data SPARQL endpoints via SPARQL queries, what is the type of reasoning provided (if any)?
For example, DBpedia SNORQL endpoint doesn't even provide the basic subclass inference (if A subClassOf B and B subClassOf C, then A subClassOf C). While FactForge SPARQL endpoint provides some inference (though it is not clear what kind of inference it is), and provides the possibility to switch that inference on and off.
My question:
How is it possible to identify the kind of inference applied? and if the inference support is limited, could it be extended using the endpoint only?

Answer 1

Inference controls will vary with the engine as well as the endpoint.
The public DBpedia SPARQL endpoint (powered by Virtuoso, from my employer, OpenLink Software) does provide various inference rules (accessible through the "Inference rules" link at the top right corner of the SPARQL endpoint query form page) which are controlled by pragmas in your SPARQL (not SNORQL, to which form you linked), such as --
DEFINE input:inference 'urn:rules.skos'
You can see the content of any predefined ruleset via SPARQL -- for the above
SELECT *
FROM <urn:rules.skos>
WHERE { ?s ?p ?o }
You can see the live query and results.
See this tutorial containing many examples.

Answer 2

While inference is not universally supported across SPARQL endpoints, most of the inferences supported by RDFS, RSFS+ and OWL 2 RL profiles are supported by SPARQL itself. For example, querying for instances of :A using your subClassOf entailment can be supported with SPARQL property paths:
SELECT ?inst
WHERE {
?cls rdfs:subClassOf* :A .
?inst a ?cls .
}
The first triple pattern gets all subclasses of :A, including :A (use + instead of * if you just want subclasses of :A), and the second triple finds all instances of all those classes.
To see how most of OWL 2 can be implemented with SPARQL, see Reasoning in OWL 2 RL and RDF Graphs using Rules. With a couple of exceptions, all of these can be implemented in SPARQL (and in fact you probably won't need some of them, such as eq-ref, (which is good for a computational lol that logicians may scoff at)).
There are few uses cases, beyond heavy-lifting classification problems, that can't be solved with a subset of the OWL 2 RL rules.
So, in the end, a recommendation is to understand what entailments you need. Chances are that OWL has totally overthought the issue and you can live with a few SPARQL patterns. And then you can hit the SPARQL endpoints without having to worry about whether specific inference profiles are supported.