bug in executing SPARQL queries

[Eloy Retamino Carrion]

Hello,

I have localized what seems to be a bug in stardog.

Given the next ontology:

@prefix : <http://www.semanticweb.org/eloy/ontologies/2016/10/untitled-ontology-9#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@base <http://www.semanticweb.org/eloy/ontologies/2016/10/untitled-ontology-9> .

<http://www.semanticweb.org/eloy/ontologies/2016/10/untitled-ontology-9> rdf:type owl:Ontology .

:A rdf:type owl:Class .
:B rdf:type owl:Class .

:a rdf:type owl:NamedIndividual ,
            :A .

:b rdf:type owl:NamedIndividual ,
            :B .

when executing the next query:

prefix : <http://www.semanticweb.org/eloy/ontologies/2016/10/untitled-ontology-9#>

select ?x ?y where
{  
    ?y a :B .
 
    { ?x a :A .
    FILTER EXISTS { ?y a :A } }
}

results are x = :a, y = :b

if I change EXISTS for NOT EXISTS no bindings are found.

ie. EXISTS is showing the opposite behavior as expected. If I remove the braces the results are correct.

This just happens in certain circumstances, which I haven't completely figured out.

SUMMARY: EXISTS shows inverse behavior when in braces in certain circumstances. This is a problem when using UNION, OPTIONAL, MINUS or in any other case which require using sub graph patterns.

Regards,
Eloy.

[Evren Sirin]

This is the expected behavior based on the SPARQL semantics; see scope
of filters [1]. The meaning of your query changes when you put the
extra grouping. First this group in your query is answered:

{ ?x a :A . FILTER EXISTS { ?y a :A } }

This will bind ?x to :a and FILTER will be satisfied due to :b but
FILTER does not bind variables. Then this solution (x = :a) will be
joined with the solution for the outer triple pattern yielding x = :a,
y = :b.

Best,
Evren

[1] https://www.w3.org/TR/sparql11-query/#scopeFilters

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[Eloy Retamino Carrion]

Thanks for the explanation! sorry I posted it as a bug.