Hi,
I am trying to use the query service provided by the EntityHub.
The JSON field query I wrote is the following
{
"selected": ["http:\/\/www.w3.org\/2000\/01\/rdf-schema#label",
"http:\/\/www.w3.org\/1999\/02\/22-rdf-syntax-ns#type"],
"offset": "0",
"limit": "3",
"constraints": [
{
"type": "reference",
"field": "http:\/\/www.w3.org\/2002\/07\/owl#sameAs",
"value": "http:\/\/www4.wiwiss.fu-berlin.de\/factbook\/resource\/United_States",
}]
}
and aims to find all the owl:sameAs entities of the entity
<http://www4.wiwiss.fu-berlin.de/factbook/resource/United_States> in DBpedia.
But instead of the graph with the expected results I always receive the error
javax.ws.rs.WebApplicationException:
org.apache.stanbol.entityhub.servicesapi.site.ReferencedSiteException: Unable
execute Query on remote site http://dbpedia.org/sparql
The sparql construct serialized by the EntityHub is the following
CONSTRUCT {
?id <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?v_1 .
?id <http://www.w3.org/2000/01/rdf-schema#label> ?v_2 .
<http://www.iks-project.eu/ontology/rick/query/QueryResultSet>
<http://www.iks-project.eu/ontology/rick/query/queryResult> ?id .
}
WHERE {
{ {?id <http://www.w3.org/2002/07/owl#sameAs>
<http://www4.wiwiss.fu-berlin.de/factbook/resource/United_States> }}
{ OPTIONAL { ?id <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
?v_1 . } }
{ OPTIONAL { ?id <http://www.w3.org/2000/01/rdf-schema#label> ?v_2 . }
}
}
ORDER BY DESC ( <LONG::IRI_RANK> (?id) ) LIMIT 3
If you try to run this construct directly on the SPARQL endpoint of dbpedia the
response is always "Virtuoso 42000 Error The estimated execution time 0 (sec)
exceeds the limit of 1500 (sec)."
It seems that the problem is in the graph patterns containing optional as they
are bound to the whole default graph ( {OPTIONAL{triple pattern} } is
equivalent to { {} OPTIONAL {triple pattern} }.
In fact the query works properly both if you substitute the three graph
patterns into one graph pattern as follows
...
WHERE {
?id <http://www.w3.org/2002/07/owl#sameAs>
<http://www4.wiwiss.fu-berlin.de/factbook/resource/United_States> .
OPTIONAL { ?id <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?v_1
. }
OPTIONAL { ?id <http://www.w3.org/2000/01/rdf-schema#label> ?v_2 . }
}
...
and if you bind the optional to the global solution of the WHERE clause, i.e.
...
WHERE {
{{?id <http://www.w3.org/2002/07/owl#sameAs>
<http://www4.wiwiss.fu-berlin.de/factbook/resource/United_States> . }}
OPTIONAL { ?id <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?v_1
. } .
OPTIONAL { ?id <http://www.w3.org/2000/01/rdf-schema#label> ?v_2 . }
}
...
Regards.
--
Andrea Giovanni Nuzzolese
Semantic Technology Laboratory (STLab)
Institute for Cognitive Science and Technology (ISTC)
National Research Council (CNR)
Via Nomentana 56, Roma - Itay
