In trying to answer a StackOverflow question [1], I ended up writing
this query for the DBpedia endpoint [2], which works:
[[[
select ?s ?p ?o where {
values ?web { dbpedia:World_Wide_Web }
values ?p { rdf:type dbpedia-owl:abstract }
?web ?p ?o bind( ?web as ?s )
}
limit 3
]]]
Note that there are _two_ values for `?p`. When I try to add a union
to the pattern, I get an error:
[[[
Virtuoso 37000 Error SP031: SPARQL compiler: Internal error:
sparp_gp_attach_filter_cbk(): attempt to attach a filter with used
variable
SPARQL query:
define sql:big-data-const 0
#output-format:text/html
define sql:signal-void-variables 1 define input:default-graph-uri
<http://dbpedia.org> select ?s ?p ?o where {
values ?web { dbpedia:World_Wide_Web }
values ?p { rdf:type dbpedia-owl:abstract }
{ ?web ?p ?o bind( ?web as ?s ) }
union
{ ?s ?p ?web bind( ?web as ?o ) }
}
limit 3
]]]
Is this a Virtuoso bug, or a bug in the DBpedia endpoint
configuration, or something else? The query (when prefixes are added)
is legal SPARQL that's accepted by the validator at sparql.org.
[1] http://stackoverflow.com/q/21002440/1281433
[2] http://dbpedia.org/sparql
--
Joshua Taylor, http://www.cs.rpi.edu/~tayloj/
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