`SERVICE` query ignores `LIMIT`

[hannahbast]

The following query should return a single row, but it takes forever because the SERVICE query is executed without the LIMIT 1.

@UNEXENU and @joka921 Do you have an idea why that is the case?

SELECT * WHERE {
  SERVICE <https://qlever.cs.uni-freiburg.de/api/wikidata> {
    SELECT ?s ?p ?o WHERE { ?s ?p ?o } LIMIT 1
  }
}

https://qlever.cs.uni-freiburg.de/wikidata/RJCTro

[UNEXENU]

I've replicated the error with the following query on the olympics dataset:

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX olympics: <http://wallscope.co.uk/ontology/olympics/>
PREFIX medal: <http://wallscope.co.uk/resource/olympics/medal/>
PREFIX dbo: <http://dbpedia.org/ontology/>
SELECT * WHERE {
  SERVICE <https://qlever.cs.uni-freiburg.de/olympics> {
    SELECT ?athlete WHERE {
      ?athlete dbo:team <http://wallscope.co.uk/resource/olympics/team/Germany> .
    }
    LIMIT 1
  }
}

Whats happening is, the Service puts a SELECT-clause with the visible variables around the query it gets passed - here SELECT ?athlete WHERE { SELECT ?athlete WHERE { ... } LIMIT 1}.

So the Service-endpoint in the query above basically computes the same incorrect result as the following nested query:

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX olympics: <http://wallscope.co.uk/ontology/olympics/>
PREFIX medal: <http://wallscope.co.uk/resource/olympics/medal/>
PREFIX dbo: <http://dbpedia.org/ontology/>
SELECT ?athlete WHERE {
  SELECT ?athlete WHERE {
    ?athlete dbo:team <http://wallscope.co.uk/resource/olympics/team/Germany> .
  }
  LIMIT 1
}

It somehow works (returns one row) when ORDER BY ?athlete is added before LIMIT 1, however i don't know why that is the case.

[hannahbast]

@UNEXENU Thanks a lot for the reply. The issue has indeed nothing to do with SERVICE. It happens exactly when there is a subquery with a single index scan and a LIMIT. A minimal dataset-independent example is:

SELECT * WHERE {
  SELECT * WHERE { ?s ?p ?o } LIMIT 1
}

https://qlever.cs.uni-freiburg.de/wikidata/IUeShp