Federated queries with wdt:P18 (image) crash

[tuukka]

I'm trying to get images related to OSM-Wikidata links, but my queries crash.

One error message (the query completes if I change the inner select to just SELECT ?item) :

Error processing query

Assertion `!isString()` failed. Please report this to the developers. In file "/local/data-ssd/qlever/qlever-code/src/parser/TripleComponent.h " at line 191
Your query was:

PREFIX geo: <http://www.opengis.net/ont/geosparql#>
PREFIX osm: <https://www.openstreetmap.org/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX osmkey: <https://www.openstreetmap.org/wiki/Key:>
PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX osm2rdfkey: <https://osm2rdf.cs.uni-freiburg.de/rdf/key#>
SELECT ?item ?osm_id ?osmName ?image ?geometry WHERE {
  SERVICE <https://qlever.cs.uni-freiburg.de/api/wikidata> {
    SELECT ?item ?image {
      ?item wdt:P31/wdt:P279* wd:Q473972 .
      ?item wdt:P18 ?image .
    }
  }
  ?osm_id osm2rdfkey:wikidata ?item .
  ?osm_id osmkey:boundary "protected_area" .
#  { ?osm_id rdf:type osm:way . } UNION { ?osm_id rdf:type osm:relation . }

  ?osm_id geo:hasGeometry/geo:asWKT ?geometry .
  ?osm_id osmkey:name ?osmName
}
ORDER BY ?item

Alternatively, if I mark the wdt:P18 as optional, I get another error message:

Error processing query

Parse error at byte position 8: A check for a required element failed The next 500 bytes are: .
Your query was:

PREFIX geo: <http://www.opengis.net/ont/geosparql#>
PREFIX osm: <https://www.openstreetmap.org/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX osmkey: <https://www.openstreetmap.org/wiki/Key:>
PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX osm2rdfkey: <https://osm2rdf.cs.uni-freiburg.de/rdf/key#>
SELECT ?item ?osm_id ?osmName ?image ?geometry WHERE {
  SERVICE <https://qlever.cs.uni-freiburg.de/api/wikidata> {
    SELECT ?item ?image {
      ?item wdt:P31/wdt:P279* wd:Q473972 .
      OPTIONAL { ?item wdt:P18 ?image . }
    }
  }
  ?osm_id osm2rdfkey:wikidata ?item .
  ?osm_id osmkey:boundary "protected_area" .
#  { ?osm_id rdf:type osm:way . } UNION { ?osm_id rdf:type osm:relation . }

  ?osm_id geo:hasGeometry/geo:asWKT ?geometry .
  ?osm_id osmkey:name ?osmName
}
ORDER BY ?item

[joka921]

Thanks for reporting this.
The underlying issue is the following:
QLever currently doesn't like if the result of a Service has contains a blank node (that is probably the cause of the !isString issue...
or if the Service returns undefined/optional values (your problem with optionals).
We currently have someone (@UNEXENU) working on the SERVICE implementation to get rid of such issues (among others).

A current workaround for your concrete query is to filter out the blank nodes (which you are probably not interested in anyway) by adding a
FILTER (!ISBLANK(?image))
inside the SERVICE clause:
[click me] (https://qlever.cs.uni-freiburg.de/osm-planet/iGdZki)

[tuukka]

Thank you! The workaround is suitable for me. To allow OPTIONAL, I can even change the missing values to a placeholder value: https://qlever.cs.uni-freiburg.de/osm-planet/wbFBTJ