KnarQL UNION: problem it ORDER BY criterion

[tobiasschweizer]

When I run the following KnarQL query, the triplestore crashes:

PREFIX beol: <http://0.0.0.0:3333/ontology/beol/simple/v2#>
    PREFIX knora-api: <http://api.knora.org/ontology/knora-api/simple/v2#>
    
    CONSTRUCT {
        ?letter knora-api:isMainResource true .
        
        ?letter beol:creationDate ?date .
    
        ?letter beol:hasAuthor  ?person1 .   

    } WHERE {
        {
            ?letter a knora-api:Resource .
            ?letter a beol:letter .
            
            ?letter beol:creationDate ?date .
        
            beol:creationDate knora-api:objectType knora-api:Date .
            ?date a knora-api:Date .
            
        
        } UNION {
        
            ?letter beol:hasAuthor ?person1 .
            
            beol:hasAuthor knora-api:objectType knora-api:Resource .
            
            ?person1 a beol:person .
            ?person1 a knora-api:Resource .
        } 
           
    } ORDER BY ?date

SPARQL that is executed by the triplestore:

 SELECT DISTINCT ?letter (GROUP_CONCAT(?person1; SEPARATOR='') AS ?person1Concat) (GROUP_CONCAT(?date; SEPARATOR='') AS ?dateConcat) (GROUP_CONCAT(?person1__LinkValue; SEPARATOR='') AS ?person1__LinkValueConcat)
 WHERE {
 {
 ?letter <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#Resource> .
 GRAPH <http://www.ontotext.com/explicit> {
     ?letter <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 ?letter <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/beol#letter> .
 ?letter <http://www.knora.org/ontology/beol#creationDate> ?date .
 GRAPH <http://www.ontotext.com/explicit> {
     ?date <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 } UNION {
 ?letter <http://www.knora.org/ontology/beol#hasAuthor> ?person1 .
 ?letter <http://www.knora.org/ontology/beol#hasAuthorValue> ?person1__LinkValue .
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#LinkValue> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> ?letter .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?person1 .
 }
 ?person1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#Resource> .
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1 <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 ?person1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/beol#person> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?date <http://www.knora.org/ontology/knora-base#valueHasStartJDN> ?date__valueHasStartJDN .
 }
 }
 
 GROUP BY ?letter ?date__valueHasStartJDN
 ORDER BY ASC(?date__valueHasStartJDN) ASC(?letter)
 OFFSET 0
 LIMIT 25

Problem: the pattern GRAPH <http://www.ontotext.com/explicit> { ?date <http://www.knora.org/ontology/knora-base#valueHasStartJDN> ?date__valueHasStartJDN . } is inserted after the last UNION block and ?date__valueHasStartJDN does not seem to be bound.

Solution: move the pattern to the end of the first block of the WHERE clause.

 SELECT DISTINCT ?letter (GROUP_CONCAT(?person1; SEPARATOR='') AS ?person1Concat) (GROUP_CONCAT(?date; SEPARATOR='') AS ?dateConcat) (GROUP_CONCAT(?person1__LinkValue; SEPARATOR='') AS ?person1__LinkValueConcat)
 WHERE {
 {
 ?letter <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#Resource> .
 GRAPH <http://www.ontotext.com/explicit> {
     ?letter <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 ?letter <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/beol#letter> .
 ?letter <http://www.knora.org/ontology/beol#creationDate> ?date .
 GRAPH <http://www.ontotext.com/explicit> {
     ?date <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
GRAPH <http://www.ontotext.com/explicit> {
     ?date <http://www.knora.org/ontology/knora-base#valueHasStartJDN> ?date__valueHasStartJDN .
 }
 } UNION {
 ?letter <http://www.knora.org/ontology/beol#hasAuthor> ?person1 .
 ?letter <http://www.knora.org/ontology/beol#hasAuthorValue> ?person1__LinkValue .
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#LinkValue> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> ?letter .
 }
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1__LinkValue <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?person1 .
 }
 ?person1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/knora-base#Resource> .
 GRAPH <http://www.ontotext.com/explicit> {
     ?person1 <http://www.knora.org/ontology/knora-base#isDeleted> "false"^^<http://www.w3.org/2001/XMLSchema#boolean> .
 }
 ?person1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.knora.org/ontology/beol#person> .
 }

 }
 
 GROUP BY ?letter ?date__valueHasStartJDN
 ORDER BY ASC(?date__valueHasStartJDN) ASC(?letter)
 OFFSET 0
 LIMIT 25

[tobiasschweizer]

see also #642

[tobiasschweizer]

I think the KnarQL query above is actually wrong, the first part should be required. As it is now, it asks for alternative patterns.

It should be rewritten to:

PREFIX beol: <http://0.0.0.0:3333/ontology/beol/simple/v2#>
    PREFIX knora-api: <http://api.knora.org/ontology/knora-api/simple/v2#>
    
    CONSTRUCT {
        ?letter knora-api:isMainResource true .
        
        ?letter beol:creationDate ?date .
    
        ?letter beol:hasAuthor  ?person1 .   

    } WHERE {
        
            ?letter a knora-api:Resource .
            ?letter a beol:letter .
            
            ?letter beol:creationDate ?date .
        
            beol:creationDate knora-api:objectType knora-api:Date .
            ?date a knora-api:Date .
            
         OPTIONAL {
        
            ?letter beol:hasAuthor ?person1 .
            
            beol:hasAuthor knora-api:objectType knora-api:Resource .
            
            ?person1 a beol:person .
            ?person1 a knora-api:Resource .
        } 
           
    } ORDER BY ?date

Then we have the same problem as reported in #686