OPTIONAL and FILTER in a single query seem to create a conflict

[niklas88]

Trying to execute the following query (generated by Aqqu when running with the QLever backend)

PREFIX fb: <http://rdf.freebase.com/ns/>

SELECT DISTINCT ?0 ?0name WHERE {
 fb:m.06w2sn5 fb:imdb.topic.name_id ?0 .
 OPTIONAL {?0 fb:type.object.name ?0name} .
 FILTER (?0 != fb:m.06w2sn5)
} LIMIT 300

QLever can't find an execution plan. Removing either the OPTIONAL or the FILTER part results in a viable plan. @floriankramer any ideas?

Also we are missing a query with OPTIONAL in the end-to-end tests (also #68)!

[niklas88]

This is a reproducer query on Scientists:

SELECT ?x ?y WHERE {
    ?x <is-a> <Scientist> .
    OPTIONAL { ?x <Spouse_(or_domestic_partner)> ?y } .
    FILTER (?x < <Ada_Lovelace>)
} ORDER BY ASC(?x)

removing either the OPTIONAL or FILTER results in a working query.

[niklas88]

Ok, so it seems this is a problem during query optimization as running with the -u flag for not optimizing OPTIONAL works around the problem.