Dear @dblp, we are very happy to see that you are deploying a QLever-based SPARQL endpoint. Would you probably mind to increase your cache size as this simple 2-hop query doesn't work:
SELECT ?p ?o ?pp ?oo WHERE {
<https://dblp.org/rec/books/acm/19/X19> ?p ?o .
OPTIONAL { ?o ?pp ?oo . }
}
https://sparql.dblp.org/n9kCi4
Thank you very much!! :)
Stop the war! @lysander07 I just checked with Hannah Bast and it seems that this is actually a limitation of how OPTIONAL is implemented in our current version of QLever. As the engine is still in active development, this is something that will be fixed in the future (fingers crossed). In the meantime, of course, there are query variants that avoid this problem, see: https://sparql.dblp.org/bp18ZE
FYI, the QLever team is collecting such valuable feedback in its GitHub repo, see https://github.com/ad-freiburg/qlever/issues
~MRA