Support OPTIONAL{}

[niklas88]

This is used by Aqqu for name relations (fb:type.object.name) so it also gets entities without a name such as dates.

As an example with QLever I currently make the optional name mandatory and get the following query

PREFIX fb: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?0 ?0name WHERE {
   fb:m.02y1vz fb:internet.website.launched ?0 .
   ?0 fb:type.object.name ?0name . 
   FILTER (?0 != fb:m.02y1vz) 
} LIMIT 300

This doesn't find an answer while what Aqqu really wanted to generate is

PREFIX fb: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?0 ?0name WHERE {
   fb:m.02y1vz fb:internet.website.launched ?0 .
   OPTIONAL {?0 fb:type.object.name ?0name . }
   FILTER (?0 != fb:m.02y1vz) 
} LIMIT 300

which gives the correct answer. The alternative would be to do this as two separate queries but this would need more special treatment for the QLever backend.

Currently this contributes to a much worse avg-F1 (36% vs 65%) with QLever even when using the exact same model (actually trained using QLever)

[Buchhold]

I still agree that this is an important feature. However, I fear I won't really have time. I really want to get the memory savvy version done before everything else