Aggregate query with only one ql:has-predicate triple fails

[hannahbast]

The following example query currently fails on Wikidata EN+DE @ vulcano:

PREFIX we: <http://www.wikidata.org/entity/>
PREFIX wp: <http://www.wikidata.org/prop/direct/>
SELECT ?r (COUNT(?r) AS ?count) WHERE {
  ?a ql:has-predicate ?r
}
GROUP BY ?r
ORDER BY DESC(?count)

The reason is that the query planner decides to use the "pattern trick", which leads to an empty query after the only triple is taken out. The correct handling would be to process ql:has-predicate without the pattern trick.