Query with one ql:has-predicate triple is slow if COUNT is in ORDER BY clause

[hannahbast]

The following query is very slow (46 seconds) on http://qlever.informatik.uni-freiburg.de/?backend=1 (Wikipedia+FreebaseEasy with up-to-date QLever build as of this writing):

SELECT ?predicate WHERE {
  ?x ql:has-predicate ?predicate
}
GROUP BY ?predicate
ORDER BY DESC((COUNT(?predicate) AS ?count))

However, the following query, which puts the COUNT in the SELECT clause instead of in the ORDER BY clause (and thus actually has more work to do) is much faster (~ 0.4 seconds):

SELECT ?predicate (COUNT(?predicate) AS ?count) WHERE {
  ?x ql:has-predicate ?predicate
}
GROUP BY ?predicate
ORDER BY DESC(?count)

The reason is obviously that the pattern trick is used for the second query, but not for the first query. The solution is obviously to also use it for the first query.