Very slow GROUP BY query, which could be much faster with better query plan

[hannahbast]

The following query is currently very slow on Wikipedia+FreebaseEasy:

SELECT ?object WHERE {
  ?subject <is-a> ?object 
}
GROUP BY ?object
ORDER BY DESC((COUNT(?object) AS ?count))

The log shows that the PSO index is scanned and then the 130K tuples from the predicate are sorted. The sorting is used to make the GROUP BY easy.

For the query above, it would clearly be better to use the POS index. Then there would be no need to sort for the GROUP BY ?object.

Note that the sorting required by the final ORDER BY is no problem, since there are only 47K distinct objects for this query.