Misleading not groupable error on query that tries to group by an aggregate

[dpriskorn]

Does anyone know why this fails?

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wikibase: <http://wikiba.se/ontology#>
#title:Medicines ordered by the most studied 
SELECT ?main_subject (count(?item) as ?c)
WHERE 
{
  ?item wdt:P31 wd:Q13442814;
        wdt:P921 ?main_subject.
  ?main_subject wdt:P31/wdt:P279* wd:Q12140.
}
group by ?main_subject ?c

"Exception: BAD QUERY (Groupby variable ?main_subject is not groupable; in ../src/engine/GroupBy.cpp, line 704, function virtual void GroupBy::computeResult(ResultTable*))"

Here is a similar query that works on WDQS

[hannahbast]

The ?c in the GROUP BY looks strange. We were not aware that this is valid SPARQL.

However, the result is exactly the same (also on the WDQS) when you remove the ?c from the GROUP BY.

Without the ?c, the query also works on QLever, just 40 times faster.

[bpelakh]

I agree with @hannahbast , I don't think it's valid to group by an aggregate (which is only derived from the first grouping). The error message is misleading, however, and that should be fixed.

[hannahbast]

This is fixed, the error message is now "GROUP BY variable ?c is not groupable"