Steps to replicate the issue (include links if applicable):

1. Login to wm superset and open sqllab ( https://superset.wmcloud.org/sqllab/ )
2. Select s4 as database and commonswiki_p as schema
3. run query select gil_wiki from globalimagelinks group by gil_wiki

What happens?:
query timeouts.

Even with limit it is still slow, though it will run it

select gil_wiki from globalimagelinks group by gil_wiki limit 100

What should have happened instead?:

It should be almost instant. When running the query in SQL (mariadb console, quarry etc) it takes <1s. Example: Quarry: 83949.

Also modifying the query in superset as subquery like this makes it almost instant in superset.wmcloud.org too:

SELECT * FROM (select gil_wiki from globalimagelinks group by gil_wiki) AS t