Then I suspect that it would try the GROUP BY on the desktop. Unfortunately I never get that far because the desktop can't handle 300M records... I'd never expect it to handle that large of a data set. That's why the query should filter and group on the database.
2. Power Query should allow a LIMIT clause to be defined such that the preview and editor don't try to bring back more than X number of records. Preview definitly does not work when working with large data sets. Well, it does, but in my case it's taking 10 min for the data to come through. I believe Power Query is assuming that all sources can stream results. This is not the case. A LIMIT clause would help the database run lighter.