What does the columnar format look like? Particularly, is the group by column compressed with RLE? That’s kind of a pre-computed group-by + count that would make this kind of query very very fast :)
This is an astute observation. Here is the accurate answer from the author of the feature:
“We do have special handling for RLE for filters and group by column but in both cases there would be per row component (updating selection vector for filter on RLE, updating aggregates for group by on RLE) and so far I saw RLE encoding usually slower than dictionary encoding for both these cases”