Apache Druid

it's pretty specific in what it's useful for: the use case is you have a situation where you can guarantee that each individual value can only occur in one segment (perhaps you achieved this through partitioning)

in that situation the extension does segment-local distinct counts, using bitmaps, then sums them up across segments

it's kind of specialty and also sharp-edgy — it does not validate that its own preconditions are met. that's up to the user

you need to lay your data out a certain way and do your queries a certain way

so i believe those two factors are why it hasn't been super popular