Comment by derefr

If the goal is to count distinct users who have seen something, but also to efficiently retrieve the actual set of users who have seen something — then why wasn't a roaring bitmap (https://pgxn.org/dist/pg_roaringbitmap/) considered as an option? Presuming user IDs are non-sparse nonnegative integers (as SERIAL/BIGSERIAL pkeys usually are), a roaring bitmap should do everything hstore does, just a lot more efficiently.