The issue (iirc) with that sort of query is that it's essentially a join, so you...

blattimwind on June 21, 2018 | parent | context | favorite | on: PostgreSQL Features You May Not Have Tried but Sho...

The issue (iirc) with that sort of query is that it's essentially a join, so you'll get count(projects)*sum(length(regions)) intermediate rows that need to be grouped back together.