It is true that DB compute is relatively expensive, but doing a join on the DB itself is obscenely more efficient vs doing it manually in a nested loop on an app server.

Also, the DB is generally a lot smarter than doing quadratic operations. The usual worst case is a Merge Join which involves sorting both tables first, to do an efficient join in nlogn time. It’s just not feasible to compete with the DB for efficiency without reimplementing a DB, which is pretty yikes.

See https://www.cybertec-postgresql.com/en/join-strategies-and-p...