> OTOH, if and only if you design your schema to exploit MySQL’s clustering index (like for 1:M, make the PK of the child table something like (FK, some_id)), your range scans will become incredibly fast. But practically no one does that.

You can achieve that with Postgres as well if you accept duplicating the data by adding an index with an include clause containing all the non-key columns you want to return in the SELECT clause. This way, you'll get fast index-only scans.