It’s not just the writes. If you’re using a non-k-sortable item (like UUIDv4) as the PK, you’re throwing away every linear read-ahead that the kernel is doing for you, because the odds of what you need being on the same page is nil. When you’re paying for IOPS, like in a cloud, that’s even worse.

You’re also causing huge amounts of WAL bloat (unless you’re running ZFS and can thus safely disable full-page writes) [1].

And on a system with a clustered index (InnoDB, SQL Server), the performance and space bloat is even worse.

[0]: https://www.cybertec-postgresql.com/en/unexpected-downsides-...

[1]: https://www.2ndquadrant.com/en/blog/on-the-impact-of-full-pa...