[anarazel]

> I can completely imagine the corner cases this ends up touching, as there must be an insane amount of logic in Postgres that assumes immutability of data on disk: if I read data block X for transaction version 1234, it will always remain the same.

Most of the code having such low level assumptions has been centralized for PG12, as part of the introduction of pluggable table storage. Also note that that assumption as stated isn't actually true - we don't expect on-disk data to be immutable. There is on-access "pruning" of dead rows, there's VACUUM, there's new row versions on the same page etc.