[TomaszZielinski]

This is pretty intuitive when you think about predicate locks that Postgres uses to detect conflicts.

If you have one SERIALIZABLE transaction that sets some locks, and one non-SERIALIZABLE that doesn't, then they can't "see" each other "by definition".

But your point stands--there could be some kind of "warning flag" somewhere, that would alert if SERIALIZABLE transactions overlap with non-SERIALIZABLE ones. Or maybe there _already_ is something like that??