Perhaps I could emphasize things differently. FWIW, besides the fact that having a foreign key constraint in that schema would prevent the badness from happening, the even bigger reason why scenarios like that are unlikely is that, realistically, for Tobi to reply to a comment, he must have seen that comment he's about to reply to, and it's very hard to imagine a scenario where he'd see it but still have the response transaction not read it (cause if the txn read it, the two transactions wouldn't be independent any more and so they'd be well ordered). The foreign key is just one way of ensuring that the read happens.