No they're not, even with a `cache` value of 1. Sequence values are issued at insert rather than commit. A transaction that commits later (which makes all updates visible) can have an earlier value than a previous transaction.
This is problematic if you try to depend on the ordering. Nothing is stopping some batch process that started an hour ago from committing a value 100k lower than where you thought the sequence was at. That's an extreme example but the consideration is the same when dealing with millisecond timeframes.
