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Also if you have a nanosecond timestamp, do you actually need a monotonically increasing row ID? What for?

Perhaps as a tie breaker if you insert multiple rows in a table within one transaction? In this situation, the timestamp returned by e.g. `now()` refers to the start of the transaction, which can cause it to be reused multiple times.

I meant in a context with a random ID and a timestamp.

If you need to retrieve rows by time/order, you use the timestamp. If you need a row by ID, you use the ID.

The use-cases where you actually need to know which row was inserted first seem exceedingly rare (mostly financial / auditing applications), and even then can probably be handled with separate transactions (as you touch on).

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