I wrote:
>> Another point that requires some thought is that switching SnapshotNow
>> to be MVCC-based will presumably result in a noticeable increase in each
>> backend's rate of wanting to acquire snapshots.
BTW, I wonder if this couldn't be ameliorated by establishing some
ground rules about how up-to-date a snapshot really needs to be.
Arguably, it should be okay for successive SnapshotNow scans to use the
same snapshot as long as we have not acquired a new lock in between.
If not, reusing an old snap doesn't introduce any race condition that
wasn't there already.
regards, tom lane