[rich_sasha]

I would imagine (but haven't looked at it at all) that it's a byproduct of an append only data format. Then having a historical PoV is cheap - you simply disregard changes after a certain time.

Append-only has many other benefits, including zero contention between many readers and (single) writers. In the vanilla version, writers still contend though.

[hansvm]

I think their point is that system timestamps for that append-only format aren't good enough. You need logical timestamps corresponding to increasing transaction ids.