[wbl]

I don't think that works. First off the server doesn't know when the client goes away so has to hold the snapshot forever. Secondly you double your replication latency and have extreme sensitivity to stragglers, unless you take the risk of hitting an unavailable cache.

[jiggawatts]

The database engine would have to be designed for this, and/or the clients could request the level of consistency that they require. Most apps only care about “this age or newer”, except when paging through a data set where consistency matters.

E.g.: indexes can include the timestamp and then queries can filter out new rows implicitly. Physically this can be implemented with tiered indexes where the topmost layer is in-memory only and queries older than what it contains are rejected. The on-disk indexes then don’t need old row versions or timestamps.