Re: Global snapshots

> On 7 May 2018, at 20:04, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> But what happens if a transaction starts on node A at time T0 but
> first touches node B at a much later time T1, such that T1 - T0 >
> global_snapshot_defer_time?
>

Such transaction will get "global snapshot too old" error.

In principle such behaviour can be avoided by calculating oldest
global csn among all cluster nodes and oldest xmin on particular
node will be held only when there is some open old transaction on
other node. It's easy to do from global snapshot point of view,
but it's not obvious how to integrate that into postgres_fdw. Probably
that will require bi-derectional connection between postgres_fdw nodes
(also distributed deadlock detection will be easy with such connection).

--
Stas Kelvich
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company