Re: Global snapshots

On Fri, Jun 19, 2020 at 05:03:20PM +0800, movead(dot)li(at)highgo(dot)ca wrote:
>
> >> would like to know if the patch related to CSN based snapshot [2] is a
> >> precursor for this, if not, then is it any way related to this patch
> >> because I see the latest reply on that thread [2] which says it is an
> >> infrastructure of sharding feature but I don't understand completely
> >> whether these patches are related?
> >I need some time to study this patch.. At first sight it is different.
>
> This patch[2] is almost base on [3], because I think [1] is talking about 2PC
> and FDW, so this patch focus on CSN only and I detach the global snapshot
> part and FDW part from the [1] patch.
>
> I notice CSN will not survival after a restart in [1] patch, I think it may not
> the
> right way, may be it is what in last mail "Needs guarantees of monotonically
> increasing of the CSN in the case of an instance restart/crash etc" so I try to
> add wal support for CSN on this patch.
>
> That's why this thread exist.

I was certainly missing how these items fit together. Sharding needs
parallel FDWs, atomic commits, and atomic snapshots. To get atomic
snapshots, we need CSN. This new sharding wiki pages has more details:

https://wiki.postgresql.org/wiki/WIP_PostgreSQL_Sharding

After all that is done, we will need optimizer improvements and shard
management tooling.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee