Re: Proposal for CSN based snapshots

On Wed, Aug 10, 2016 at 12:26 PM, Greg Stark <stark@mit.edu> wrote:

> Complexity like this makes it hard to implement other features such as
> CSNs. IIRC this already bit hot standby as well. I think it would be a
> big improvement if we had a clear, well defined commit order that was
> easy to explain and easy to reason about when new changes are being
> made.

And here I was getting concerned that there was no mention of
"apparent order of execution" for serializable transactions --
which does not necessarily match either the order of LSNs from
commit records nor CSNs.  The order in which transactions become
visible is clearly a large factor in determining AOoE, but it is
secondary to looking at whether a transaction modified data based
on reading the "before" image of a data set modified by a
concurrent transaction.

I still think that our best bet for avoiding anomalies when using
logical replication in complex environments is for logical
replication to apply transactions in apparent order of execution.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company