On 28 February 2016 at 06:38, Kevin Grittner <kgrittn@gmail.com> wrote:
> For logical replay, applying in batches is actually a good thing since it > allows parallelism. We can remove them all from the target's procarray all > at once to avoid intermediate states becoming visible. So that would be the > preferred mechanism.
That could be part of a solution. What I sketched out with the "apparent order of execution" ordering of the transactions (basically, commit order except when one SERIALIZABLE transaction needs to be dragged in front of another due to a read-write dependency) is possibly the simplest approach, but batching may well give better performance.
I'd be really interested in some ideas on how that information might be usefully accessed. If we could write info on when to apply commits to the xlog in serializable mode that'd be very handy, especially when looking to the future with logical decoding of in-progress transactions, parallel apply, etc.
For parallel apply I anticipated that we'd probably have workers applying xacts in parallel and committing them in upstream commit order. They'd sometimes deadlock with each other; when this happened all workers whose xacts committed after the first aborted xact would have to abort and start again. Not ideal, but safe.
Being able to avoid that by using SSI information was in the back of my mind, but with no idea how to even begin to tackle it. What you've mentioned here is helpful and I'd be interested if you could share a bit more of your experience in the area.