RE: Perform streaming logical transactions by background workers and parallel apply

On Friday, February 3, 2023 11:04 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Feb 2, 2023 at 4:52 AM Peter Smith <smithpb2250(at)gmail(dot)com>
> wrote:
> >
> > Some minor review comments for v91-0001
> >
>
> Pushed this yesterday after addressing your comments!

Thanks for pushing.

Currently, we have two remaining patches which we are not sure whether it's worth
committing for now. Just share them here for reference.

0001:

Based on our discussion[1] on -hackers, it's not clear that if it's necessary
to add the sub-feature to stop extra worker when
max_apply_workers_per_suibscription is reduced. Because:

- it's not clear whether reducing the 'max_apply_workers_per_suibscription' is very
common.
- even when the GUC is reduced, at that point in time all the workers might be
in use so there may be nothing that can be immediately done.
- IIUC the excess workers (for a reduced GUC) are going to get freed naturally
anyway over time as more transactions are completed so the pool size will
reduce accordingly.

And given that the logic of this patch is simple, it would be easy to add this
at a later point if we really see a use case for this.

0002:

Since all the deadlock errors and other errors that caused by parallel streaming
will be logged and user can check this kind of ERROR and disable the parallel
streaming mode to resolve this. Besides, for this retry feature, It will
be hard to distinguish whether the ERROR is caused by parallel streaming, and we
might need to retry in serialize mode for all kinds of ERROR. So, it's not very
clear if automatic use serialize mode to retry in case of any ERROR in parallel
streaming is necessary or not. And we can also add this when we see a use case.

[1] https://www.postgresql.org/message-id/CAA4eK1LotEuPsteuJMNpixxTj6R4B8k93q-6ruRmDzCxKzMNpA%40mail.gmail.com

Best Regards,
Hou zj