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

On Tuesday, April 19, 2022 2:58 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Apr 14, 2022 at 9:12 AM houzj(dot)fnst(at)fujitsu(dot)com
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > On Friday, April 8, 2022 5:14 PM houzj(dot)fnst(at)fujitsu(dot)com
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > Attach a new version patch which improved the error handling and handled
> the case
> > when there is no more worker available (will spill the data to the temp file in
> this case).
> >
> > Currently, it still doesn't support skip the streamed transaction in bgworker,
> because
> > in this approach, we don't know the last lsn for the streamed transaction
> being applied,
> > so cannot get the lsn to SKIP. I will think more about it and keep testing the
> patch.
> >
>
> I think we can avoid performing the streaming transaction by bgworker
> if skip_lsn is set. This needs some more thought but anyway I see
> another problem in this patch. I think we won't be able to make the
> decision whether to apply the change for a relation that is not in the
> 'READY' state (see should_apply_changes_for_rel) as we won't know
> 'remote_final_lsn' by that time for streaming transactions. I think
> what we can do here is that before assigning the transaction to
> bgworker, we can check if any of the rels is not in the 'READY' state,
> we can make the transaction spill the changes as we are doing now.
> Even if we do such a check, it is still possible that some rel on
> which this transaction is performing operation can appear to be in
> 'non-ready' state after starting bgworker and for such a case I think
> we need to give error and restart the transaction as we have no way to
> know whether we need to perform an operation on the 'rel'. This is
> possible if the user performs REFRESH PUBLICATION in parallel to this
> transaction as that can add a new rel to the pg_subscription_rel.

Changed as suggested.

Attach the new version patch which cleanup some code and fix above problem. For
now, it won't apply streaming transaction in bgworker if skiplsn is set or any
table is not in 'READY' state.

Besides, extent the subscription streaming option to ('on/off/apply(apply in
bgworker)/spool(spool to file)') so that user can control whether to apply The
transaction in a bgworker.

Best regards,
Hou zj