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

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.

--
With Regards,
Amit Kapila.