Re: Skipping logical replication transactions on subscriber side

On Sat, Jan 22, 2022 at 9:51 PM David G. Johnston
<david.g.johnston@gmail.com> wrote:
>
> So long as the ALTER command errors when asked to skip those IDs there isn't any reason for an end-user, who likely
doesn'tknow or care that 1 and 2 are special, to be concerned about them (the only two invalid values) while reading
thedocs. 
>

In this matter, I don't see any problem with the current text proposed
and there are many others who have also reviewed it. I am fine to
change if others also think that the current text needs to be changed.

>>
>> > Additionally, the description for pg_stat_subscription_workers should describe what happens once the transaction
representedby last_error_xid has either been successfully processed or skipped.  Does this "last error" stick around
untilanother error happens (which is hopefully very rare) or does it reset to blanks? 
>> >
>>
>> It will be reset only on subscription drop, otherwise, it will stick
>> around until another error happens.
>
>
> I really dislike the user experience this provides, and given it is new in v15 (and right now this table seems to
existsolely to support this feature) changing this seems within the realm of possibility. I have to imagine these
workershave a sense of local state that would just be "no errors, no need to touch pg_stat_subscription_workers at the
endof this transaction's commit".  It would save a local state of the error_xid and if a successfully committed
transactionhas that xid it would clear the error.  The skip code path would also check for and see the matching xid
valueand clear the error.  Even if the local state thing doesn't work, one catalog lookup per transaction seems like
potentiallyreasonable overhead to incur here. 
>

Are you telling to update the catalog to save error_xid when an error
occurs? If so, that has many challenges like we are not supposed to
perform any such operations when the transaction is in an error state.
We have discussed this and other ideas in the beginning. I don't find
any of your arguments convincing to change the basic approach here but
I would like to see what others think on this matter?

--
With Regards,
Amit Kapila.