Re: Why ALTER SUBSCRIPTION ... SET (slot_name='none') requires subscription disabled?

On Thu, 08 Jul 2021 at 17:51, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Wed, Jul 7, 2021 at 7:25 PM Japin Li <japinli(at)hotmail(dot)com> wrote:
>>
>> Hi, hackers
>>
>> The documentation [1] says:
>>
>> When dropping a subscription that is associated with a replication slot on the
>> remote host (the normal state), DROP SUBSCRIPTION will connect to the remote
>> host and try to drop the replication slot as part of its operation. This is
>> necessary so that the resources allocated for the subscription on the remote
>> host are released. If this fails, either because the remote host is not
>> reachable or because the remote replication slot cannot be dropped or does not
>> exist or never existed, the DROP SUBSCRIPTION command will fail. To proceed in
>> this situation, disassociate the subscription from the replication slot by
>> executing ALTER SUBSCRIPTION ... SET (slot_name = NONE).
>>
>> However, when I try this, it complains the subscription is enabled, this command
>> requires the subscription disabled. Why we need this limitation?
>>
>
> If we don't have this limitation then even after you have set the slot
> name to none, the background apply worker corresponding to that
> subscription will continue to stream changes via the previous slot.
>

Yeah, thanks for your explain! Should we add some comments here?

>> OTOH, when I execute ALTER SUBSCRIPTION ... SET (slot_name=''), it doesn't complain. However,
>> SELECT select pg_create_logical_replication_slot('', 'pgoutput') complains slot name is too
>> short. Although, the slot will be created at publisher, and validate the slot name, IMO, we
>> can also validate the slot_name in parse_subscription_options() to get the error early.
>> Attached fixes it. Any thoughts?
>>
>
> Oh, I think this should be fixed. Can anyone else think this to be
> valid behavior?
>

Ranier Vilela provides a v2 patch and reduce the overhead of
ReplicationSlotValidateName() in thread [1].

[1] https://www.postgresql.org/message-id/CAEudQAqLtNJ1wvMKLK8ZH27SGJW5OjizgyMq28bFj-_5QG1G+A@mail.gmail.com

--
Regrads,
Japin Li.
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