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

On Thu, 08 Jul 2021 at 18:17, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Thu, Jul 8, 2021 at 3:43 PM Japin Li <japinli(at)hotmail(dot)com> wrote:
>>
>> 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?
>>
>
> Sure, but let's keep that as a separate HEAD-only patch.

Please consider review v3 patch. v3-0001 adds slot_name verification in
parse_subscription_options() and comments for why we need disable subscription
where set slot_name to NONE. v3-0002 comes from Ranier Vilela, it reduce the
overhead strlen in ReplicationSlotValidateName().

--
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.