Re: Copy function for logical replication slots

On Wed, Aug 29, 2018 at 9:39 AM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> On Tue, Aug 28, 2018 at 10:34 PM, Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>> On Tue, Aug 28, 2018 at 04:14:04PM +0900, Masahiko Sawada wrote:
>>> I think the copying from a slot that already reserved WAL would be
>>> helpful for backup cases (maybe you suggested?). Also, either way we
>>> need to make a safe logic of acquring and releasing the source slot
>>> for logical slots cases. Or you meant to restrict the case where the
>>> copying a slot that doesn't reserve WAL?
>>
>> I mean the latter, as-known-as there is no actual point in being able to
>> copy WAL which does *not* reserve WAL.
>
> Agreed. I'll restrict that case in the next version
>
>>
>>>> Does it actually make sense to allow copy of temporary slots or change
>>>> their persistence? Those don't live across sessions so they'd need to
>>>> be copied in the same session which created them.
>>>
>>> I think the copying of temporary slots would be an impracticable
>>> feature but the changing their persistence might be helpful for some
>>> cases, especially copying from persistent to temporary.
>>
>> The session doing the copy of a permanent slot to the temporary slot has
>> to be the one also consuming it as the session which created the slot
>> owns it, and the slot would be dropped when the session ends. For
>> logical slots perhaps you have something in mind? Like copying a slot
>> which is not active to check where it is currently replaying, and using
>> the copy for sanity checks?
>
> Yeah, I imagined such case. If we want to do backup/logical decoding
> from the same point as the source permanent slot we might want to use
> temporary slots so that it will be dropped surely after that.
>

Attached a new version patch incorporated the all comments I got.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center