Re: Copy function for logical replication slots

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.

Regards,

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