Re: pg_rewind does not rewind diverging timelines

On 7/29/26 14:55, Andreas Karlsson wrote:
> On 7/27/26 01:36, Michael Paquier wrote:
>> On Mon, Jun 01, 2026 at 03:30:58PM +0900, Kyotaro Horiguchi wrote:
>>> I wonder whether strengthening the history-based matching would be
>>> sufficient instead. If timelines with the same TLI but different
>>> histories can be treated as distinct and pg_rewind continues walking
>>> the history chain until it finds a common ancestor, that seems like a
>>> fairly natural fit with the existing timeline model.
>>
>> Yeah, perhaps there is something that we could do here better in
>> pg_rewind in terms of TLI history.  A second software layer to ensure
>> TLI unicity feels just like a shortcut: we already have a LSN.
>
> As Ants said, the LSN can easily be the same in both.
>
>>> UUIDs would certainly make identification straightforward, although
>>> they would also introduce longer identifiers that are a bit less
>>> convenient for humans to work with. My initial thought is that it may
>>> be worth exploring how far we can get with the existing history
>>> information before introducing a new identifier.
>>
>> This.  For this reason, I am not convinced that this proposal is
>> neither acceptable or something that we need to do at all.
>>
>> Why should we bear the burden of introducing a second level of
>> timeline identification knowing that by design we have to rely on a
>> *single* archive location for a new timeline selection when a standby
>> is triggered for promotion?  My opinion is that this is trying to fix
>> a problem for something that it not actually a problem.  If you play
>> with HA scenarios where there is a risk of two standbys reusing the
>> same timeline number, just don't do that.  We've historically claimed
>> that the archive strategy is wrong if your deployments cannot
>> guarantee a unique TLI assignment.  A new sub-identification system
>> will not provide more guarantees.
>
> Having had to debug issues with pg_rewind and timelines in our
> Kubernetes Operator these UUIDs would probably have helped me a lot.
> Maybe the proposed fix is wrong (maybe like Ants says it does not got
> far enough) but from personal experience I disagree with that there is
> no problem here.
>
> I will try to find time to play around with the patch and see if it
> would have helped us.

Thanks Andreas,

I would love to hear if this would have been a help in those scenarios.

Best wishes,
Mats Kindahl, Multigres team, Supabase