Message-ID: <0bf0971a-d72d-436e-b8aa-b34abdb1c3de@gmail.com>
Date: Wed, 12 Aug 2026 08:33:19 +0200
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: pg_rewind does not rewind diverging timelines
To: Andreas Karlsson <andreas@proxel.se>,
 Michael Paquier <michael@paquier.xyz>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: japinli@hotmail.com, suryapoondla4@gmail.com,
 pgsql-hackers@lists.postgresql.org
References: 
 <SY7PR01MB10921E0F8383139EB27B33C11B6162@SY7PR01MB10921.ausprd01.prod.outlook.com>
 <9ce0d2b9-7a41-4a8a-b299-da295bb4514f@gmail.com>
 <SY7PR01MB10921F0B09B693C8D5149880EB6152@SY7PR01MB10921.ausprd01.prod.outlook.com>
 <20260601.153058.2271004477925758292.horikyota.ntt@gmail.com>
 <amaZ6bN0pJM75OEs@paquier.xyz>
 <49b84ecf-21a2-4e2b-ab66-fbba8ee6540e@proxel.se>
Content-Language: en-US
From: Mats Kindahl <mats.kindahl@gmail.com>
In-Reply-To: <49b84ecf-21a2-4e2b-ab66-fbba8ee6540e@proxel.se>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Archived-At: 
 <https://www.postgresql.org/message-id/0bf0971a-d72d-436e-b8aa-b34abdb1c3de%40gmail.com>
Precedence: bulk


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