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Date: Sat, 3 Jan 2026 00:38:10 +0100
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Subject: Re: Parallelizing startup with many databases
To: Babak Ghadiri <bbkghadiri6@gmail.com>, pgsql-hackers@lists.postgresql.org
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From: Andreas Karlsson <andreas@proxel.se>
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On 1/2/26 8:55 AM, Babak Ghadiri wrote:
> In PostgreSQL 16, startup appears to initialize databases sequentially and
> primarily uses a single CPU core. In clusters with a very large number of
> databases (around 5,000 in our case), this results in noticeably long
> startup times after restarts or crash recovery.

Have you measured what is actually causing the slow startup? Without 
knowing what is actually slow it is hard to say if threading would even 
help.

How slow are we talking about and have you managed to create a minimal 
case for reproducing the issue?

> - Is the largely single-threaded startup behavior a fundamental 
> architectural
>    constraint (e.g. catalog dependencies, locking, recovery ordering), 
> or mainly
>    an unimplemented optimization?

PostgreSQL does not support threading, it uses a multi-process model to 
implement for example parallel queries. And there is no way threading 
would be introduced just to improved startup performance.

> - Are there any existing discussions, patches, versions (18+) to 
> parallelize parts of startup or otherwise improve startup scalability 
> with many databases?

Not as far as I am aware but you can search our archives.

> - Are there any PostgreSQL configuration settings known to dramatically 
> reduce startup time, or is startup performance mostly fixed by 
> architecture in this scenario?

I would first start trying to figure out why startup is slow before 
doing anything else.

Andreas