Subject: Re: Is there any documentation on how to correctly create extensions in HA(primary-standby) setup?

From: Julien Rouhaud <[email-masked]>
Date: Tue, 18 Jan 2022 15:55:58 +0800

Hi,

On Thu, Dec 09, 2021 at 08:19:06AM +0530, Bharath Rupireddy wrote:
> 
> Thanks. Attaching v1 patch specifying the notes there. Please review.

I think that the common terminology is "module", not "extension".  That's
especially important here as this information is also relevant for modules that
may come with an SQL level extension.  This should be made clear in that new
documentation, same for the CREATE EXTENSION part that may not be relevant.

It also seems that this documentation is only aimed for physical replication.
It should also be explicitly stated as it might not be obvious for the intended
readers.

+     [...] set it either via <link linkend="sql-altersystem">ALTER SYSTEM</link>
+     command or <filename>postgresql.conf</filename> file on both primary and
+     standys, reload the <filename>postgresql.conf</filename> file and restart
+     the servers.

Isn't the reload a terrible advice?  By definition changing
shared_preload_libraries isn't compatible with a simple reload and will emit
some error.

+     [...] Create the extension on the primary, there is no need to
+     create it on the standbys as the <link linkend="sql-createextension"><command>CREATE EXTENSION</command></link>
+     command is replicated.

The "no need" here is quite ambiguous, as it seems to indicate that trying to
create the extension on the standby will work but is unnecessary.