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Date: Mon, 7 Apr 2025 13:44:08 -0700
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Subject: Re: find replication slots that "belong" to a publication
To: Willy-Bas Loos <willybas@gmail.com>,
 Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Justin <zzzzz.graf@gmail.com>,
 pgsql-general <pgsql-general@lists.postgresql.org>
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From: Adrian Klaver <adrian.klaver@aklaver.com>
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On 4/7/25 13:32, Willy-Bas Loos wrote:
> Hi Laurenz,
> 
> Thanks for answering!
> I find it very strange, because the publication is needed to make a 
> subscription, which makes the slot.

 From here:

https://www.postgresql.org/docs/current/logical-replication-subscription.html

"A subscription defines the connection to another database and set of 
publications (one or more) to which it wants to subscribe."

and here:

"PUBLICATION publication_name [, ...]

     Names of the publications on the publisher to subscribe to.
"

Finding the subscriptions for a given publication and deleting those 
slots may break the subscription on the receiving side if it is looking 
for data from more then one publication.

> Thanks for looking into it and helping me understand.
> 
> Cheers!
> Willy-Bas Loos
> 
> 
> On Mon, Apr 7, 2025 at 3:31 PM Laurenz Albe <laurenz.albe@cybertec.at 
> <mailto:laurenz.albe@cybertec.at>> wrote:
> 
>     On Mon, 2025-04-07 at 12:16 +0200, Willy-Bas Loos wrote:
>      > My question is not so much about "can i drop a certain
>     replication slot",
>      > more about "does this publication still have any replication slots?".
>      > Or, if you will: "what's the publication for this replication slot?".
>      >
>      > I've double checked the views that you suggested, and I found
>     that I can relate
>      > the WAL sender processes to replication slots through
>     pg_replication_slots.active_pid .
>      > I've also looked into replication origins.
>      >
>      > But I can't find a link to the publication. And that's what I
>     need to know.
> 
>     I don't think that there is a connection between a publication and a
>     replication slot.  That connection is only made when a subscriber
>     connects
>     and runs the START_REPLICATION command [1] and specifies the "pgoutput"
>     plugin with the "publication_names" option [2].
> 
>     I don't think you can see that information reflected in a system view
>     on the primary.  You'd have to query "pg_subscription" on the standby.
> 
>     Yours,
>     Laurenz Albe
> 
> 
>       [1]:
>     https://www.postgresql.org/docs/current/protocol-replication.html#PROTOCOL-REPLICATION-START-REPLICATION-SLOT-LOGICAL <https://www.postgresql.org/docs/current/protocol-replication.html#PROTOCOL-REPLICATION-START-REPLICATION-SLOT-LOGICAL>
>       [2]:
>     https://www.postgresql.org/docs/current/protocol-logical-replication.html#PROTOCOL-LOGICAL-REPLICATION-PARAMS <https://www.postgresql.org/docs/current/protocol-logical-replication.html#PROTOCOL-LOGICAL-REPLICATION-PARAMS>
> 
> 
> 
> -- 
> Willy-Bas Loos

-- 
Adrian Klaver
adrian.klaver@aklaver.com