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From: Alvaro Herrera <[email protected]>
To: Tomas Vondra <[email protected]>
Cc: Amit Kapila <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: Pg Hackers <[email protected]>
Subject: Re: bogus: logical replication rows/cols combinations
Date: Mon, 2 May 2022 12:17:54 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 2022-May-02, Tomas Vondra wrote:
> On 5/2/22 07:31, Amit Kapila wrote:

> > Yeah, or don't allow to define such publications in the first place so
> > that different subscriptions can't combine them but I guess that might
> > forbid some useful cases as well where publication may not get
> > combined with other publications.
> 
> But how would you check that? You don't know which publications will be
> combined by a subscription until you create the subscription, right?

... and I think this poses a problem: if the publisher has multiple
publications and the subscriber later uses those to create a combined
subscription, we can check at CREATE SUBSCRIPTION time that they can be
combined correctly.  But if the publisher decides to change the
publications changing the rules and they are no longer consistent, can
we throw an error at ALTER PUBLICATION point?  If the publisher can
detect that they are being used together by some subscription, then
maybe we can check consistency in the publication side and everything is
all right.  But I'm not sure that the publisher knows who is subscribed
to what, so this might not be an option.

The latter ultimately means that we aren't sure that a combined
subscription is safe.  And in turn this means that a pg_dump of such a
database cannot be restored (because the CREATE SUBSCRIPTION will be
rejected as being inconsistent).


-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/

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