Re: bogus: logical replication rows/cols combinations

On 5/2/22 12:17, Alvaro Herrera wrote:
> 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.
> 

AFAIK we don't track that (publication/subscription mapping). The
publications are listed in publication_names parameter of the
START_REPLICATION command.

> 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).
> 

We could do this check when executing the START_REPLICATION command, no?


regards

-- 
Tomas Vondra
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