Re: Column Filtering in Logical Replication

On Fri, Jan 14, 2022 at 7:08 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>
> On 2022-Jan-14, Amit Kapila wrote:
>
> > 1. Replica Identity handling: Currently the column filter patch gives
> > an error during create/alter subscription if the specified column list
> > is invalid (Replica Identity columns are missing). It also gives an
> > error if the user tries to change the replica identity. However, it
> > doesn't deal with cases where the user drops and adds a different
> > primary key that has a different set of columns which can lead to
> > failure during apply on the subscriber.
>
> Hmm, yeah, I suppose we should check that the primary key is compatible
> with the column list in all publications. (I wonder what happens in the
> interim, that is, what happens to tuples modified after the initial PK
> is dropped and before the new PK is installed. Are these considered to
> have "replica identiy nothing"?)
>

I think so.

> > I think another issue w.r.t column filter patch is that even while
> > creating publication (even for 'insert' publications) it should check
> > that all primary key columns must be part of published columns,
> > otherwise, it can fail while applying on subscriber as it will try to
> > insert NULL for the primary key column.
>
> I'm not so sure about the primary key aspects, actually; keep in mind
> that the replica can have a different table definition, and it might
> have even a completely different primary key. I think this part is up
> to the user to set up correctly; we have enough with just trying to make
> the replica identity correct.
>

But OTOH, the primary key is also considered default replica identity,
so I think users will expect it to work. You are right this problem
can also happen if the user defined a different primary key on a
replica but that is even a problem in HEAD (simple inserts will fail)
but I am worried about the case where both the publisher and
subscriber have the same primary key as that works in HEAD.

--
With Regards,
Amit Kapila.