On Mon, Jul 26, 2021 at 10:45 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> I was thinking more about this idea, but IMHO, unless we send the key
> toasted tuple from the publisher how is the subscriber supposed to
> fetch it. Because that is the key value for finding the tuple on the
> subscriber side and if we haven't sent the key value, how are we
> supposed to find the tuple on the subscriber side?
>
I was thinking of using toast pointer but that won't work because it
can be different on the subscriber-side. I don't see any better ideas
to fix this issue. This problem seems to be from the time Logical
Replication has been introduced, so adding others (who are generally
involved in this area) to see what they think about this bug? I think
people might not be using toasted columns for Replica Identity due to
which this problem has been reported yet but I feel this is quite a
fundamental issue and we should do something about this.
Let me summarize the problem for the ease of others.
The logical replica can go out of sync for UPDATES when there is a
toast column as part of REPLICA IDENTITY. In such cases, updates are
not replicated if the key column doesn't change because we don't log
the actual key value for the unchanged toast key. It is neither logged
as part of old_key_tuple nor for new tuple due to which we are not
able to find the tuple to be updated on the subscriber-side and the
update is ignored on the subscriber-side. We log this in DEBUG1 mode
but I don't think the user can do anything about this and the replica
will go out-of-sync. This works when the replica identity column value
is not toasted because then it will be part of the new tuple and we
use that to fetch the tuple on the subscriber.
Now, it is not clear if the key-value (for the toast column which is
part of replica identity) is not present in WAL how we can find the
tuple to update on subscriber? We can't use the toast pointer in the
new tuple to fetch the toast information as that can be different on
subscribers. The simple way is to WAL LOG the unchanged toasted value
as part of old_key_tuple, this will be required only for toast
columns.
Note that Delete works because we WAL Log the unchanged key tuple in that case.
--
With Regards,
Amit Kapila.
