Re: Conflict detection and logging in logical replication

On Fri, Aug 16, 2024 at 10:46 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> 3)
> For update_exists(), we dump:
> Key (a, b)=(2, 1)
>
> For delete_missing, update_missing, update_differ, we dump:
> Replica identity (a, b)=(2, 1).
>
> For update_exists as well, shouldn't we dump 'Replica identity'? Only
> for insert case, it should be referred as 'Key'.
>

On rethinking, is it because for update_exists case 'Key' dumped is
not the one used to search the row to be updated? Instead it is the
one used to search the conflicting row. Unlike update_differ, the row
to be updated and the row currently conflicting will be different for
update_exists case. I earlier thought that 'KEY' and 'Existing local
tuple' dumped always belong to the row currently being
updated/deleted/inserted. But for 'update_eixsts', that is not the
case. We are dumping 'Existing local tuple' and 'Key' for the row
which is conflicting and not the one being updated. Example:

ERROR: conflict detected on relation "public.tab_1": conflict=update_exists
Key (a, b)=(2, 1); existing local tuple (2, 1); remote tuple (2, 1).

Operations performed were:
Pub: insert into tab values (1,1);
Sub: insert into tab values (2,1);
Pub: update tab set a=2 where a=1;

Here Key and local tuple are both 2,1 instead of 1,1. While replica
identity value (used to search original row) will be 1,1 only.

It may be slightly confusing or say tricky to understand when compared
to other conflicts' LOGs. But not sure what better we can do here.

--------------------

One more comment:

5)
For insert/update_exists, the sequence is:
Key .. ; existing local tuple .. ; remote tuple ...

For rest of the conflicts, sequence is:
Existing local tuple .. ; remote tuple .. ; replica identity ..

Is it intentional? Shall the 'Key' or 'Replica Identity' be the first
one to come in all conflicts?

thanks
Shveta