Re: [BUG]Update Toast data failure in logical replication

On Thu, Jun 3, 2021 at 5:15 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Wed, Jun 2, 2021 at 7:23 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Wed, Jun 2, 2021 at 7:20 PM Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Jun 2, 2021 at 3:10 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > > Yes, you are right. I will change it in the next version, along with
> > > > the test case.
> > > >
> > > + /*
> > > + * if the key hasn't changed and we're only logging the key, we're done.
> > > + * But if tuple has external data then we might have to detoast the key.
> > > + */
> > > This doesn't really mention why we need to detoast the key even when
> > > the key remains the same. I guess we can add some more details here.
> >
> > Noted, let's see what others have to say about fixing this, then I
> > will fix this along with one other pending comment and I will also add
> > the test case. Thanks for looking into this.
>
> I have fixed all the pending issues, I see there is already a test
> case for this so I have changed the output for that.
>

IIUC, this issue occurs because we don't log the actual key value for
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 it in the
apply-side when we searched it via FindReplTupleInLocalRel. Now, I
think it will work if we LOG the entire unchanged toasted value as you
have done in the patch but can we explore some other way to fix it. In
the subscriber-side, can we detect that the key column has toasted
value in the new tuple and try to first fetch it and then do the index
search for the fetched toasted value? I am not sure about the
feasibility of this but wanted to see if we can someway avoid logging
unchanged toasted key value as that might save us from additional WAL.

--
With Regards,
Amit Kapila.