Re: row filtering for logical replication

On Sat, Dec 18, 2021 at 1:33 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Dec 17, 2021 at 5:29 PM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
> >
> > On Fri, Dec 17, 2021 at 7:20 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Dec 17, 2021 at 5:46 PM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
> > >
> > > > So using the v47 patch-set, I still find that the UPDATE above results in publication of an INSERT of (2,1), rather than an UPDATE of (1,1) to (2,1).
> > > > This is according to the 2nd UPDATE rule below, from patch 0003.
> > > >
> > > > + * old-row (no match) new-row (no match) -> (drop change)
> > > > + * old-row (no match) new row (match) -> INSERT
> > > > + * old-row (match) new-row (no match) -> DELETE
> > > > + * old-row (match) new row (match) -> UPDATE
> > > >
> > > > This is because the old row (1,1) doesn't match the UPDATE filter "(a>1)", but the new row (2,1) does.
> > > > This functionality doesn't seem right to me. I don't think it can be assumed that (1,1) was never published (and thus requires an INSERT rather than UPDATE) based on these checks, because in this example, (1,1) was previously published via a different operation - INSERT (and using a different filter too).
> > > > I think the fundamental problem here is that these UPDATE rules assume that the old (current) row was previously UPDATEd (and published, or not published, according to the filter applicable to UPDATE), but this is not necessarily the case.
> > > > Or am I missing something?
> > >
> > > But it need not be correct in assuming that the old-row was part of a
> > > previous INSERT either (and published, or not published according to
> > > the filter applicable to an INSERT).
> > > For example, change the sequence of inserts and updates prior to the
> > > last update:
> > >
> > > truncate tbl1 ;
> > > insert into tbl1 values (1,5); ==> not replicated since insert and ! (b < 2);
> > > update tbl1 set b = 1; ==> not replicated since update and ! (a > 1)
> > > update tbl1 set a = 2; ==> replicated and update converted to insert
> > > since (a > 1)
> > >
> > > In this case, the last update "update tbl1 set a = 2; " is updating a
> > > row that was previously updated and not inserted and not replicated to
> > > the subscriber.
> > > How does the replication logic differentiate between these two cases,
> > > and decide if the update was previously published or not?
> > > I think it's futile for the publisher side to try and figure out the
> > > history of published rows. In fact, if this level of logic is required
> > > then it is best implemented on the subscriber side, which then defeats
> > > the purpose of a publication filter.
> > >
> >
> > I think it's a concern, for such a basic example with only one row,
> > getting unpredictable (and even wrong) replication results, depending
> > upon the order of operations.
> >
>
> I am not sure how we can deduce that. The results are based on current
> and new values of row which is what I think we are expecting here.
>
> > Doesn't this problem result from allowing different WHERE clauses for
> > different pubactions for the same table?
> > My current thoughts are that this shouldn't be allowed, and also WHERE
> > clauses for INSERTs should, like UPDATE and DELETE, be restricted to
> > using only columns covered by the replica identity or primary key.
> >
>
> Hmm, even if we do that one could have removed the insert row filter
> by the time we are evaluating the update. So, we will get the same
> result. I think the behavior in your example is as we expect as per
> the specs defined by the patch and I don't see any problem, in this
> case, w.r.t replication results. Let us see what others think on this?
>

I think currently there could be a problem with user perceptions. IMO
a user would be mostly interested in predictability and getting
results that are intuitive.

So, even if all strange results can (after careful examination) be
after-the-fact explained away as being "correct" according to a spec,
I don't think that is going to make any difference. e.g. regardless of
correctness, even if it just "appeared" to give unexpected results
then a user may just decide that row-filtering is not worth their
confusion...

Perhaps there is a slightly dumbed-down RF design that can still be
useful, but which can give much more comfort to the user because the
replica will be more like what they were expecting?

------
Kind Regards,
Peter Smith.
Fujitsu Australia