Re: row filtering for logical replication

On Tue, Jul 13, 2021 at 10:24 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Jul 12, 2021 at 3:01 PM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
> > In terms of implementation, I think there are two basic options - either
> > we can define a new "expression" type in gram.y, which would be a subset
> > of a_expr etc. Or we can do it as some sort of expression walker, kinda
> > like what the transform* functions do now.
> >
>
> I think it is better to use some form of walker here rather than
> extending the grammar for this. However, the question is do we need
> some special kind of expression walker here or can we handle all
> required cases via transformWhereClause() call as the patch is trying
> to do. AFAIU, the main things we want to prohibit in the filter are:
> (a) it doesn't refer to any relation other than catalog in where
> clause, (b) it doesn't use UDFs in any way (in expressions, in
> user-defined operators, user-defined types, etc.), (c) the columns
> referred to in the filter should be part of PK or Replica Identity.
> Now, if all such things can be detected by the approach patch has
> taken then why do we need a special kind of expression walker? OTOH,
> if we can't detect some of this then probably we can use a special
> walker.
>
> I think in the long run one idea to allow UDFs is probably by
> explicitly allowing users to specify whether the function is
> publication predicate safe and if so, then we can allow such functions
> in the filter clause.
>

Another idea here could be to read the publication-related catalog
with the latest snapshot instead of a historic snapshot. If we do that
then if the user faces problems as described by Petr [1] due to
missing dependencies via UDFs then she can Alter the Publication to
remove/change the filter clause and after that, we would be able to
recognize the updated filter clause and the system will be able to
move forward.

I might be missing something but reading publication catalogs with
non-historic snapshots shouldn't create problems as we use the
historic snapshots are required to decode WAL.

I think the problem described by Petr[1] is also possible today if the
user drops the publication and there is a corresponding subscription,
basically, the system will stuck with error: "ERROR: publication
"mypub" does not exist. I think allowing to use non-historic snapshots
just for publications will resolve that problem as well.

[1] - https://www.postgresql.org/message-id/92e5587d-28b8-5849-2374-5ca3863256f1%402ndquadrant.com

--
With Regards,
Amit Kapila.