Re: row filtering for logical replication

On Fri, Sep 24, 2021 at 10:50 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

> > 12) misuse of REPLICA IDENTITY
> >
> > The more I think about this, the more I think we're actually misusing
> > REPLICA IDENTITY for something entirely different. The whole purpose of
> > RI was to provide a row identifier for the subscriber.
> >
> > But now we're using it to ensure we have all the necessary columns,
> > which is entirely orthogonal to the original purpose. I predict this
> > will have rather negative consequences.
> >
> > People will either switch everything to REPLICA IDENTITY FULL, or create
> > bogus unique indexes with extra columns. Which is really silly, because
> > it wastes network bandwidth (transfers more data) or local resources
> > (CPU and disk space to maintain extra indexes).
> >
> > IMHO this needs more infrastructure to request extra columns to decode
> > (e.g. for the filter expression), and then remove them before sending
> > the data to the subscriber.
> >
>
> Yeah, but that would have an additional load on write operations and I
> am not sure at this stage but maybe there could be other ways to
> extend the current infrastructure wherein we build the snapshots using
> which we can access the user tables instead of only catalog tables.
> Such enhancements if feasible would be useful not only for allowing
> additional column access in row filters but for other purposes like
> allowing access to functions that access user tables. I feel we can
> extend this later as well seeing the usage and requests. For the first
> version, this doesn't sound too limiting to me.

I agree with one point from Tomas, that if we bind the row filter with
the RI, then if the user has to use the row filter on any column 1)
they have to add an unnecessary column to the index 2) Since they have
to add it to RI so now we will have to send it over the network as
well. 3). We anyway have to WAL log it if it is modified because now
we forced users to add some columns to RI because they wanted to use
the row filter on that. Now suppose we remove that limitation and we
somehow make these changes orthogonal to RI, i.e. if we have a row
filter on some column then we WAL log it, so now the only extra cost
we are paying is to just WAL log that column, but the user is not
forced to add it to index, not forced to send it over the network.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com