On Tue, Jul 20, 2021 at 11:38 AM Greg Nancarrow <gregn4...@gmail.com> wrote:
>
> On Tue, Jul 20, 2021 at 2:25 PM Amit Kapila <amit.kapil...@gmail.com> wrote:
> >
> > Today, while studying the behavior of this particular operation in
> > other databases, I found that IBM's InfoSphere Data Replication does
> > exactly this. See [1]. I think there is a merit if want to follow this
> > idea.
> >
>
> So in this model (after initial sync of rows according to the filter),
> for UPDATE, the OLD row is checked against the WHERE clause, to know
> if the row had been previously published. If it hadn't, and the NEW
> row satisfies the WHERE clause, then it needs to be published as an
> INSERT. If it had been previously published, but the NEW row doesn't
> satisfy the WHERE condition, then it needs to be published as a
> DELETE. Otherwise, if both OLD and NEW rows satisfy the WHERE clause,
> it needs to be published as an UPDATE.
>
Yeah, this is what I also understood.
> At least, that seems to be the model when the WHERE clause refers to
> the NEW (updated) values, as used in most of their samples (i.e. in
> that database "the current log record", indicated by a ":" prefix on
> the column name).
> I think that allowing the OLD values ("old log record") to be
> referenced in the WHERE clause, as that model does, could be
> potentially confusing.
>
I think in terms of referring to old and new rows, we already have
terminology which we used at various other similar places. See Create
Rule docs [1]. For where clause, it says "Within condition and
command, the special table names NEW and OLD can be used to refer to
values in the referenced table. NEW is valid in ON INSERT and ON
UPDATE rules to refer to the new row being inserted or updated. OLD is
valid in ON UPDATE and ON DELETE rules to refer to the existing row
being updated or deleted.". We need similar things for the WHERE
clause in publication if we want special syntax to refer to old and
new rows.
I think if we use some existing way to refer to old/new values then it
shouldn't be confusing to users.
[1] - https://www.postgresql.org/docs/devel/sql-createrule.html
--
With Regards,
Amit Kapila.
