Re: Ideas about a better API for postgres_fdw remote estimates

On 9/4/20 6:23 PM, Ashutosh Bapat wrote:
>
>
> On Thu, 3 Sep 2020 at 10:44, Andrey V. Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru <mailto:a(dot)lepikhov(at)postgrespro(dot)ru>> wrote:
>
> On 8/31/20 6:19 PM, Ashutosh Bapat wrote:
> > On Mon, Aug 31, 2020 at 3:36 PM Andrey V. Lepikhov
> > <a(dot)lepikhov(at)postgrespro(dot)ru <mailto:a(dot)lepikhov(at)postgrespro(dot)ru>> wrote:
> >>
> >> Thanks for this helpful feedback.
> > I think the patch has some other problems like it works only for
> > regular tables on foreign server but a foreign table can be pointing
> > to any relation like a materialized view, partitioned table or a
> > foreign table on the foreign server all of which have statistics
> > associated with them. I didn't look closely but it does not consider
> > that the foreign table may not have all the columns from the relation
> > on the foreign server or may have different names. But I think those
> > problems are kind of secondary. We have to agree on the design first.
> >
> In accordance with discussion i made some changes in the patch:
> 1. The extract statistic routine moved into the core.
>
>
> Bulk of the patch implements the statistics conversion to and fro json
> format. I am still not sure whether we need all of that code here.
Yes, i'm sure we'll replace it with something.

Right now, i want to discuss format of statistics dump. Remind, that a
statistics dump is needed not only for fdw, but it need for the pg_dump.
And in the dump will be placed something like this:
'SELECT store_relation_statistics(rel, serialized_stat)'

my reasons for using JSON:
* it have conversion infrastructure like json_build_object()
* this is flexible readable format, that can be useful in text dumps of
relations.

> Can we re-use pg_stats view? That is converting some of the OIDs to names. I
> agree with anyarray but if that's a problem here it's also a problem for
> pg_stats view, isn't it?
Right now, I don't know if it is possible to unambiguously convert the
pg_stats information to a pg_statistic tuple.

> If we can reduce the stats handling code to a
> minimum or use it for some other purpose as well e.g. pg_stats
> enhancement, the code changes required will be far less compared to the
> value that this patch provides.
+1

--
regards,
Andrey Lepikhov
Postgres Professional