Re: Foreign Data Wrapper behavior?

On 10/11/24 04:08, Koen De Groote wrote:
> In the release notes for postgres 17 I'm reading:
>
> > The PostgreSQL foreign data wrapper (postgres_fdw), used to execute
> queries on remote PostgreSQL instances, can now push EXISTS and IN
> subqueries to the remote server for more efficient processing.
>
> I'm confused as to what this means. In older versions, are parts of
> queries not sent to the foreign server? Or is this change meant to imply
> the sending of only the subqueries, the result of which is then directly
> used in pushing the entire query?
>
> Or am I still wrong as to what this means?
>
> I looked at the documentation and there doesn't seem to be any
> indication of particular queries not being pushed to the foreign server,
> so this wording that "can now push EXISTS and IN subqueries to the
> remote server" is confusing.
>
> What am I missing?

Read:

https://www.postgresql.org/docs/current/postgres-fdw.html#POSTGRES-FDW-REMOTE-QUERY-OPTIMIZATION

F.36.5. Remote Query Optimization

As to the change in the Release Note see the --hackers discussion:

https://www.postgresql.org/message-id/c9e2a757cf3ac2333714eaf83a9cc184%40postgrespro.ru

>
> Regards,
> Koen De Groote

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com