Re: Problems with plan estimates in postgres_fdw

On Wed, Jan 30, 2019 at 6:12 AM Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
wrote:

> (2018/12/28 15:50), Etsuro Fujita wrote:
> > Attached is a new version of the
> > patch.
>
> Here is an updated version of the patch set. Changes are:
>
> * In the previous version, LIMIT without OFFSET was not performed
> remotely as the costs was calculated the same as the costs of performing
> it locally. (Actually, such LIMIT was performed remotely in a case in
> the postgres_fdw regression test, but that was due to a bug :(.) I
> think we should prefer performing such LIMIT remotely as that avoids
> extra row fetches from the remote that performing it locally might
> cause, so I tweaked the costs estimated in estimate_path_cost_size(), to
> ensure that we'll prefer performing such LIMIT remotely.
>

With your tweaks, I'm still not seeing the ORDER-less LIMIT get pushed down
when using use_remote_estimate in a simple test case, either with this set
of patches, nor in the V4 set. However, without use_remote_estimate, the
LIMIT is now getting pushed with these patches when it does not in HEAD.

See attached test case, to be run in new database named 'foo'.

Cheers,

Jeff