Re: Index Skip Scan

> > On Sat, Jun 22, 2019 at 12:17 PM Floris Van Nee <florisvannee(at)optiver(dot)com> wrote:
> > The following sql statement seems to have incorrect results - some logic in
> > the backwards scan is currently not entirely right.
>
> Thanks for testing! You're right, looks like in the current implementation in
> case of backwards scan there is one unnecessary extra step forward. It seems
> this mistake was made, since I was concentrating only on the backward scans
> with a cursor, and used not exactly correct approach to wrap up after a scan
> was finished. Give me a moment, I'll tighten it up.

Here is finally a new version of the patch, where all the mentioned issues
seems to be fixed and the corresponding new tests should keep it like that
(I've skipped all the pubs at PostgresLondon for that). Also I've addressed the
most of feedback from Tomas, except the points about planning improvements
(which is still in our todo list). By no means it's a final result (e.g. I
guess `_bt_read_closest` must be improved), but I hope making progress
step-by-step will help anyway. Also I've fixed some, how it is popular to say
here, brain fade, where I mixed up scan directions.

> On Tue, Jul 2, 2019 at 11:00 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> On Fri, Jun 21, 2019 at 1:20 AM Jesper Pedersen
> <jesper(dot)pedersen(at)redhat(dot)com> wrote:
> > Attached is v20, since the last patch should have been v19.
>
> I took this for a quick spin today. The DISTINCT ON support is nice
> and I think it will be very useful. I've signed up to review it and
> will have more to say later. But today I had a couple of thoughts
> after looking into how src/backend/optimizer/plan/planagg.c works and
> wondering how to do some more skipping tricks with the existing
> machinery.
>
> On Thu, Jul 4, 2019 at 1:00 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> I mention all this stuff not because I want us to run before we can
> walk, but because to be ready to commit the basic distinct skip scan
> feature, I think we should know approximately how it'll handle the
> future stuff we'll need.

Great, thank you! I agree with Jesper that probably some parts of this are
outside of scope for the first patch, but we definitely can take a look at what
needs to be done to make the current implementation more flexible, so the
follow up would be just natural.