Re: Index Skip Scan

> On Fri, Feb 1, 2019 at 8:24 PM Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com> wrote:
>
> Dmitry and I will look at this and take it into account for the next
> version.

In the meantime, just to not forget, I'm going to post another version with a
fix for cursor fetch backwards, which was crashing before. And talking about
this topic I wanted to ask to clarify a few points, since looks like I'm
missing something:

One of not yet addressed points in this patch is amcanbackward. From the
historical thread, mentioned in the first email:

> On 2016-11-25 at 01:33 Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> + if (ScanDirectionIsForward(dir))
> + {
> + so->currPos.moreLeft = false;
> + so->currPos.moreRight = true;
> + }
> + else
> + {
> + so->currPos.moreLeft = true;
> + so->currPos.moreRight = false;
> + }
>
>
>
> The lack of comments makes it hard for me to understand what the
> motivation for this is, but I bet it's wrong. Suppose there's a
> cursor involved here and the user tries to back up. Instead of having
> a separate amskip operation, maybe there should be a flag attached to
> a scan indicating whether it should return only distinct results.
> Otherwise, you're allowing for the possibility that the same scan
> might sometimes skip and other times not skip, but then it seems hard
> for the scan to survive cursor operations. Anyway, why would that be
> useful?

I assume that "sometimes skip and other times not skip" refers to the
situation, when we did fetch forward and jump something over, and then right
away doing fetch backwards, when we don't actually need to skip anything and
can get the result right away, right? If so, I can't find any comments about
why is it should be a problem for cursor operations?