Re: Index Skip Scan

> On Thu, Feb 06, 2020 at 09:22:20PM +0900, Kyotaro Horiguchi wrote:
> At Thu, 6 Feb 2020 11:57:07 +0100, Dmitry Dolgov <9erthalion6@gmail.com> wrote in 
> > > On Thu, Feb 06, 2020 at 10:24:50AM +0900, Kyotaro Horiguchi wrote:
> > > At Wed, 5 Feb 2020 17:37:30 +0100, Dmitry Dolgov <9erthalion6@gmail.com> wrote in
> > > We could add an additional parameter "in_cursor" to
> > > ExecSupportBackwardScan and let skip scan return false if in_cursor is
> > > true, but I'm not sure it's acceptable.
> > 
> > I also was thinking about whether it's possible to use
> > ExecSupportBackwardScan here, but skip scan is just a mode of an
> > index/indexonly scan. Which means that ExecSupportBackwardScan also need
> > to know somehow if this mode is being used, and then, since this
> > function is called after it's already decided to use skip scan in the
> > resulting plan, somehow correct the plan (exclude skipping and try to
> > find next best path?) - do I understand your suggestion correct?
> 
> I didn't thought so hardly, but a bit of confirmation told me that
> IndexSupportsBackwardScan returns fixed flag for AM.  It seems that
> things are not that simple.

Yes, I've mentioned that already in one of the previous emails :) The
simplest way I see to achieve what we want is to do something like in
attached modified version with a new hasDeclaredCursor field. It's not a
final version though, but posted just for discussion, so feel free to
suggest any improvements or alternatives.