Re: Bug in nbtree optimization to skip > operator comparisons (or < comparisons in backwards scans)

On Wed, 6 Dec 2023 at 14:11, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Tue, Dec 5, 2023 at 8:15 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > Just to be clear, you're raising a concern that seems to me to apply
> > to "the other optimization" from the same commit, specifically -- the
> > precheck optimization. Not the one I found a problem in. (They're
> > closely related but distinct optimizations.)
>
> It isn't very clear from the commit message that this commit is doing
> two different things, and in fact I'm still unclear on what exactly
> the other optimization is.

I feel that Peter refered to these two distinct optimizations:

1. When scanning an index in ascending order using scankey a > 1 (so,
one that defines a start point of the scan), we don't need to check
items for consistency with that scankey once we've found the first
value that is consistent with the scankey, as all future values will
also be consistent with the scankey (if we assume no concurrent page
deletions).

2. When scanning an index in ascending order using scankey a < 10 (one
that defines an endpoint of the scan), we can look ahead and check if
the last item on the page is consistent. If so, then all other items
on the page will also be consistent with that scankey.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)