On Wed, Nov 21, 2018 at 12:55 PM Dmitry Dolgov <9erthalion6@gmail.com> wrote:
> Well, no, it's callled with ScanDirectionIsForward(dir). But as far as I
> remember from the previous discussions the entire topic of backward scan is
> questionable for this patch, so I'll try to invest some time in it.
Another thing that I think is related to skip scans that you should be
aware of is dynamic prefix truncation, which I started a thread on
just now [1]. While I see one big problem with the POC patch I came up
with, I think that that optimization is going to be something that
ends up happening at some point. Repeatedly descending a B-Tree when
the leading column is very low cardinality can be made quite a lot
less expensive by dynamic prefix truncation. Actually, it's almost a
perfect case for it.
I'm not asking anybody to do anything with that information. "Big
picture" thinking seems particularly valuable when working on the
B-Tree code; I don't want anybody to miss a possible future
opportunity.
[1] https://postgr.es/m/CAH2-Wzn_NAyK4pR0HRWO0StwHmxjP5qyu+X8vppt030XpqrO6w@mail.gmail.com
--
Peter Geoghegan