On 2019-02-01 16:04:58 -0500, James Coleman wrote:
> On Thu, Jan 31, 2019 at 1:32 AM Kyotaro HORIGUCHI
> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> > Also as mentioned upthread by Peter Geoghegan, this could easly
> > give worse plan by underestimation. So I also suggest that this
> > has dynamic fallback function. In such perspective it is not
> > suitable for AM API level feature.
> >
> > If all leaf pages are on the buffer and the average hopping
> > distance is less than (expectedly) 4 pages (the average height of
> > the tree), the skip scan will lose. If almost all leaf pages are
> > staying on disk, we could win only by 2-pages step (skipping over
> > one page).
> >
> > =====
> > As I'm writing the above, I came to think that it's better
> > implement this as an pure executor optimization.
> >
> > Specifically, let _bt_steppage count the ratio of skipped pages
> > so far then if the ratio exceeds some threshold (maybe around
> > 3/4) it gets into hopscotching mode, where it uses index scan to
> > find the next page (rather than traversing). As mentioned above,
> > I think using skip scan to go beyond the next page is a good
> > bet. If the success ration of skip scans goes below some
> > threshold (I'm not sure for now), we should fall back to
> > traversing.
> >
> > Any opinions?
>
> I'd like to offer a counterpoint: in cases where this a huge win we
> definitely do want this to affect cost estimation, because if it's
> purely an executor optimization the index scan path may not be chosen
> even when skip scanning would be a dramatic improvement.
+many.
Greetings,
Andres Freund