Re: Tid scan improvements

On Tue, 26 Mar 2019 at 11:54, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > I was kinda hoping to keep block numbers out of the "main" APIs, to
> > avoid assuming everything is BLCKSZ based. I don't have a particular
> > problem allowing an optional setscanlimits type callback that works with
> > block numbers. The planner could check its presence and just not build
> > tid range scans if not present. Alternatively a bespoke scan API for
> > tid range scans, like the later patches in the tableam series for
> > bitmap, sample, analyze scans, might be an option.
>
> Given Andres' API concerns, and the short amount of time remaining in
> this CF, I'm not sure how much of this patch set we can expect to land
> in v12. It seems like it might be a good idea to scale back our ambitions
> and see whether there's a useful subset we can push in easily.

I agree. It'll take some time to digest Andres' advice and write a
better patch.

Should I set update CF app to a) set the target version to 13, and/or
move it to next commitfest?

> With that in mind, I went ahead and pushed 0001+0004, since improving
> the planner's selectivity estimate for a "ctid vs constant" qual is
> likely to be helpful whether the executor is smart about it or not.

Cool.

> FWIW, I don't really see the point of treating 0002 as a separate patch.
> If it had some utility on its own, then it'd be sensible, but what
> would that be? Also, it looks from 0002 like you are trying to overload
> rs_startblock with a different meaning than it has for syncscans, and
> I think that might be a bad idea.

Yeah I don't think either patch is useful without the other. They
were separate because, initially, only some of the TidRangeScan
functionality depended on it, and I was particularly uncomfortable
with what I was doing to heapam.c.

The changes in heapam.c were required for backward scan support, as
used by ORDER BY ctid DESC and MAX(ctid); and also for FETCH LAST and
FETCH PRIOR. I have removed the backward scans functionality from the
current set of patches, but support for backward cursor fetches
remains.

I guess to brutally simplify the patch further, we could give up
backward cursor fetches entirely? This means such cursors that end up
using a TidRangeScan will require SCROLL to go backwards (which is a
small pain for user experience), but TBH I don't think backwards-going
cursors on CTID will be hugely common.

I'm still not familiar enough with heapam.c to have any better ideas
on how to support backward scanning a limited range.