Re: [RFC] speed up count(*)

On 10/20/21 20:33, John Naylor wrote:
>
> On Wed, Oct 20, 2021 at 2:23 PM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com <mailto:tomas(dot)vondra(at)enterprisedb(dot)com>>
> wrote:
> >
> > Couldn't we simply inspect the visibility map, use the index data only
> > for fully visible/summarized ranges, and inspect the heap for the
> > remaining pages? That'd still be a huge improvement for tables with most
> > only a few pages modified recently, which is a pretty common case.
> >
> > I think the bigger issue is that people rarely do COUNT(*) on the whole
> > table. There are usually other conditions and/or GROUP BY, and I'm not
> > sure how would that work.
>
> Right. My (possibly hazy) recollection is that people don't have quite
> as high an expectation for queries with more complex predicates and/or
> grouping. It would be interesting to see what the balance is.
>

I don't know where the balance is, and I doubt it's possible to answer
that in general - I'm sure some workloads might benefit significantly.

I wonder if multi-column BRIN indexes would help in cases with
additional predicates. Seems possible.

BTW you mentioned using BRIN indexes for min/max - I've been thinking
about using BRIN indexes for ordering/sorting, which seems related. And
I think it's actually doable, so I wonder why you concluded using BRIN
indexes for min/max is not possible?

regards

--
Tomas Vondra
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