Re: [RFC] speed up count(*)

On 10/20/21 2:33 PM, 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 think you are exactly correct. People seem to understand that with a
predicate it is harder, but they expect

select count(*) from foo;

to be nearly instantaneous, and they don't really need it to be exact.
The stock answer for that has been to do

select reltuples from pg_class
where relname = 'foo';

But that is unsatisfying because the problem is often with some
benchmark or another that cannot be changed.

I'm sure this idea will be shot down in flames <donning flameproof
suit>, but what if we had a default "off" GUC which could be turned on
causing the former to be transparently rewritten into the latter
</donning flameproof suit>?

Joe

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