Re: [RFC] speed up count(*)

On 10/20/21 19:57, Tom Lane wrote:
> John Naylor <john(dot)naylor(at)enterprisedb(dot)com> writes:
>> Perennially our users have complaints about slow count(*) when coming from
>> some other systems. Index-only scans help, but I think we can do better. I
>> recently wondered if a BRIN index could be used to answer min/max aggregate
>> queries over the whole table, and it turns out it doesn't. However, then it
>> occurred to me that if we had an opclass that keeps track of the count in
>> each page range, that would be a way to do a fast count(*) by creating the
>> right index. That would require planner support and other work, but it
>> seems doable. Any opinions on whether this is worth the effort?
>
> The core reason why this is hard is that we insist on giving the right
> answer. In particular, count(*) is supposed to count the rows that
> satisfy the asker's snapshot. So I don't see a good way to answer it
> from an index only, given that we don't track visibility accurately
> in indexes.
>

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.

regards

--
Tomas Vondra
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