Re: [RFC] speed up count(*)

Hi,

On October 20, 2021 10:57:50 AM PDT, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 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.

Yeah.

If we really wanted to, we could accelerate unqualified count(*) substantially by computing the count inside heapam. There's a *lot* of overhead associated with returning tuples, grouping them, etc. Especially with all_visible set that's bound to be way faster (I'd guess are least 3-5x) if done in heapam (like we do the visibility determinations in heapgetpage for all tuples on a page at once).

But it's doubtful the necessary infrastructure is worth it. Perhaps that changes with the infrastructure some columnar AMs are asking for. They have a need to push more stuff down to the AM that's more generic than just count(*).

Regards,

Andres
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