Re: Bitmap index scans use of filters on available columns
| От | Amit Kapila |
|---|---|
| Тема | Re: Bitmap index scans use of filters on available columns |
| Дата | |
| Msg-id | CAA4eK1+OfKgSTSsMCu7zFmze8UwSRdx+Em1PFtbcrWZi71GBEQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Bitmap index scans use of filters on available columns (Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>) |
| Список | pgsql-hackers |
On Fri, Nov 6, 2015 at 10:28 AM, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>
> Hello,
>
> At Fri, 6 Nov 2015 09:49:30 +0530, Amit Kapila <amit.kapila16@gmail.com> wrote in <CAA4eK1L_5T-UYsQeMOrX54g3QeXGhhAk5YFmzZqu5MidzxzkRg@mail.gmail.com>
> > Apart from other problems discussed, I think it could also lead to
> > a performance penality for the cases when the qual condition is
> > costly as evaluating such a qual against lot of dead rows would be a
> > time consuming operation. I am not sure, but I think some of the
> > other well know databases might not have any such problems as
> > they store visibility info inside index due to which they don't need to
> > fetch the heap tuple for evaluating such quals.
>
> I was junst thinking of the same thing. Can we estimate the
> degree of the expected penalty using heap statistics? Of couse
> not so accurate though.
>
>
> Hello,
>
> At Fri, 6 Nov 2015 09:49:30 +0530, Amit Kapila <amit.kapila16@gmail.com> wrote in <CAA4eK1L_5T-UYsQeMOrX54g3QeXGhhAk5YFmzZqu5MidzxzkRg@mail.gmail.com>
> > Apart from other problems discussed, I think it could also lead to
> > a performance penality for the cases when the qual condition is
> > costly as evaluating such a qual against lot of dead rows would be a
> > time consuming operation. I am not sure, but I think some of the
> > other well know databases might not have any such problems as
> > they store visibility info inside index due to which they don't need to
> > fetch the heap tuple for evaluating such quals.
>
> I was junst thinking of the same thing. Can we estimate the
> degree of the expected penalty using heap statistics? Of couse
> not so accurate though.
>
I think so. Information related to number of tuples inserted, updated,
hot-updated, deleted and so on is maintained and is shown by
pg_stat_all_tables. By the way, whats your point here, do you mean to
say that we can estimate approximate penality and then decide whether
quals should be evaluated at index level?
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