Re: So, is COUNT(*) fast now?

On Saturday, October 22, 2011 05:20:26 PM Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On Friday, October 21, 2011 08:14:12 PM Robert Haas wrote:
> >> On Fri, Oct 21, 2011 at 2:08 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >>> It's not "touching six times less data". It's touching the exact same
> >>> number of tuples either way, just index tuples in one case and heap
> >>> tuples in the other.
> >>
> >> Yeah, but it works out to fewer pages.
> >
> > But access to those is not sequential. I guess if you measure cache hit
> > ratios the index scan will come out significantly worse.
>
> Huh? In the case he's complaining about, the index is all in RAM.
> Sequentiality of access is not an issue (at least not at the page
> level --- within a page I suppose there could be cache-line effects).
I was talking about L2/L3 caches...

Andres