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

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:
> > Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> >> On Fri, Oct 21, 2011 at 1:18 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >>> I don't know why you'd imagine that touching an index is free, or even
> >>> cheap, CPU-wise. The whole point of the index-only optimization is to
> >>> avoid I/O. When you try it on a case where there's no I/O to be saved,
> >>> and no shared-buffers contention to be avoided, there's no way it's
> >>> going to be a win.
> >>
> >> Well, call me naive, but I would have thought touching six times less
> >> data would make the operation run faster, not slower.
> >
> > 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.

Andres