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

2011/10/22 Andres Freund <andres(at)anarazel(dot)de>

> 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
>
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"But access to those is not sequential" yes, I am agree.
In my opinion the problem is that. If the query needs to scan all the b-tree
index without to
access the table rows, the better way to read the index is like sequential
one,
in fact , query like count(*) or other not need the data are in "order" so I
think we
could read all blocks (better, "only" the leaf blocks) without to touching
too much the branch blocks.

For example query like this :

select column_a from table ;

is better to read the data from indexes like sequential

For query like this :

select column_a from table order by column_a ;

is better to read the data from indexes in range scan from root block to
first branch blocks and their leaf blocks, so we could "save"
the sorting.

Mat