2011/10/22 Andres Freund
<andres@anarazel.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@sss.pgh.pa.us> wrote:
> > Robert Haas <
robertmhaas@gmail.com> writes:
> >> On Fri, Oct 21, 2011 at 1:18 PM, Tom Lane <
tgl@sss.pgh.pa.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,
> >>> 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
"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