Re: Slow counting still true?

Em 17/09/2012 06:13, Chris Travers escreveu:
>
>
> On Mon, Sep 17, 2012 at 1:07 AM, Thomas Guettler <hv(at)tbz-pariv(dot)de
> <mailto:hv(at)tbz-pariv(dot)de>> wrote:
>
> Release 9.2 should increase count(*) performance. Is this wiki
> page still valid?
>
> http://wiki.postgresql.org/wiki/Slow_Counting
>
> Please update the content.
>
> As I understand it, covering indexes don't currently help with
> count(*) because indexes can't be traversed in physical order, so it
> is a matter of trading random disk I/O for a much larger amount of
> sequential disk I/O.
>
> Best Wishes,
> Chris Travers

I'm just a little bit curious, and since count(*) affects a lot my
applications (every web system has a paginating feature that depends on
count(*) to calculate number of pages without loading everything), I'm
also interested in this topic.

The wiki page in question has been updated today, and I see the alert in
top of page "Note that the following article only applies to versions of
PostgreSQL prior to 9.2. Index-only scans are now implemented."

So seems that traversing indexes for count(*) would be faster on 9.2, right?

AFAIK, for count(*) doesn't matter the order data is stored - just need
to load index leaf pages and count from there, right?

Edson