Re: Slow counting still true?

If the numbers do not have to be exact the web applications could use the cardinality estimates stored in the system tables.

 

On Mon, Sep 17, 2012 at 1:07 AM, Thomas Guettler <hv@tbz-pariv.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