| From: | Andrew Dunstan <andrew(at)dunslane(dot)net> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, josh(at)agliodbs(dot)com, Greg Stark <gsstark(at)mit(dot)edu>, Marko Ristola <marko(dot)ristola(at)kolumbus(dot)fi>, pgsql-perform <pgsql-performance(at)postgresql(dot)org>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: [HACKERS] Bad n_distinct estimation; hacks suggested? |
| Date: | 2005-04-26 21:41:20 |
| Message-ID: | 426EB580.9040606@dunslane.net |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers pgsql-performance |
Simon Riggs wrote:
>The comment
> * Every value in the sample appeared more than once. Assume
> * the column has just these values.
>doesn't seem to apply when using larger samples, as Josh is using.
>
>Looking at Josh's application it does seem likely that when taking a
>sample, all site visitors clicked more than once during their session,
>especially if they include home page, adverts, images etc for each page.
>
>Could it be that we have overlooked this simple explanation and that the
>Haas and Stokes equation is actually quite good, but just not being
>applied?
>
>
>
>
No, it is being aplied. If every value in the sample appears more than
once, then f1 in the formula is 0, and the result is then just d, the
number of distinct values in the sample.
cheers
andrew
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Rob Butler | 2005-04-26 21:43:27 | Re: DO INSTEAD and conditional rules |
| Previous Message | Simon Riggs | 2005-04-26 21:02:31 | Re: [HACKERS] Bad n_distinct estimation; hacks suggested? |
| From | Date | Subject | |
|---|---|---|---|
| Next Message | John A Meinel | 2005-04-26 21:51:09 | Re: speed up query with max() and odd estimates |
| Previous Message | Simon Riggs | 2005-04-26 21:02:31 | Re: [HACKERS] Bad n_distinct estimation; hacks suggested? |