| From: | Bruno Wolff III <bruno(at)wolff(dot)to> |
|---|---|
| To: | Dror Matalon <dror(at)zapatec(dot)com> |
| Cc: | "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: count(*) slow on large tables |
| Date: | 2003-10-02 19:39:05 |
| Message-ID: | 20031002193905.GD18417@wolff.to |
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| Lists: | pgsql-hackers pgsql-performance |
On Thu, Oct 02, 2003 at 12:15:47 -0700,
Dror Matalon <dror(at)zapatec(dot)com> wrote:
> Hi,
>
> I have a somewhat large table, 3 million rows, 1 Gig on disk, and growing. Doing a
> count(*) takes around 40 seconds.
>
> Looks like the count(*) fetches the table from disk and goes through it.
> Made me wonder, why the optimizer doesn't just choose the smallest index
> which in my case is around 60 Megs and goes through it, which it could
> do in a fraction of the time.
Because it can't tell from the index if a tuple is visible to the current
transaction and would still have to hit the table to check this. So that
performance would be a lot worse instead of better.
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