Re: count(*) slow on large tables

On Thu, Oct 02, 2003 at 12:15:47 -0700,
  Dror Matalon <dror@zapatec.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.