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Date: Fri, 4 Oct 2002 10:31:08 -0500
From: Bruno Wolff III <bruno@wolff.to>
To: Michael Contzen <Michael.Contzen@dohle.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: How slow is distinct - 2nd
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On Tue, Oct 01, 2002 at 14:18:50 +0200,
  Michael Contzen <Michael.Contzen@dohle.com> wrote:
> Here the table:
> 
> mc=# \d egal
>      Table "public.egal"
>  Column |  Type   | Modifiers 
> --------+---------+-----------
>  i      | integer | 
> 
> mc=# select count(*) from egal;
>   count  
> ---------
>  7227744
> (1 row)
> 
> mc=# select count(distinct i) from egal;
>  count 
> -------
>     67
> (1 row)

This suggests that the best way to do this is with a hash instead of a sort.

If you have lots of memory you might try increasing the sort memory size.