Re: INDEX suggestion needed

On Fri, 13 Dec 2002 13:00:14 -0300, Alvaro Herrera
<alvherre@dcc.uchile.cl> wrote:
>Now this catched my attention (in the questions' side, sorry, not the
>answers').  Why the aggregate takes 10 times the time needed for the
>indexscan?

Good point!

>  One would think that a function like count() should be
>pretty cheap,

COUNT is cheap.  But COUNT(DISTINCT something) is not trivial, it has
to keep a list of all values it has already counted.  I didn't look at
the implementation.  Do we have O(n^2) cost here?

Thomas, could you EXPLAIN ANALYZE some test cases with

    SELECT COUNT(*) FROM (
        SELECT DISTINCT a_id
          FROM stat_pages
         WHERE ...
    ) AS x;

and compare them to the results of SELECT COUNT(DISTINCT ...)?

So now you are back where you started.  At least you have an index on
"visit" now ;-)

Servus
 Manfred