Re: Help with a seq scan on multi-million row table

Hi,

Thanks Bruno. That was indeed a redundant DISTINCT. It did reduce the amount of work, but as you said it doesn't get rid of the sequential scan, which is the real problem with this query.

Otis

----- Original Message ----
From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: ogjunk-pgjedan(at)yahoo(dot)com
Cc: pgsql-sql(at)postgresql(dot)org
Sent: Wednesday, May 10, 2006 3:23:29 PM
Subject: Re: [SQL] Help with a seq scan on multi-million row table

On Wed, May 10, 2006 at 13:13:59 -0500,
ogjunk-pgjedan(at)yahoo(dot)com wrote:
> Hello,
>
> I have a little 2-table JOIN, GROUP BY, ORDER BY query that does a sequential scan on a multi-million row table. I _thought_ I had all the appropriate indices, but apparently I do not. I was wondering if anyone can spot a way I can speed up this query.
> The query currently takes... *gulp*: 381119.201 ms :(
>
> There are only 2 tables in the game: user_url and user_url_tag. The latter has FKs pointing to the former. The sequential scan happens on the latter - user_url_tag:
>
> EXPLAIN ANALYZE select DISTINCT userurltag0_.tag as x0_0_, COUNT(*) as x1_0_ from user_url_tag userurltag0_, user_url userurl1_ WHERE (((userurl1_.user_id=1 )) AND ((userurltag0_.user_url_id=userurl1_.id ))) GROUP BY userurltag0_.tag ORDER BY count(*) DESC;

While this isn't a big issue, it looks like DISTINCT is redundant in your
query and seems to be adding some extra work.

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