Date: Fri, 14 Feb 2003 20:21:13 +0100
To: Brad Hilton <bhilton@vpop.net>
Cc: pgsql-sql@postgresql.org
Subject: Re: sub-query optimization
Message-ID: <20030214192113.GA26987@serwer>
References: <1045246939.29966.22.camel@aragorn.vpop.net>
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From: jasiek@serwer.skawsoft.com.pl

On Fri, Feb 14, 2003 at 10:22:19AM -0800, Brad Hilton wrote:
> Hello,
>
> I am hoping someone can help explain why modifying the following query
> can effect such a huge change in speed.  The query is:
>
>       select * from articles
>       where exists
>        ( select 1 from article_categories
>          where
>          article_categories.article_id = articles.id and
>          article_categories.category_id is null
>        )

Can you test these two queries?
 select * from
  (select article_id from article_categories where category_id is null
   group by article_id) X
    join articles using (article_id);

  select <fields> from
   article_categories
   join articles using (article_id)
  where category_id is null
   group by <fields>

 Above queries will need index on article_id
 I'm not sure if it helps, but they are the only solutions in my
 mind ;-)

 Can you say anything about data statistics in your tables? How
 many rows are with category_id=null?

 I looked into query definition once again. Your query doesn't make
 sense - article_categories have not null category_id... What do you really
 want to do?

Regards,
Tomasz Myrta