Re: sub-query optimization

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
groupby 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_idI'm not sure if it helps, but they are the only solutions in mymind ;-)
Can you say anything about data statistics in your tables? Howmany rows are with category_id=null?
I looked into query definition once again. Your query doesn't makesense - article_categories have not null
category_id...What do you reallywant to do?
 

Regards,
Tomasz Myrta