Re: sub-query optimization

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