Re: sub-query optimization

On Fri, 2003-02-14 at 10:22, 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
> )

To add one more detail: in simplifying my query for the list, I should
have said:

article_categories.category_id = 0

instead of

article_categories.category_id is NULL

Then, with an index on article_categories (category_id) you get the
following results for the two queries:

Without adding the "articles" table to the inner query: 1329 msec
With the "articles" table in the inner query: 0.28 msec!

That highlights the difference a bit more dramatically.

Any ideas?

Thanks,
-Brad