Subject: Re: sub-query optimization
From: Brad Hilton <bhilton@vpop.net>
To: pgsql-sql@postgresql.org
In-Reply-To: <1045246939.29966.22.camel@aragorn.vpop.net>
References: <1045246939.29966.22.camel@aragorn.vpop.net>
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Date: 14 Feb 2003 10:38:51 -0800

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