From: Brad Hilton <bhilton@vpop.net>
To: pgsql-sql@postgresql.org
Subject: Re: sub-query optimization
Date: 14 Feb 2003 10:38:51 -0800
Message-ID: <1045247931.29968.28.camel@aragorn.vpop.net> (raw)
In-Reply-To: <1045246939.29966.22.camel@aragorn.vpop.net>
References: <1045246939.29966.22.camel@aragorn.vpop.net>
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
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To: pgsql-sql@postgresql.org
Cc: bhilton@vpop.net
Subject: Re: sub-query optimization
In-Reply-To: <1045247931.29968.28.camel@aragorn.vpop.net>
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