From: Tom Lane <tgl@sss.pgh.pa.us>
To: Brad Hilton <bhilton@vpop.net>
Cc: pgsql-sql@postgresql.org
Subject: Re: sub-query optimization
Date: Fri, 14 Feb 2003 17:08:27 -0500
Message-ID: <11481.1045260507@sss.pgh.pa.us> (raw)
In-Reply-To: <1045246939.29966.22.camel@aragorn.vpop.net>
References: <1045246939.29966.22.camel@aragorn.vpop.net>
Brad Hilton <bhilton@vpop.net> writes:
> 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
> )
> ... If I modify the query slightly:
> --------
> select 1 from article_categories
> -->
> select 1 from articles, article_categories
> ---------
> the query takes 98 msec.
Yeah, because then the sub-query is a constant (it doesn't depend on the
current outer row at all) and so it is only evaluated once, not once per
outer row. Unfortunately, that approach probably gives the wrong
answers...
regards, tom lane
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, bhilton@vpop.net
Subject: Re: sub-query optimization
In-Reply-To: <11481.1045260507@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox