From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: List pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: WIP Join Removal
Date: Tue, 02 Sep 2008 13:20:36 +0300
Message-ID: <48BD1374.4030504@enterprisedb.com> (raw)
In-Reply-To: <1220348487.4371.307.camel@ebony.2ndQuadrant>
References: <1220176372.4371.118.camel@ebony.2ndQuadrant>
<48BC4127.30706@enterprisedb.com>
<1220348487.4371.307.camel@ebony.2ndQuadrant>
Simon Riggs wrote:
> It turns out that a join like this
>
> select a.col2
> from a left outer join b on a.col1 = b.col1
> where b.col2 = 1;
>
> can be cheaper if we don't remove the join, when there is an index on
> a.col1 and b.col2, because the presence of b allows the values returned
> from b to be used for an index scan on a.
Umm, you *can't* remove that join. Because of the condition "b.col2 =
1", which implies that "b.col1 IS NOT NULL", that's actually equal to:
select a.col2
from a inner join b on a.col1 = b.col1
where b.col2 = 1;
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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-patches@postgresql.org
Cc: heikki.linnakangas@enterprisedb.com, simon@2ndQuadrant.com
Subject: Re: WIP Join Removal
In-Reply-To: <48BD1374.4030504@enterprisedb.com>
* 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