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 13:41:22 +0300
Message-ID: <48BD1852.1070904@enterprisedb.com> (raw)
In-Reply-To: <1220352112.4371.349.camel@ebony.2ndQuadrant>
References: <1220176372.4371.118.camel@ebony.2ndQuadrant>
<48BC4127.30706@enterprisedb.com>
<1220348487.4371.307.camel@ebony.2ndQuadrant>
<48BD1374.4030504@enterprisedb.com>
<1220352112.4371.349.camel@ebony.2ndQuadrant>
Simon Riggs wrote:
> On Tue, 2008-09-02 at 13:20 +0300, Heikki Linnakangas wrote:
>> 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.
>
> Yes, you can. The presence or absence of rows in b is not important to
> the result of the query because of the "left outer join".
>
> I spent nearly a whole day going down that deadend also.
Oh. How does the query look like after removing the join, then?
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-patches@postgresql.org
Cc: heikki.linnakangas@enterprisedb.com, simon@2ndQuadrant.com
Subject: Re: WIP Join Removal
In-Reply-To: <48BD1852.1070904@enterprisedb.com>
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