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Re: WIP Join Removal

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: List pgsql-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: WIP Join Removal
Date: 2008-09-02 10:41:22
Message-ID: 48BD1852.1070904@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-patches
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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Subject: Re: WIP Join Removal
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