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Re: new correlation metric

From: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, pgsql-hackers(at)postgresql(dot)org, npboley(at)gmail(dot)com
Subject: Re: new correlation metric
Date: 2008-10-27 17:36:27
Message-ID: 4905FC1B.7030202@cheapcomplexdevices.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> writes:
>> ...bitmap cost estimates didn't also change much....
> 
> By definition, a bitmap scan's cost isn't affected by index order
> correlation.

No?   I think I understand that for index scans the correlation
influenced how many data pages are estimated to get sucked in.

Wouldn't a bitmap scan using a single index also fetch roughly
the same number of data pages as an index scan?

I'm not complaining, since 8.3's doing great on all my real-world
queries.   And sorry for my naive questions - feel free to tell
me to just read the code if this is something I should be able
to figure out myself.


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