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Re: More thoughts about planner's cost estimates

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: More thoughts about planner's cost estimates
Date: 2006-06-02 02:32:10
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Lists: pgsql-hackers
Mark Kirkwood <markir(at)paradise(dot)net(dot)nz> writes:
> Tom Lane wrote:
>> With this model, the disk cost to fetch a single
>> index entry will be estimated as random_page_cost (default 4.0) rather
>> than the current fixed 2.0.  This shouldn't hurt things too much for
>> simple indexscans --- especially since anyone running with a reduced
>> random_page_cost won't see as much change.  And it will increase the costs
>> for bitmap scans that cross many index pages, which is what we need in
>> light of Philippe's numbers.

> This sounds good to me, although the 2.0 -> 4.0 cost jump may cause 
> (more) cases of people seeing their index scans in pre-8.2 versions 
> becoming some other type of access in 8.2. I guess a comment about 
> testing existing applications could be placed in the 8.2 release notes?

Yeah, that comes with the territory.  One point to note is that with
this model, setting random_page_cost below 2.0 will actually make small
indexscans look *cheaper* than they do now.  So it'll certainly be
possible to make the thing jump in that direction if you need to.

			regards, tom lane

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