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Re: [PATCHES] Proposed patch: synchronized_scanning GUCvariable

From: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Neil Conway <neilc(at)samurai(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [PATCHES] Proposed patch: synchronized_scanning GUCvariable
Date: 2008-01-29 00:13:09
Message-ID: 479E6F95.4010901@cheapcomplexdevices.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Jeff Davis wrote:
> On Mon, 2008-01-28 at 23:13 +0000, Heikki Linnakangas wrote:
>   
>> "clusteredness" didn't get screwed up by a table that looks like this: 
>> "5 6 7 8 9 1 2 3 4"
>>     
> ...test table with a similar
> distribution to your example, and it shows a correlation of about -0.5,
> but it should be as good as something near -1 or +1.
>
> I am not a statistics expert, but it seems like a better measurement
> would be: "what is the chance that, if the tuples are close together in
> index order, the corresponding heap tuples are close together?".
>   
Same applies for data clustered by zip-code.

All rows for any State or City or County or SchoolZone
are "close together" on the same pages; yet postgres's
stats think they're totally unclustered.
> The answer to that question in your example is "very likely", so there
> would be no problem.
> Is there a reason we don't do this?
>   
I've been tempted to do things like

   update pg_statistic set stanumbers3='{1.0}' where starelid=2617 and
staattnum=7;

after every analyze when I have data like this from tables clustered
by zip.  Seems it'd help more plans than it hurts, but haven't been
brave enough to try in production.


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