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Re: [PATCHES] Proposed patch: synchronized_scanningGUCvariable

From: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>
To: "Jeff Davis" <pgsql(at)j-davis(dot)com>
Cc: "Simon Riggs" <simon(at)2ndquadrant(dot)com>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"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_scanningGUCvariable
Date: 2008-01-29 08:20:41
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Lists: pgsql-hackerspgsql-patches
Jeff Davis wrote:
> On Mon, 2008-01-28 at 23:13 +0000, Heikki Linnakangas wrote:
>> It's a good point that we don't want pg_dump to screw up the cluster 
>> order, but that's the only use case I've seen this far for disabling 
>> sync scans. Even that wouldn't matter much if our estimate for 
>> "clusteredness" didn't get screwed up by a table that looks like this: 
>> "5 6 7 8 9 1 2 3 4"
> It doesn't seem like there is any reason for the estimate to get
> confused, but it apparently does. I loaded a 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?".
> 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?

It has been discussed before, but no-one has come up with a good 
measurement for that.

   Heikki Linnakangas

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