Skip site navigation (1) Skip section navigation (2)

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
Message-ID: 479EE1D9.9090106@enterprisedb.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:
>> 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
   EnterpriseDB   http://www.enterprisedb.com

In response to

pgsql-hackers by date

Next:From: Kris JurkaDate: 2008-01-29 08:34:19
Subject: GSSAPI and V2 protocol
Previous:From: Florian WeimerDate: 2008-01-29 08:10:13
Subject: Re: [GENERAL] SHA1 on postgres 8.3

pgsql-patches by date

Next:From: Zeugswetter Andreas ADI SDDate: 2008-01-29 09:40:40
Subject: Re: [PATCHES] Proposed patch: synchronized_scanning GUCvariable
Previous:From: Kris JurkaDate: 2008-01-29 05:09:28
Subject: Re: Proposed patch: synchronized_scanning GUC variable

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group