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Re: Difference between 8.1 & 8.3

From: "Gregory Williamson" <Gregory(dot)Williamson(at)digitalglobe(dot)com>
To: "Patrick Vachon" <pvachon(dot)videotron(at)videotron(dot)ca>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: Difference between 8.1 & 8.3
Date: 2008-07-16 21:59:42
Message-ID: 8B319E5A30FF4A48BE7EEAAF609DB233021F3037@COMAIL01.digitalglobe.com (view raw or flat)
Thread:
Lists: pgsql-performance
Patrick Vachon wrote:

> Hi guys,
> 
> I've got a query that is running slower on 8.3 than on 8.1 (with 
> equivalent server config),
> because the join ordering is not the same, at least that's my guess... ;-)
> 
> In 8.1.4, table A had 122880 pages, B 112690 pages and C 80600 pages.
> Now in 8.3.3, table A has only 77560 pages, B 69580 but C remains at 
> 80600 pages.
> 
> In 8.1 the tables were joined in that way (using explain analyse):
> C join A join B
> now in 8.3:
> B join A join C
> Beside that, the plan is very similar, but the indexes used are not the 
> same.
> 
> Could the number of disk pages of a table influence the
> order in which it is joined, even when it is scanned with an index?
> 
> I'm pretty sure it is because of the reduced table sizes,
> since the server configuration is the same.
> 
> Thoughts?

8.3 has fewer automatic casts to text types; perhaps you have indexes which are not being used because of mismatched types ? Perhaps an EXPLAIN ANALYZE from both, if possible, would clairfy.

HTH,

Greg Williamson
Senior DBA
DigitalGlobe

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