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Re: I/O on select count(*)

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Matthew Wakeling <matthew(at)flymine(dot)org>, pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-16 18:23:55
Message-ID: 1210962235.18922.109.camel@ebony.site (view raw or flat)
Thread:
Lists: pgsql-performance
On Fri, 2008-05-16 at 14:05 -0400, Greg Smith wrote:
> After reading the code and that documentation a bit, the part I'm
> still not sure about is whether the CLOG entry is created when the XID
> is assigned and then kept current as the state changes, or whether
> that isn't even in CLOG until the transaction is committed.  It seems
> like the latter, but there's some ambiguity in the wording and too
> many code paths for me to map right now.

Alvaro already said this, I thought? The clog is updated only at sub or
main transaction end, thank goodness. When the transactionid is assigned
the page of the clog that contains that transactionid is checked to see
if it already exists and if not, it is initialised.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support


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