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Re: I/O on select count(*)

From: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
To: Matthew Wakeling <matthew(at)flymine(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-15 13:50:44
Message-ID: 482C3FB4.6050102@cheapcomplexdevices.com (view raw or flat)
Thread:
Lists: pgsql-performance
Matthew Wakeling wrote:
> On Thu, 15 May 2008, Luke Lonergan wrote:
>> ...HINT bit optimization, but avoids this whole ³write the data,
>> write it to the log also, then write it again just for good measure²
> ...
> The hint data will be four bits per tuple plus overheads, so it could be 
> made very compact, and therefore likely to stay in the cache fairly 
> well. 

Does it seem like these HINT bits would be good candidates to move
off to map forks similar to how the visibility map stuff will be handled?

Since (if I understand right) only the hint bits change during the
select(*) it seems a lot less write-IO would happen if such a map
were updated rather than the data pages themselves.

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