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Re: I/O on select count(*)

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-15 14:41:06
Message-ID: 20080515144106.GA8221@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-performance
Greg Smith escribió:
> On Thu, 15 May 2008, Pavan Deolasee wrote:
>
>> I had suggested in the past that whenever we set hint bits for a tuple, 
>> we should check all other tuples in the page and set their hint bits 
>> too to avoid multiple writes of the same page. I guess the idea got 
>> rejected because of lack of benchmarks to prove the benefit.
>
> From glancing at http://www.postgresql.org/docs/faqs.TODO.html I got the  
> impression the idea was to have the background writer get involved to 
> help with this particular situation.

The problem is that the bgwriter does not understand about the content
of the pages it is writing -- they're opaque pages for all it knows.  So
it cannot touch the hint bits.

I agree with Pavan that it's likely that setting hint bits in batches
instead of just for the tuple being examined is a benefit.  However,
it's perhaps not so good to be doing it in a foreground process, because
you're imposing extra cost to the client queries which we want to be as
fast as possible.  Perhaps the thing to do is have a "database-local
bgwriter" which would scan pages and do this kind of change ...
a different kind of process to be launched by autovacuum perhaps.

If we had the bitmask in a separate map fork, this could be cheap.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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