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Re: I/O on select count(*)

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
Cc: Greg Smith <gsmith(at)gregsmith(dot)com>, 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 15:49:32
Message-ID: 20080515154932.GB8221@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-performance
Heikki Linnakangas escribió:
> Alvaro Herrera wrote:

>> 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.
>
> We know what kind of a relation we're dealing with in ReadBuffer, so we  
> could add a flag to BufferDesc to mark heap pages.

Hmm, I was thinking that it would need access to the catalogs to know
where the tuples are, but that's certainly not true, so perhaps it could
be made to work.

>> If we had the bitmask in a separate map fork, this could be cheap.
>
> I don't buy that. The point of a hint bit is that it's right there along  
> with the tuple you're looking at. If you have to look at a separate  
> buffer, you might as well just look at clog.

True -- I was confusing this with the idea of having the tuple MVCC
header (xmin, xmax, etc) in a separate fork, which would make the idea
of index-only scans more feasible at the expense of seqscans.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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Subject: Re: I/O on select count(*)
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