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Re: I/O on select count(*)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Matthew Wakeling <matthew(at)flymine(dot)org>, pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-19 15:37:58
Message-ID: 5697.1211211478@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Bruce Momjian <bruce(at)momjian(dot)us> writes:
> Matthew Wakeling wrote:
>> Does it really take that long to zero out 8kB of RAM? I thought CPUs were 
>> really quick at doing that!

> Yea, that was my assumption too.

You have to write the page (to be sure there is space for it on disk)
not only zero it.

This design is kind of a holdover, though, from back when we had one
ever-growing clog file.  Today I'd be inclined to think about managing
it more like pg_xlog, ie, have some background process pre-create a
whole segment file at a time.

			regards, tom lane

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