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Re: TOAST usage setting

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Zeugswetter Andreas ADI SD <ZeugswetterA(at)spardat(dot)at>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: TOAST usage setting
Date: 2007-06-01 16:38:09
Message-ID: 200706011638.l51Gc9v08061@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Gregory Stark wrote:
> "Gregory Stark" <stark(at)enterprisedb(dot)com> writes:
> 
> > "Bruce Momjian" <bruce(at)momjian(dot)us> writes:
> >
> >> shared_buffers again was 32MB so all the data was in memory.
> >
> > The case where all the data is in memory is simply not interesting. The cost
> > of TOAST is the random access seeks it causes. You seem to be intentionally
> > avoiding testing the precise thing we're interested in.
> 
> Also, something's not right with these results. 100,000 tuples --even if all
> they contain is a toast pointer-- won't fit on a single page. And the toast
> tables should vary in size depending on how many toast chunks are created.

The test creates _one_ row of length 100,000 and then finds out how long
it takes to access it twenty times.

I don't see how having the data outside cache helps us.  For a large row
with 2k chunks, I assume all the 2k chunks are going to be in the same
8k page.  What I want to measure is the cost of accessing four 2k chunks
vs. one 8k chunk, and I think we can conclude that is 6% of the access
time.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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Subject: Re: TOAST usage setting
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