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Re: TOAST usage setting

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, Zeugswetter Andreas ADI SD <ZeugswetterA(at)spardat(dot)at>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: TOAST usage setting
Date: 2007-05-30 16:43:58
Message-ID: 200705301643.l4UGhwX10751@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> > Whereas if you set toast_tuples_per_page to 8k then the only option for
> > Postgres will be to put each datum in its own page and waste 1-3k on every
> > page.
> 
> No, because actually the code is designed to make the toast chunk size
> just enough less than 8K that the tuples fit.
> 
> The shorter-than-normal tuples carrying the last chunk of any particular
> datum are going to result in wasted space to the extent that we can't
> pack them together on a page, but that's true now.  Right now, if you
> have a large toasted datum, it mostly will consist of just-under-2K
> tuples that are sized so that there's no noticeable wasted space on a
> page with 4 of them.  There isn't any advantage to that compared to one
> just-under-8K tuple AFAICS, and it takes 4 times as much work to insert
> or retrieve 'em.

Uh, am I supposed to be running more TOAST tests?  Would someone explain
what they want tested?

-- 
  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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