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Re: TOAST usage setting

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: TOAST usage setting
Date: 2007-05-29 16:09:16
Message-ID: 20070529160916.GF4667@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-hackers
Bruce Momjian wrote:
> Gregory Stark wrote:
> > "Bruce Momjian" <bruce(at)momjian(dot)us> writes:
> > 
> > > Gregory Stark wrote:
> > >> "Bruce Momjian" <bruce(at)momjian(dot)us> writes:
> > >> 
> > >> > I tested TOAST using a method similar to the above method against CVS
> > >> > HEAD, with default shared_buffers = 32MB and no assert()s.  I created
> > >> > backends with power-of-2 seetings for TOAST_TUPLES_PER_PAGE (4(default),
> > >> > 8, 16, 32, 64) which gives TOAST/non-TOAST breakpoints of 2k(default),
> > >> > 1k, 512, 256, and 128, roughly.
> > >> >
> > >> > The results are here:
> > >> >
> > >> > 	http://momjian.us/expire/TOAST/
> > >> >
> > >> > Strangely, 128 bytes seems to be the break-even point for TOAST and
> > >> > non-TOAST, even for sequential scans of the entire heap touching all
> > >> > long row values.  I am somewhat confused why TOAST has faster access
> > >> > than inline heap data.
> > 
> > Is your database initialized with C locale? If so then length(text) is
> > optimized to not have to detoast:
> > 
> > 	if (pg_database_encoding_max_length() == 1)
> > 		PG_RETURN_INT32(toast_raw_datum_size(str) - VARHDRSZ);
> 
> Wow, we optimized length().  OK, will run the test with
> substring(t,1,1).

Be careful about the compression!  It might be a good idea to run the
test once with the column set to uncompressible (SET STORAGE EXTERNAL?),
and again with it as compressible.

-- 
Alvaro Herrera       Valdivia, Chile   ICBM: S 39º 49' 18.1", W 73º 13' 56.4"
Syntax error: function hell() needs an argument.
Please choose what hell you want to involve.

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