Re: TOAST usage setting

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.

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