Re: docco on external storage?

Thanks. We don't really expect DBAs to go hunting through the source code,
though, do we?

cheers

andrew


----- Original Message ----- 
From: "Joe Conway" <mail@joeconway.com>
To: "Andrew Dunstan" <andrew@dunslane.net>
Cc: "Postgresql Hackers" <pgsql-hackers@postgresql.org>
Sent: Monday, November 03, 2003 9:50 PM
Subject: Re: [HACKERS] docco on external storage?


> Andrew Dunstan wrote:
> > I have a table with two modest-length bytea fields (around 9K and 2K
> > respectively) of already compressed data. I was looking for a clue as to
> > whether or not these should use external storage - I suspect they should
> > but I didn't see anything that gave me a definite answer.
>
>  From src/include/access/tuptoaster.h:
> -------------------------------------
> /*
>   * These symbols control toaster activation.  If a tuple is larger than
>   * TOAST_TUPLE_THRESHOLD, we will try to toast it down to no more than
>   * TOAST_TUPLE_TARGET bytes.  Both numbers include all tuple header and
>   * alignment-padding overhead.
>   *
>   * The numbers need not be the same, though they currently are.
>   */
> #define TOAST_TUPLE_THRESHOLD   (MaxTupleSize / 4)
> #define TOAST_TUPLE_TARGET      (MaxTupleSize / 4)
>
>
> And from src/include/access/htup.h:
> -----------------------------------
> /*
>   * MaxTupleSize is the maximum allowed size of a tuple, including header
>   * and MAXALIGN alignment padding.  Basically it's BLCKSZ minus the
>   * other stuff that has to be on a disk page.  The "other stuff"
>   * includes access-method- dependent "special space", which we assume
>   * will be no more than MaxSpecialSpace bytes (currently, on heap pages
>   * it's actually zero).
>   *
>   * NOTE: we do not need to count an ItemId for the tuple because
>   * sizeof(PageHeaderData) includes the first ItemId on the page.
>   */
> #define MaxSpecialSpace  32
> #define MaxTupleSize    \
>          (BLCKSZ - MAXALIGN(sizeof(PageHeaderData) + MaxSpecialSpace))
>
>
> And from src/include/pg_config_manual.h:
> ----------------------------------------
> #define BLCKSZ  8192
>
>
> So, it looks like the threshold for external storage is somewhere around
> 2000 bytes.
>
> Joe
>
>
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