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Re: docco on external storage?

From: Joe Conway <mail(at)joeconway(dot)com>
To: Andrew Dunstan <andrew(at)dunslane(dot)net>
Cc: Postgresql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: docco on external storage?
Date: 2003-11-04 02:50:33
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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.


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