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

In response to

• docco on external storage? at 2003-11-03 17:53:46 from Andrew Dunstan

Responses

• Re: docco on external storage? at 2003-11-04 12:18:45 from Andrew Dunstan

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