Re: 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