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
itdown 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'sBLCKSZ 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
actuallyzero). * * NOTE: we do not need to count an ItemId for the tuple because * sizeof(PageHeaderData) includes the
firstItemId 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