Thanks. We don't really expect DBAs to go hunting through the source code, though, do we?
cheers andrew ----- Original Message ----- From: "Joe Conway" <[EMAIL PROTECTED]> To: "Andrew Dunstan" <[EMAIL PROTECTED]> Cc: "Postgresql Hackers" <[EMAIL PROTECTED]> 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 > > > ---------------------------(end of broadcast)--------------------------- > TIP 9: the planner will ignore your desire to choose an index scan if your > joining column's datatypes do not match ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])