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Decouple the values of TOAST_TUPLE_THRESHOLD and TOAST_MAX_CHUNK_SIZE.

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Add the latter to the values checked in pg_control, since it can't be changed
without invalidating toast table content.  This commit in itself shouldn't
change any behavior, but it lays some necessary groundwork for experimentation
with these toast-control numbers.

Note: while TOAST_TUPLE_THRESHOLD can now be changed without initdb, some
thought still needs to be given to needs_toast_table() in toasting.c before
unleashing random changes.
This commit is contained in:
Tom Lane
2007-04-03 04:14:26 +00:00
parent 4fea0ca8f3
commit b3005276eb
8 changed files with 113 additions and 47 deletions
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55
src/include/access/tuptoaster.h
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@@ -6,7 +6,7 @@
*
* Copyright (c) 2000-2007, PostgreSQL Global Development Group
*
* $PostgreSQL: pgsql/src/include/access/tuptoaster.h,v 1.33 2007/03/29 00:15:39 tgl Exp $
* $PostgreSQL: pgsql/src/include/access/tuptoaster.h,v 1.34 2007/04/03 04:14:26 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -29,19 +29,26 @@
* TOAST_TUPLE_TARGET bytes. Both numbers include all tuple header overhead
* and between-fields alignment padding, but we do *not* consider any
* end-of-tuple alignment padding; hence the values can be compared directly
* to a tuple's t_len field. We choose TOAST_TUPLE_THRESHOLD with the
* knowledge that toast-table tuples will be exactly that size, and we'd
* like to fit four of them per page with minimal space wastage.
* to a tuple's t_len field.
*
* The numbers need not be the same, though they currently are.
* The numbers need not be the same, though they currently are. It doesn't
* make sense for TARGET to exceed THRESHOLD, but it could be useful to make
* it be smaller.
*
* Note: sizeof(PageHeaderData) includes the first ItemId, but we have
* to allow for 3 more, if we want to fit 4 tuples on a page.
* Currently we choose both values to match the largest tuple size for which
* TOAST_TUPLES_PER_PAGE tuples can fit on a disk page.
*
* XXX while these can be modified without initdb, some thought needs to be
* given to needs_toast_table() in toasting.c before unleashing random
* changes.
*/
#define TOAST_TUPLES_PER_PAGE 4
/* Note: sizeof(PageHeaderData) includes the first ItemId on the page */
#define TOAST_TUPLE_THRESHOLD \
MAXALIGN_DOWN((BLCKSZ - \
MAXALIGN(sizeof(PageHeaderData) + 3 * sizeof(ItemIdData))) \
/ 4)
MAXALIGN(sizeof(PageHeaderData) + (TOAST_TUPLES_PER_PAGE-1) * sizeof(ItemIdData))) \
/ TOAST_TUPLES_PER_PAGE)
#define TOAST_TUPLE_TARGET TOAST_TUPLE_THRESHOLD
@@ -56,20 +63,26 @@
* When we store an oversize datum externally, we divide it into chunks
* containing at most TOAST_MAX_CHUNK_SIZE data bytes. This number *must*
* be small enough that the completed toast-table tuple (including the
* ID and sequence fields and all overhead) is no more than MaxHeapTupleSize
* bytes. It *should* be small enough to make toast-table tuples no more
* than TOAST_TUPLE_THRESHOLD bytes, else heapam.c will uselessly invoke
* the toaster on toast-table tuples. The current coding ensures that the
* maximum tuple length is exactly TOAST_TUPLE_THRESHOLD bytes.
* ID and sequence fields and all overhead) will fit on a page.
* The coding here sets the size on the theory that we want to fit
* EXTERN_TUPLES_PER_PAGE tuples of maximum size onto a page.
*
* NB: you cannot change this value without forcing initdb, at least not
* if your DB contains any multi-chunk toasted values.
* NB: Changing TOAST_MAX_CHUNK_SIZE requires an initdb.
*/
#define TOAST_MAX_CHUNK_SIZE (TOAST_TUPLE_THRESHOLD - \
MAXALIGN(offsetof(HeapTupleHeaderData, t_bits)) - \
sizeof(Oid) - \
sizeof(int32) - \
VARHDRSZ)
#define EXTERN_TUPLES_PER_PAGE 4 /* tweak only this */
/* Note: sizeof(PageHeaderData) includes the first ItemId on the page */
#define EXTERN_TUPLE_MAX_SIZE \
MAXALIGN_DOWN((BLCKSZ - \
MAXALIGN(sizeof(PageHeaderData) + (EXTERN_TUPLES_PER_PAGE-1) * sizeof(ItemIdData))) \
/ EXTERN_TUPLES_PER_PAGE)
#define TOAST_MAX_CHUNK_SIZE \
(EXTERN_TUPLE_MAX_SIZE - \
MAXALIGN(offsetof(HeapTupleHeaderData, t_bits)) - \
sizeof(Oid) - \
sizeof(int32) - \
VARHDRSZ)
/* ----------