Clean up a couple of ad-hoc computations of the maximum number of tuples

on a page, as suggested by ITAGAKI Takahiro.  Also, change a few places
that were using some other estimates of max-items-per-page to consistently
use MaxOffsetNumber.  This is conservatively large --- we could have used
the new MaxHeapTuplesPerPage macro, or a similar one for index tuples ---
but those places are simply declaring a fixed-size buffer and assuming it
will work, rather than actively testing for overrun.  It seems safer to
size these buffers in a way that can't overflow even if the page is
corrupt.

src/backend/access/gist/gistvacuum.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/gist/gistvacuum.c,v 1.6 2005/06/30 17:52:14 teodor Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/gist/gistvacuum.c,v 1.7 2005/09/02 19:02:19 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -52,7 +52,7 @@ gistVacuumUpdate( GistVacuum *gv, BlockNumber blkno, bool needunion ) {
 	int 		lenaddon=4, curlenaddon=0, ntodelete=0;
 	IndexTuple	idxtuple, *addon=NULL;
 	bool		needwrite=false;
-	OffsetNumber    todelete[ BLCKSZ/SizeOfIptrData ];
+	OffsetNumber    todelete[MaxOffsetNumber];
 	ItemPointerData	*completed=NULL;
 	int 		ncompleted=0, lencompleted=16;
 
@@ -439,7 +439,7 @@ gistbulkdelete(PG_FUNCTION_ARGS) {
 		page   = (Page) BufferGetPage(buffer);
 
 		if ( GistPageIsLeaf(page) ) {
-			OffsetNumber todelete[BLCKSZ/SizeOfIptrData];
+			OffsetNumber todelete[MaxOffsetNumber];
 			int ntodelete = 0;
 
 			LockBuffer(buffer, GIST_UNLOCK);

src/backend/access/nbtree/nbtree.c

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.130 2005/05/11 06:24:53 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.131 2005/09/02 19:02:19 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -584,7 +584,7 @@ btbulkdelete(PG_FUNCTION_ARGS)
 	IndexBulkDeleteResult *result;
 	double		tuples_removed;
 	double		num_index_tuples;
-	OffsetNumber deletable[BLCKSZ / sizeof(OffsetNumber)];
+	OffsetNumber deletable[MaxOffsetNumber];
 	int			ndeletable;
 	Buffer		buf;
 	BlockNumber num_pages;

src/backend/commands/vacuum.c

@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.313 2005/08/20 00:39:54 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.314 2005/09/02 19:02:19 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2407,7 +2407,7 @@ repair_frag(VRelStats *vacrelstats, Relation onerel,
 		{
 			Buffer		buf;
 			Page		page;
-			OffsetNumber unused[BLCKSZ / sizeof(OffsetNumber)];
+			OffsetNumber unused[MaxOffsetNumber];
 			OffsetNumber offnum,
 						maxoff;
 			int			uncnt;
@@ -2896,7 +2896,7 @@ vacuum_heap(VRelStats *vacrelstats, Relation onerel, VacPageList vacuum_pages)
 static void
 vacuum_page(Relation onerel, Buffer buffer, VacPage vacpage)
 {
-	OffsetNumber unused[BLCKSZ / sizeof(OffsetNumber)];
+	OffsetNumber unused[MaxOffsetNumber];
 	int			uncnt;
 	Page		page = BufferGetPage(buffer);
 	ItemId		itemid;

src/backend/commands/vacuumlazy.c

@@ -31,7 +31,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.57 2005/08/20 23:26:13 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.58 2005/09/02 19:02:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -60,9 +60,6 @@
 #define REL_TRUNCATE_MINIMUM	1000
 #define REL_TRUNCATE_FRACTION	16
 
-/* MAX_TUPLES_PER_PAGE can be a conservative upper limit */
-#define MAX_TUPLES_PER_PAGE		((int) (BLCKSZ / sizeof(HeapTupleHeaderData)))
-
 
 typedef struct LVRelStats
 {
@@ -259,7 +256,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		 * dead-tuple TIDs, pause and do a cycle of vacuuming before we
 		 * tackle this page.
 		 */
-		if ((vacrelstats->max_dead_tuples - vacrelstats->num_dead_tuples) < MAX_TUPLES_PER_PAGE &&
+		if ((vacrelstats->max_dead_tuples - vacrelstats->num_dead_tuples) < MaxHeapTuplesPerPage &&
 			vacrelstats->num_dead_tuples > 0)
 		{
 			/* Remove index entries */
@@ -554,7 +551,7 @@ static int
 lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
 				 int tupindex, LVRelStats *vacrelstats)
 {
-	OffsetNumber unused[BLCKSZ / sizeof(OffsetNumber)];
+	OffsetNumber unused[MaxOffsetNumber];
 	int			uncnt;
 	Page		page = BufferGetPage(buffer);
 	ItemId		itemid;
@@ -960,7 +957,7 @@ lazy_space_alloc(LVRelStats *vacrelstats, BlockNumber relblocks)
 	maxtuples = (maintenance_work_mem * 1024L) / sizeof(ItemPointerData);
 	maxtuples = Min(maxtuples, INT_MAX);
 	/* stay sane if small maintenance_work_mem */
-	maxtuples = Max(maxtuples, MAX_TUPLES_PER_PAGE);
+	maxtuples = Max(maxtuples, MaxHeapTuplesPerPage);
 
 	vacrelstats->num_dead_tuples = 0;
 	vacrelstats->max_dead_tuples = (int) maxtuples;

src/backend/nodes/tidbitmap.c

@@ -23,7 +23,7 @@
  * Copyright (c) 2003-2005, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/nodes/tidbitmap.c,v 1.6 2005/08/28 22:47:20 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/nodes/tidbitmap.c,v 1.7 2005/09/02 19:02:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,7 +42,7 @@
  * the per-page bitmaps variable size.  We just legislate that the size
  * is this:
  */
-#define MAX_TUPLES_PER_PAGE  ((BLCKSZ - 1) / MAXALIGN(offsetof(HeapTupleHeaderData, t_bits) + sizeof(ItemIdData)) + 1)
+#define MAX_TUPLES_PER_PAGE  MaxHeapTuplesPerPage
 
 /*
  * When we have to switch over to lossy storage, we use a data structure

src/include/access/htup.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.76 2005/08/20 00:39:59 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.77 2005/09/02 19:02:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -323,6 +323,16 @@ do { \
 #define MaxTupleSize	\
 	(BLCKSZ - MAXALIGN(sizeof(PageHeaderData) + MaxSpecialSpace))
 
+/*
+ * MaxHeapTuplesPerPage is an upper bound on the number of tuples that can
+ * fit on one heap page.  (Note that indexes could have more, because they
+ * use a smaller tuple header.)  We arrive at the divisor because each tuple
+ * must be maxaligned, and it must have an associated item pointer.
+ */
+#define MaxHeapTuplesPerPage	\
+	((int) ((BLCKSZ - offsetof(PageHeaderData, pd_linp)) / \
+			(MAXALIGN(offsetof(HeapTupleHeaderData, t_bits)) + sizeof(ItemIdData))))
+
 /*
  * MaxAttrSize is a somewhat arbitrary upper limit on the declared size of
  * data fields of char(n) and similar types.  It need not have anything