It turns out that the item size limit for btree indexes is about BLCKSZ/3,

not BLCKSZ/2 as some of us thought.  Add check for oversize item so that
failure is detected before corrupting the index, not after.

src/backend/access/nbtree/nbtinsert.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.51 1999/11/22 17:55:54 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.52 1999/12/26 03:48:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -267,6 +267,18 @@ _bt_insertonpg(Relation rel,
 										 * this but we need to be
 										 * consistent */
 
+	/*
+	 * Check whether the item can fit on a btree page at all.
+	 * (Eventually, we ought to try to apply TOAST methods if not.)
+	 * We actually need to be able to fit three items on every page,
+	 * so restrict any one item to 1/3 the per-page available space.
+	 * Note that at this point, itemsz doesn't include the ItemId.
+	 */
+	if (itemsz > (PageGetPageSize(page)-sizeof(PageHeaderData)-MAXALIGN(sizeof(BTPageOpaqueData)))/3 - sizeof(ItemIdData))
+		elog(ERROR, "btree: index item size %d exceeds maximum %d",
+			 itemsz,
+			 (PageGetPageSize(page)-sizeof(PageHeaderData)-MAXALIGN(sizeof(BTPageOpaqueData)))/3 - sizeof(ItemIdData));
+
 	/*
 	 * If we have to insert item on the leftmost page which is the first
 	 * page in the chain of duplicates then: 1. if scankey == hikey (i.e.
@@ -342,12 +354,16 @@ _bt_insertonpg(Relation rel,
 	{
 		OffsetNumber offnum = (P_RIGHTMOST(lpageop)) ? P_HIKEY : P_FIRSTKEY;
 		OffsetNumber maxoff = PageGetMaxOffsetNumber(page);
+
+		if (offnum < maxoff)	/* can't split unless at least 2 items... */
+		{
 			ItemId		itid;
 			BTItem		previtem,
 						chkitem;
 			Size		maxsize;
 			Size		currsize;
 
+			/* find largest group of identically-keyed items on page */
 			itid = PageGetItemId(page, offnum);
 			previtem = (BTItem) PageGetItem(page, itid);
 			maxsize = currsize = (ItemIdGetLength(itid) + sizeof(ItemIdData));
@@ -368,11 +384,13 @@ _bt_insertonpg(Relation rel,
 			}
 			if (currsize > maxsize)
 				maxsize = currsize;
+			/* Decide to split if largest group is > 1/2 page size */
 			maxsize += sizeof(PageHeaderData) +
 				MAXALIGN(sizeof(BTPageOpaqueData));
 			if (maxsize >= PageGetPageSize(page) / 2)
 				do_split = true;
 		}
+	}
 
 	if (do_split)
 	{