nbtree: Use only one notnullkey ScanKeyData.

_bt_first need only store one ScanKeyData struct on the stack for the
purposes of building an IS NOT NULL key based on an implied NOT NULL
constraint.  We don't need INDEX_MAX_KEYS-many ScanKeyData structs.

This saves us a little over 2KB in stack space.  It's possible that this
has some performance benefit.  It also seems simpler and more direct.

It isn't possible for more than a single index attribute to need its own
implied IS NOT NULL key: the first such attribute/IS NOT NULL key always
makes _bt_first stop adding additional boundary keys to startKeys[].
Using INDEX_MAX_KEYS-many ScanKeyData entries was (at best) misleading.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Mircea Cadariu <cadariu.mircea@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wzm=1kJMSZhhTLoM5BPbwQNWxUj-ynOEh=89ptDZAVgauw@mail.gmail.com

src/backend/access/nbtree/nbtsearch.c

@@ -892,9 +892,9 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	OffsetNumber offnum;
 	BTScanInsertData inskey;
 	ScanKey		startKeys[INDEX_MAX_KEYS];
-	ScanKeyData notnullkeys[INDEX_MAX_KEYS];
+	ScanKeyData notnullkey;
 	int			keysz = 0;
-	StrategyNumber strat_total;
+	StrategyNumber strat_total = InvalidStrategy;
 	BlockNumber blkno = InvalidBlockNumber,
 				lastcurrblkno;
 
@@ -1034,7 +1034,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	 * need to be kept in sync.
 	 *----------
 	 */
-	strat_total = BTEqualStrategyNumber;
 	if (so->numberOfKeys > 0)
 	{
 		AttrNumber	curattr;
@@ -1122,16 +1121,15 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 					 ScanDirectionIsForward(dir) :
 					 ScanDirectionIsBackward(dir)))
 				{
-					/* Yes, so build the key in notnullkeys[keysz] */
-					bkey = &notnullkeys[keysz];
+					/* Final startKeys[] entry will be deduced NOT NULL key */
+					bkey = &notnullkey;
 					ScanKeyEntryInitialize(bkey,
 										   (SK_SEARCHNOTNULL | SK_ISNULL |
 											(impliesNN->sk_flags &
 											 (SK_BT_DESC | SK_BT_NULLS_FIRST))),
 										   curattr,
-										   ((impliesNN->sk_flags & SK_BT_NULLS_FIRST) ?
-											BTGreaterStrategyNumber :
-											BTLessStrategyNumber),
+										   ScanDirectionIsForward(dir) ?
+										   BTGreaterStrategyNumber : BTLessStrategyNumber,
 										   InvalidOid,
 										   InvalidOid,
 										   InvalidOid,