nbtree: Use only one notnullkey ScanKeyData

Author: petergeoghegan

src/backend/access/nbtree/nbtsearch.c

@@ -892,7 +892,7 @@ _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;
 	BlockNumber blkno = InvalidBlockNumber,
@@ -1122,8 +1122,8 @@ _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 &