Revert buggy optimization of index scans

606c0123 attempted to reduce cost of index scans using > and <
strategies, though got that completely wrong in a few complex cases.

Revert whole patch until we find a safe optimization.

src/backend/access/nbtree/nbtsearch.c

@@ -1026,33 +1026,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	if (goback)
 		offnum = OffsetNumberPrev(offnum);
 
-	/*
-	 * By here the scan position is now set for the first key.  If all further
-	 * tuples are expected to match we set the SK_BT_MATCHED flag to avoid
-	 * re-checking the scan key later.  This is a big win for slow key matches
-	 * though is still significant even for fast datatypes.
-	 */
-	switch (startKeys[0]->sk_strategy)
-	{
-		case BTEqualStrategyNumber:
-			break;
-
-		case BTGreaterEqualStrategyNumber:
-		case BTGreaterStrategyNumber:
-			if (ScanDirectionIsForward(dir))
-				startKeys[0]->sk_flags |= SK_BT_MATCHED;
-			break;
-
-		case BTLessEqualStrategyNumber:
-		case BTLessStrategyNumber:
-			if (ScanDirectionIsBackward(dir))
-				startKeys[0]->sk_flags |= SK_BT_MATCHED;
-			break;
-
-		default:
-			break;
-	}
-
 	/* remember which buffer we have pinned, if any */
 	Assert(!BTScanPosIsValid(so->currPos));
 	so->currPos.buf = buf;

src/backend/access/nbtree/nbtutils.c

@@ -1429,13 +1429,6 @@ _bt_checkkeys(IndexScanDesc scan,
 		bool		isNull;
 		Datum		test;
 
-		/*
-		 * If the scan key has already matched we can skip this key, as long
-		 * as the index tuple does not contain NULL values.
-		 */
-		if (key->sk_flags & SK_BT_MATCHED && !IndexTupleHasNulls(tuple))
-			continue;
-
 		/* row-comparison keys need special processing */
 		if (key->sk_flags & SK_ROW_HEADER)
 		{

src/include/access/nbtree.h

@@ -646,7 +646,6 @@ typedef BTScanOpaqueData *BTScanOpaque;
  */
 #define SK_BT_REQFWD	0x00010000		/* required to continue forward scan */
 #define SK_BT_REQBKWD	0x00020000		/* required to continue backward scan */
-#define SK_BT_MATCHED	0x00040000		/* required to skip further key match */
 #define SK_BT_INDOPTION_SHIFT  24		/* must clear the above bits */
 #define SK_BT_DESC			(INDOPTION_DESC << SK_BT_INDOPTION_SHIFT)
 #define SK_BT_NULLS_FIRST	(INDOPTION_NULLS_FIRST << SK_BT_INDOPTION_SHIFT)