Reduce btree scan overhead for < and > strategies

For <, <=, > and >= strategies, mark the first scan key
as already matched if scanning in an appropriate direction.
If index tuple contains no nulls we can skip the first
re-check for each tuple.

Author: Rajeev Rastogi
Reviewer: Haribabu Kommi
Rework of the code and comments by Simon Riggs

src/backend/access/nbtree/nbtsearch.c

@@ -996,6 +996,33 @@ _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;
+	}
+
 	/*
 	 * Now load data from the first page of the scan.
 	 */

src/backend/access/nbtree/nbtutils.c

@@ -1429,6 +1429,13 @@ _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)
 		{