Index: src/where.c
==================================================================
--- src/where.c
+++ src/where.c
@@ -1883,21 +1883,23 @@
 
 /*
 ** Return TRUE if all of the following are true:
 **
 **   (1)  X has the same or lower cost that Y
-**   (2)  X users fewer WHERE clause terms than Y
+**   (2)  X uses fewer WHERE clause terms than Y
 **   (3)  Every WHERE clause term used by X is also used by Y
 **   (4)  X skips at least as many columns as Y
 **   (5)  If X is a covering index, than Y is too
 **
 ** Conditions (2) and (3) mean that X is a "proper subset" of Y.
 ** If X is a proper subset of Y then Y is a better choice and ought
 ** to have a lower cost.  This routine returns TRUE when that cost 
 ** relationship is inverted and needs to be adjusted.  Constraint (4)
 ** was added because if X uses skip-scan less than Y it still might
-** deserve a lower cost even if it is a proper subset of Y.
+** deserve a lower cost even if it is a proper subset of Y.  Constraint (5)
+** was added because a covering index probably deserves to have a lower cost
+** than a non-covering index even if it is a proper subset.
 */
 static int whereLoopCheaperProperSubset(
   const WhereLoop *pX,       /* First WhereLoop to compare */
   const WhereLoop *pY        /* Compare against this WhereLoop */
 ){