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 */
){