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 */ ){