Add test cases and fix a comment.

FossilOrigin-Name: 50f8ea37fb9647c4a9da2c269a4d6f54b10ce96b

src/expr.c

@@ -3994,10 +3994,10 @@ static int exprIdxCover(Walker *pWalker, Expr *pExpr){
 }
 
 /*
-** Determine if an index on table iCur that contains the columns in
-** Bitmask m will cover the expression pExpr.  Return true if the index
-** does cover the expression and false if the expression references
-** table columns that are not found in the index.
+** Determine if an index pIdx on table with cursor iCur contains will
+** the expression pExpr.  Return true if the index does cover the
+** expression and false if the pExpr expression references table columns
+** that are not found in the index pIdx.
 **
 ** An index covering an expression means that the expression can be
 ** evaluated using only the index and without having to lookup the

src/where.c

@@ -2478,11 +2478,11 @@ static int whereLoopAddBtreeIndex(
     pNew->nSkip++;
     pNew->aLTerm[pNew->nLTerm++] = 0;
     pNew->wsFlags |= WHERE_SKIPSCAN;
-    nIter = pProbe->aiRowLogEst[saved_nEq] - pProbe->aiRowLogEst[saved_nEq+1];
+    nIter = pProbe->aiRowLogEst[saved_nEq]+1 - pProbe->aiRowLogEst[saved_nEq+1];
     pNew->nOut -= nIter;
     /* TUNING:  Because uncertainties in the estimates for skip-scan queries,
     ** add a 1.375 fudge factor to make skip-scan slightly less likely. */
-    nIter += 5;
+    nIter += 4;
     whereLoopAddBtreeIndex(pBuilder, pSrc, pProbe, nIter + nInMul);
     pNew->nOut = saved_nOut;
     pNew->u.btree.nEq = saved_nEq;