Extra steps taken to avoid using low-quality indexes in a query plan.

This branch accomplishes the same end as the nearby enhanced-stat1 branch,
but with much less change and hence less risk.

FossilOrigin-Name: c030e646262fee43a59b45fdc1630d972f8bf88ac3c142b6bdaf4cbb36695a4f

src/analyze.c

@@ -1556,6 +1556,16 @@ static void decodeIntArray(
       while( z[0]!=0 && z[0]!=' ' ) z++;
       while( z[0]==' ' ) z++;
     }
+
+    /* Set the bLowQual flag if the peak number of rows obtained
+    ** from a full equality match is so large that a full table scan
+    ** seems likely to be faster than using the index.
+    */
+    if( aLog[0] > 66              /* Index has more than 100 rows */
+     && aLog[0] <= aLog[nOut-1]   /* And only a single value seen */
+    ){
+      pIndex->bLowQual = 1;
+    }
   }
 }