diff --git a/benchmarks/commonUtilities.js b/benchmarks/commonUtilities.js
index 05a4a73..7224ef5 100644
--- a/benchmarks/commonUtilities.js
+++ b/benchmarks/commonUtilities.js
@@ -31,17 +31,22 @@ module.exports.prepareDb = function (filename, cb) {
 
 /**
  * Return an array with the numbers from 0 to n-1, in a random order
+ * Uses Fisher Yates algorithm
  * Useful to get fair tests
  */
 function getRandomArray (n) {
-  var res, next;
+  var res = []
+    , i, j, temp
+    ;
 
-  if (n === 0) { return []; }
-  if (n === 1) { return [0]; }
+  for (i = 0; i < n; i += 1) { res[i] = i; }
 
-  res = getRandomArray(n - 1);
-  next = Math.floor(Math.random() * n);
-  res.splice(next, 0, n - 1);   // Add n-1 at a random position in the array
+  for (i = n - 1; i >= 1; i -= 1) {
+    j = Math.floor((i + 1) * Math.random());
+    temp = res[i];
+    res[i] = res[j];
+    res[j] = temp;
+  }
 
   return res;
 };
diff --git a/benchmarks/insert.js b/benchmarks/insert.js
index bcf2de5..e10797c 100644
--- a/benchmarks/insert.js
+++ b/benchmarks/insert.js
@@ -21,12 +21,19 @@ console.log("Test with " + n + " documents");
 console.log(program.withIndex ? "Use an index" : "Don't use an index");
 console.log("----------------------------");
 
+
 async.waterfall([
   async.apply(commonUtilities.prepareDb, benchDb)
 , function (cb) {
     d.loadDatabase(function (err) {
       if (err) { return cb(err); }
-      if (program.withIndex) { d.ensureIndex({ fieldName: 'docNumber' }); }
+      if (program.withIndex) {
+        d.ensureIndex({ fieldName: 'docNumber' });
+        n = 2 * n;   // We will actually insert twice as many documents
+                     // because the index is slower when the collection is already
+                     // big. So the result given by the algorithm will be a bit worse than
+                     // actual performance
+      }
       cb();
     });
   }