diff --git a/core/Archive.php b/core/Archive.php
index b95544dfbc6..1f9d571fb8d 100644
--- a/core/Archive.php
+++ b/core/Archive.php
@@ -610,7 +610,7 @@ private function cacheArchiveIdsAfterLaunching($archiveGroups, $plugins)
                     continue;
                 }
 
-                $this->prepareArchives($archiveGroups, $site, $period);
+                $this->prepareArchive($archiveGroups, $site, $period);
             }
         }
     }
@@ -779,7 +779,7 @@ private static function getPluginForReport($report)
      * @param $site
      * @param $period
      */
-    private function prepareArchives(array $archiveGroups, Site $site, Period $period)
+    private function prepareArchive(array $archiveGroups, Site $site, Period $period)
     {
         $parameters = new ArchiveProcessor\Parameters($site, $period, $this->params->getSegment());
         $archiveLoader = new ArchiveProcessor\Loader($parameters);
diff --git a/core/ArchiveProcessor.php b/core/ArchiveProcessor.php
index 189f861b4c4..b901c7fb6ab 100644
--- a/core/ArchiveProcessor.php
+++ b/core/ArchiveProcessor.php
@@ -365,6 +365,10 @@ protected function getOperationForColumns($columns, $defaultOperation)
 
     protected function enrichWithUniqueVisitorsMetric(Row $row)
     {
+        if(!$this->getParams()->isSingleSite() ) {
+            // we only compute unique visitors for a single site
+            return;
+        }
         if ( $row->getColumn('nb_uniq_visitors') !== false) {
             if (SettingsPiwik::isUniqueVisitorsEnabled($this->getParams()->getPeriod()->getLabel())) {
                 $uniqueVisitors = (float)$this->computeNbUniqVisitors();
@@ -470,9 +474,7 @@ protected function getAggregatedNumericMetrics($columns, $operationToApply)
         if($rowMetrics === false) {
             $rowMetrics = new Row;
         }
-        if($this->getParams()->isSingleSite() ) {
-            $this->enrichWithUniqueVisitorsMetric($rowMetrics);
-        }
+        $this->enrichWithUniqueVisitorsMetric($rowMetrics);
         $this->renameColumnsAfterAggregation($results);
 
         $metrics = $rowMetrics->getColumns();
diff --git a/core/ArchiveProcessor/Parameters.php b/core/ArchiveProcessor/Parameters.php
index eaec67751ff..70bb06e0ece 100644
--- a/core/ArchiveProcessor/Parameters.php
+++ b/core/ArchiveProcessor/Parameters.php
@@ -14,6 +14,7 @@
 use Piwik\Date;
 use Piwik\Log;
 use Piwik\Period;
+use Piwik\Piwik;
 use Piwik\Segment;
 use Piwik\Site;
 
@@ -97,7 +98,12 @@ public function getSubPeriods()
     public function getIdSites()
     {
         $idSite = $this->getSite()->getId();
-        return array($idSite);
+
+        $idSites = array($idSite);
+
+        Piwik::postEvent('ArchiveProcessor.Parameters.getIdSites', array( &$idSites ) );
+
+        return $idSites;
     }
 
     /**