_exportIterateCollection performance optimization (OpenMage#1467)

* _exportIterateCollection performance optimization

* Update _exportIterateCollection

Author: midlan

app/code/core/Mage/Adminhtml/Block/Widget/Grid.php

@@ -963,36 +963,29 @@ protected function _getExportTotals()
      *
      * @param string $callback
      * @param array $args additional arguments for callback method
-     * @return $this
      */
     public function _exportIterateCollection($callback, array $args)
     {
         $originalCollection = $this->getCollection();
         $count = null;
         $page  = 1;
-        $lPage = null;
-        $break = false;
 
-        while ($break !== true) {
+        do {
             $collection = clone $originalCollection;
             $collection->setPageSize($this->_exportPageSize);
             $collection->setCurPage($page);
             $collection->load();
-            if (is_null($count)) {
-                $count = $collection->getSize();
-                $lPage = $collection->getLastPageNumber();
-            }
-            if ($lPage == $page) {
-                $break = true;
-            }
-            $page ++;
+
+            $count = $collection->count();
+
+            $page++;
 
             foreach ($collection as $item) {
                 call_user_func_array(array($this, $callback), array_merge(array($item), $args));
             }
             $collection->clear();
             unset($collection);
-        }
+        } while($count == $this->_exportPageSize);
     }
 
     /**