public function optimizeIndex()
 {
     // optimize lucene index for better performance
     $this->index->optimize();
     //clean up
     if (is_object($this->index) and $this->index instanceof \Zend_Search_Lucene_Proxy) {
         $this->index->removeReference();
         unset($this->index);
         \Pimcore\Logger::debug('LuceneSearch: Closed frontend index references');
     }
     \Pimcore\Logger::debug('LuceneSearch: optimizeIndex.');
 }