private function _loadDictionaryIndex()
    {
        // Check, if index is already serialized
        if ($this->_directory->fileExists($this->_name . '.sti')) {
            // Load serialized dictionary index data
            $stiFile = $this->_directory->getFileObject($this->_name . '.sti');
            $stiFileData = $stiFile->readBytes($this->_directory->fileLength($this->_name . '.sti'));

            // Load dictionary index data
            if (($unserializedData = @unserialize($stiFileData)) !== false) {
                list($this->_termDictionary, $this->_termDictionaryInfos) = $unserializedData;
                return;
            }
        }

        // Load data from .tii file and generate .sti file

        // Prefetch dictionary index data
        $tiiFile = $this->openCompoundFile('.tii');
        $tiiFileData = $tiiFile->readBytes($this->compoundFileLength('.tii'));

        /** Zend_Search_Lucene_Index_DictionaryLoader */
        require_once 'Zend/Search/Lucene/Index/DictionaryLoader.php';

        // Load dictionary index data
        list($this->_termDictionary, $this->_termDictionaryInfos) =
                    Zend_Search_Lucene_Index_DictionaryLoader::load($tiiFileData);

        $stiFileData = serialize(array($this->_termDictionary, $this->_termDictionaryInfos));
        $stiFile = $this->_directory->createFile($this->_name . '.sti');
        $stiFile->writeBytes($stiFileData);
    }