private function _buildStructure()
    {
        $this->_query('DROP INDEX tag_id_index');
        $this->_query('DROP INDEX tag_name_index');
        $this->_query('DROP INDEX cache_id_expire_index');
        $this->_query('DROP TABLE version');
        $this->_query('DROP TABLE cache');
        $this->_query('DROP TABLE tag');
        $this->_query('CREATE TABLE version (num INTEGER PRIMARY KEY)');
        $this->_query('CREATE TABLE cache (id TEXT PRIMARY KEY, content BLOB, lastModified INTEGER, expire INTEGER)');
        $this->_query('CREATE TABLE tag (name TEXT, id TEXT)');
        $this->_query('CREATE INDEX tag_id_index ON tag(id)');
        $this->_query('CREATE INDEX tag_name_index ON tag(name)');
        $this->_query('CREATE INDEX cache_id_expire_index ON cache(id, expire)');
        $this->_query('INSERT INTO version (num) VALUES (1)');
    }