private function generateIndexSQL($info) {
        $keys = $info['columns'];
        $sql = '';
        if (!isset($info['type'])) $info['type'] = ($info['name'] == 'primary' ? 'primary' : 'simple');
        switch ($info['type']) {
            case 'primary':
                $sql .= 'PRIMARY KEY (`'.(is_array($keys) ? implode('`, `', $keys) : $keys).'`)';
                break;
            case 'unique':
            case 'simple':
                $sql .= ($info['type'] == 'unique' ? 'UNIQUE ' : '').'KEY '.'`'.$info['name'].'` (`'.(is_array($keys) ? implode('`, `', $keys) : $keys).'`)';
                break;
        }
        return $sql;
    }