Is index like key(timetime) usable when we query: SELECT * ....... ORDER BY timetime DESC
Yes, it does. MySQL can use the index and use it for scanning/seeking in either direction. You can read in MySQL documentation: How MySQL Uses Indexes


  To sort or group a table if the sorting or grouping is done on a leftmost prefix of a usable key (for example, ORDER BY key_part1, key_part2). If all key parts are followed by DESC, the key is read in reverse order. See Section 8.13.9, “ORDER BY Optimization”, and Section 8.13.10, “GROUP BY Optimization”.