Is index like key(timetime) usable when we query: SELECT * ....... ORDER BY timetime DESC When we index a timestamp field like timetime. Can we use it for ORDER BY timetime DESC or is it just usable for ORDER BY timetime ASC OR ORDER BY timetime? Does mySQL use this index when doing vise verse ordering?
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”.