See the details at https://github.com/mybatis/mybatis-3/issues?milestone=6&page=1&state=closed

Provides some new features:

• Caches nested selects
• Lazy loading can be enabled only for specific relations
• Supports returning multiple fields in a select key
• Some other minor improvements

A note for users who implemented their own caches or 3rd party cache adapters. MyBatis no longer calls Cache#getReadWriteLock(). Any locking needed by the cache should be implemented internally by the cache. So in case you coded your own cache make sure it is properly synchronized. See issue #159.