Each element of a database may have multiple versions; the versions are partitioned into branches, and versions of a branch are ordered linearly according to their timestamps. Branches are timestamped and related to one another by a version graph. Each version of an element of a database is represented...http://www.google.com.hk/patents/US5893117?utm_source=gb-gplus-share專利 US5893117 - Time-stamped database transaction and version management system
