Y.: Transaction time support inside a database engine

Transaction time databases retain and provide access to prior states of a database. An update “inserts ” a new record while preserving the old version. Immortal DB builds transaction time database support into a database engine, not in middleware. It supports as of queries re-turning records current at the specified time. It also sup-ports snapshot isolation concurrency control. Versions are stamped with the “clock times ” of their updating transac-tions. The timestamp order agrees with transaction serial-ization order. Lazy timestamping propagates timestamps to transaction updates after commit. Versions are kept in an integrated storage structure, with historical versions ini-tially stored with current data. Time-splits of pages permit large histories to be maintained, and enable time based in-dexing, which is essential for high performance historical queries. Experiments show that Immortal DB introduces little overhead for accessing recent database states while providing access to past states