Storage of multiple pre-modification short duration copies of database information in short term memory

摘要

A method and apparatus for storing snapshots (pre-modification copies of data) in short term memory is disclosed. In an embodiment, if short term storage is clogged, snapshot chains are copied to longer term memory. In an embodiment, snapshot chains that are older than any active transaction are discarded, and the storage space which the discarded snapshot chains occupy is reclaimed. In an embodiment, the snapshots are stored in chains in which the snapshots are ordered chronologically, and each snapshot is explicitly linked to the next oldest snapshot (if one exists) in the snapshot chain.