a two copy versioning scheme is proposed . the scheme provides efficient implementation for a high read load database and eliminates read dependencies . the versioning scheme provides efficient implementation for transactions executing under ansi read committed isolation , and is extensible to serializable isolation . the two - copy versioning scheme is garbage free meaning that all versions of data records are used during execution and is adapted to index scanning operations . a “ deferred delete ” scheme allows read transactions to complete data deallocation operations . statements that require “ snapshots ” are efficiently supported .