An Eecient Transient Versioning Method

High-performance database systems have to support ad-hoc queries while providing high transaction throughput. Transient versioning methods, where prior versions are maintained temporarily, are developed to execute queries without aaecting concurrent transactions. However , transactions in these methods do not exploit the prior versions. This paper presents a transient-versioning method, called Dynamic Versioning, which uses the transient versions to not only support queries but also increase concurrency among transactions. Data connicts due to incompatible requests are resolved by dynamically creating dependence orders among the requesting transactions and queries. These orders help precisely identify the prior versions that have to be kept to ensure serializability for the queries. Since only the required versions are stored, the storage overhead for the transient versions is reduced to the minimum possible. We also propose techniques to eeciently support queries, based on their lengths and access patterns. We present the results of simulation experiments that show that our method provides high transaction concurrency and supports queries almost as eeciently as the other transient-versioning methods, while incurring only a fraction of the storage overhead.