On mixing queries and transactions via multiversion locking

The authors discuss a novel approach to multiversion concurrency control that allows high-performance transaction systems to support long-running queries. The approach extends the multiversion locking algorithm developed by Computer Corporation of America by using record-level versioning and reserving a portion of each data page for caching prior versions that are potentially needed for the serializable execution of queries; on-page caching also enables an efficient approach to garbage collection of old versions. In addition, view sharing is introduced, which has the potential for reducing the cost of versioning by grouping together queries to run against the same transaction-consistent view of the database. Results from a simulation study that indicate that the approach is a viable alternative to level-one and level-two consistency locking when the portion of each data reserved for prior versions is chosen appropriately are presented