Reads, and Phantoms. This paper shows that these phenomena and the ANSI SQL definitions fail to properly characterize several popular isolation levels, including the standard Ioeking implementations of the levels covered. Ambiguity in the statement of the phenomena is investigated and a more formal statement is arrived at; in addition new phenomena that better characterize isolation types are introduced. Finally, an important multiversion isolation type, called Snapshot Isolation, is defined. 1.

Multiversion two-phase locking (MV2PL) has been incorporated in some commercial transaction processing systems to support the serializable execution of queries. A drawback to this algorithm is the potentially high cost that it adds to maintain and access prior versions of data In this paper, we present a new multiversion locking algorithm, multiversion query locking (MVQL), that reduces the cost of versioning by accepting weaker forms of consistency for queries than MV2PL. Nevertheless, queries are guaranteed to see transactionconsistent data. We present results from a detailed performance study that show that, under a wide range of conditions, MVQL provides higher throughput to queries and update transactions with a lower storage cost than MV2PL. In the worst case, the performance of MVQL approaches that of MV2PL. 1.

A caching scheme that uses query predicates to cache data on the client-side in a client-server relational database system was presented in [15]. The client-side cache (henceforth referred to as a SQL*Cache), loads query results dynamically in the course of transaction execution, and formulates a cache description based on the query predicates. SQL*Cache is associative in nature, in that it supports content-based reasoning and local execution of SQL queries on the cached data, thereby attempting to reduce query response times and increase server throughput. Local caching involves cache consistency maintenance and complicates transaction concurrency control --- in this paper, we examine serializability questions that arise in such a system. We first analyze the issues in supporting different (0/1/2/3) degrees of isolation that client transactions may specify, and describe algorithms to achieve these isolation levels. Then, we propose new levels of isolation that take into account the di...

. Multiversion two-phase locking (MV2PL) provides on-line serializable queries without introducing the long blocking delays that can occur with conventional two-phase locking (2PL). MV2PL requires indexing structures, however, that are capable of supporting multiple versions of data. In this paper, we present several options for extending single-version indexing schemes for use with MV2PL. These basic approaches are largely orthogonal to the underlying indexing structure (e.g., hashing or B+ trees). The options considered differ in where they place version selection information (i.e., references to individual versions); this information is placed either with the data or with the index entries of one or more of the indices. We also present the results from a performance study that show that placing the version selection information with the data is usually the best option, since it keeps the indices smaller and thus enables a larger fraction of the index pages to remain cached in the bu...

This paper is concerned with the problem of integrating a number of existing, off-the-shelf local database systems into a multidatabase system that maintains consistency in the face of concurrency and failures. The major difficulties in designing such systems stem from the requirements that local transactions be allowed to execute outside the multidatabase system control, and that the various local database systems cannot participate in the execution of a global commit protocol. A scheme based on the assumption that the component local database systems use the strict two-phase locking protocol is developed. Two major problems are addressed. How to ensure global transaction atomicity without the provision of a commit protocol, and how to ensure freedom from global deadlocks. 1 This paper is based in part on [8]. 2 This material is based in part upon work supported by the Center for Manufacturing and Robotics of the University of Kentucky and by NSF Grant IRI-8904932. 3 This materia...