We consider the problem of computing answers to queries by using materialized views. Aside from its potential in optimizing query evaluation, the problem also arises in applications such as Global Information Systems, Mobile Computing and maintaining physical data independence. We consider the problem of finding a rewriting of a query that uses the materialized views, the problem of finding minimal rewritings, and finding complete rewritings (i.e., rewritings that use only the views). We show that all the possible rewritings can be obtained by considering containment mappings from the views to the query, and that the problems we consider are NP-complete when both the query and the views are conjunctive and don't involve built-in comparison predicates. We show that the problem has two independent sources of complexity (the number of possible containment mappings, and the complexity of deciding which literals from the original query can be deleted). We describe a polynomial time algorith...

While much work has addressed the problem of maintaining materialized views, the important question of optimizing queries in the presence of materialized views has not been resolved. In this paper, we analyze the optimization question and provide a comprehensive and efficient solution. Our solution has the desirable property that it is a simple generalization of the traditional query optimization algorithm. 1 Introduction The idea of using materialized views for the benefit of improved query processing has been proposed in the literature more than a decade ago. In this context, problems such as definition of views, composition of views, maintenance of views [BC79, KP81, SI84, BLT86, CW91, Rou91, GMS93] have been researched but one topic has been conspicuous by its absence. This concerns the problem of the judicious use of materialized views in answering a query. It may seem that materialized views should be used to evaluate a query whenever they are applicable. In fact, blind applicat...

Relational database systems provide a powerful abstraction mechanism: any query, since it returns a relation, can be used to define a view, that becomes a derived (or virtual) relation. Views are defined by a statement such as "define view <name> as <query>." Views can be used to tailor the global database schema. . Query formulation is simplified, if frequent subexpressions are predefined. . Application programs are insulated from changes to the underlying schema. . Information can be restructured to better suit an application programs' requirements. . Derived information is kept consistent with base data. . Access restrictions (for authorization) can be enforced by hiding data. In contrast to base relations, views are typically not stored permanently, but rather computed on demand. Queries to view relations are modified by query substitution so as to operate on the underlying b

, Exception, Clock, Externalg Granularity ae fMember, Subset, Setg Type ae fPrimitive, Composite g Operators ae for, and, seq, closure, times, not g Consumption mode ae fRecent, Chronicle, Cumulative, Continuous g Role 2 fMandatory, Optional, Noneg Condition Role 2 fMandatory, Optional, Noneg Context ae fDB T , BindE , DBE , DBC g Action Options ae fStructure Operation, Behavior Invocation, Update-Rules, Abort Inform, External, Do Instead g Context ae fDB T , BindE , BindC , DBE , DBC , DBA g ---behavior invocation, in which case the event is raised by the execution of some user-defined operation (e.g. the message display is sent to an object of type widget). It is common for event languages to allow events to be raised before or after an operation has been executed. ---transaction, in which case the event is raised by transaction commands (e.g. abort, commit, begin-transaction) ---abstract or user-defined, in which case a programming mechanism is used that allows an appli...

We consider a variant of the view maintenance problem: How does one keep a materialized view up-to-date when the view definition itself changes? Can one do better than recomputing the view from the base relations? Traditional view maintenance tries to maintain the materialized view in response to modifications to the base relations; we try to “adapt ” the view in response to changes in the view definition. Such techniques are needed for applications where the user can change queries dynamically and see the changes in the results fast. Data archaeology, data visualization, and dynamic queries are examples of such applications. We consider all possible redefinitions of SQL SELECT-FROM-UHERE-GROUPBY, UNION, and EXCEPT views, and show how these views can be adapted using the old materialization for the cases where it is possible to do so. We identify extra information that can be kept with a materialization to facilitate redefinition. Multiple simultaneous changes to a view can be handled without necessarily materializing intermediate results. We iden-tify guidelines for users and database administrators that can be used to facilitate efficient view adaptation. 1

We present novel algorithms for the problem of using materialized views to compute answers to SQL queries with grouping and aggregation, in the presence of multiset tables. In addition to its obvious potential in query optimization, this problem is important in many applications, such as data warehousing, very large transaction recording systems, global information systems and mobile computing, where access to local or cached materialized views may be cheaper than access to the underlying database. Our contributions are the following: First, we show that in the case where the query has grouping and aggregation but the views do not, a view is usable in answering a query only if there is an isomorphism between the view and a portion of the query. Second, when the views also have grouping and aggregation we identify conditions under which the aggregation information present in a view is sufficient to perform the aggregation computations required in the query. The algorithms we describe fo...

The declarativeness of relational query languages is very attractive for developing applications. However, many applications also need to invoke external functions or to access data that is not stored in the database. It is not hard to express references to such foreign functions in the query language. However, the issue of cost-based optimization of relational queries in the presence of such foreign functions has not previously been addressed satisfactorily. In this paper, we describe a comprehensive approach to this problem. Our key observation is that the optimization must take into account semantic information about foreign functions. Therefore, we provide a simple declarative rule language to express such semantics. We present algorithms necessary for applying the rules and for generating the space of equivalent queries. The equivalent queries provide the optimizer with an enriched execution space. We show how we can modify the traditional join reordering algorithm based on dynami...

Information Superhighway environments such as the Internet have brought us ready access to large amount of information. However, Internet data is notoriously unorganized and autonomously managed in a distributed fashion. Large scale information monitoring in the Internet environment requires support beyond traditional database techniques. Two of the key issues are the increasing reward in monitoring a fast growing information base and the similarly increasing processing cost. To improve the expressiveness of queries for information monitoring, we define continual queries as a useful tool for monitoring of updated information. Continual queries are standing queries that monitor the source data and notify the users whenever new data matches the query. In addition to periodic refresh, continual queries include Epsilon Transaction concepts to allow users to specify query refresh based on the magnitude of updates. To support efficient processing of continual queries, we propose a different...

Derived data is maintained in a database system to correlate and summarize base data which record real world facts. As base data changes, derived data needs to be recomputed. A high performance system should execute all these updates and recomputations in a timely fashion so that the data remains fresh and useful, while at the same time executing user transactions quickly. This paper studies the intricate balance between recomputing derived data and transaction execution. Our focus is on efficient recomputation strategies --- how and when recomputations should be done to reduce their cost without jeopardizing data timeliness. We propose the Forced Delay recomputation algorithm and show how it can exploit update locality to improve both data freshness and transaction response time. Keywords: derived data, view maintenance, active database system, transaction scheduling, update locality. 1 Introduction Active rule-based systems are often employed in dynamic environments to monitor the...

1 Relational integrity checking technology can be transfered to deductive object bases by utilizing a simple logical framework for objects. The principles of object identity, aggregation and classification allow a more efficient constraint control by finer granularity of updates, composite updates and semantic constraint simplification. In many cases, meta-level constraints and deductive rules can be handled efficiently by a stepwise compilation approach. An extended integrity subsystem with these features has been implemented in the deductive object base ConceptBase. 1 This work was supported in part by the Commission of the European Community under ESPRIT Basic Research Action 3012 (CompuLog). A version of this paper will also appear in the Proc. Second Int. Conf. on Deductive and Object-Oriented Databases, Munich, Dec. 1991 1. Introduction Comprehensive and efficient integrity maintenance has been quoted as one of the major problems in next-generation databases. Systems l...