tailieunhanh - Lore: A Database Management System for Semistructured Data

A materialized view (MV) is a more complex physical design structure than an index since a materialized view may be defined over multiple tables, and can involve selection, projection, join and group by. This richness of structure of MVs makes the problem of selecting materialized views significantly more complex than that of index selection. First, for a given query (and hence workload) the space of materialized views that must be considered is much larger than the space of indexes. For example, MVs on any subset of tables referenced in the query may be relevant. For each such subset many MVs. | Lore A Database Management System for Semistructured Data Jason McHugh Serge Abiteboul Roy Goldman Dalian Quass Jennifer Widom Stanford University mchughj abitebou royg quass widom @ http lore Abstract Lore for Lightweight Object Repository is a DBMS designed specifically for managing semistructured information. Implementing Lore has required rethinking all aspects of a DBMS including storage management indexing query processing and optimization and user interfaces. This paper provides an overview of these aspects of the Lore system as well as other novel features such as dynamic structural summaries and seamless access to data from external sources. 1 Introduction Traditional database systems force all data to adhere to an explicitly specified rigid schema. For many new database applications there can be two significant drawbacks to this approach The data may be irregular and thus not conform to a rigid schema. In relational systems null values typically are used when data is irregular a well-known headache. While complex types and inheritance in object-oriented databases clearly enable more flexibility it can still be difficult to design an appropriate object-oriented schema to accommodate irregular data. It may be difficult to decide in advance on a single correct schema. The structure of the data may evolve rapidly data elements may change types or data not conforming to the previous structure may be added. These characteristics result in frequent schema modifications another well-known headache in traditional database systems. Because of these limitations many applications involving semistructured data Abi97 are forgoing the use of a database management system despite the fact that many strengths of a DBMS ad-hoc queries efficient access concurrency control crash recovery security etc. would be very useful to those applications. As a popular first example consider data stored on the World-Wide Web. At a typical Web site data