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Study Of Query Rewrite Based On Semantic Information

Posted on:2003-12-04Degree:MasterType:Thesis
Country:ChinaCandidate:L P SongFull Text:PDF
GTID:2168360092966058Subject:Computer system architecture
Abstract/Summary:PDF Full Text Request
It is very important to quickly answer queries in database especially in data warehouse environment. Besides query optimization, another efficient way is query rewrite using materialized views. Materialized views are different from ordinary views. Not only the definitions of materialized views, but also the results are stored in database. Users give their queries against tables without knowing the materialized views, the queries are transparently rewritten using materialized views and the time consumed by queries is reduced by avoiding accessing huge raw records and performing time-consuming join and aggregate operations.Rewrite and the original query generate the same multi-set answer. The paper studies conditions and algorithms of query rewrite using materialized views. The author introduces conditions and algorithms of query rewrite based on semantic information, extending the usability of materialized views. The main contributions are following:â‘  The paper studies query rewrite in database environment based on foreign key information. Detail table may join the master table by foreign key losslessly and access the columns in master table. The paper uses this property to extend the usability of materialized views.1) Introduces the concept of join-lossless tables. If the extra tables in the materialized view but not in the query are join-lossless tables, the materialized view may be used to rewrite the query.2) Introduces the concept of expanded tables. If the columns required by the query are not in the output of a materialized view but in the expanded tables, the materialized view can also be used to rewrite queries. 3) Divides the WHERE conditions of the materialized views and the queries into three categories, introduces the concept of equivalent classes and specifies how to derive the query conditions by the materialized view conditions and how to generate the compensated conditions.4) Supplies the conditions and algorithms of query rewrite based on foreignkey semantic information.â‘¡ The dimensions in data warehouse have many dimension levels. Due to dependencies between dimension levels, there have derivation relationships between column sets (granularities). The paper studies query rewrite in data warehouse environment based on granularity information.1) Due to join-lossless between fact-table and dimension tables, the paper introduces the equivalent form of the aggregate materialized view and the aggregate query, the table mapping and column mapping is direct between them.2) Based on the derivability of dimension levels, the paper introduced the concept of derived dimension tables. The paper extends the usability of materialized views by using derived dimension tables.3) Supplies the conditions and algorithms of query rewrite based on granularity semantic information.â‘¢ The paper provides the data structures and procedures of query rewrite.
Keywords/Search Tags:query rewrite, materialized views, semantic, foreign key, granularity
PDF Full Text Request
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