DDB分布式数据库系统_第三讲__Ddb-L_design.ppt

Total Cost the query processing cost of application qi the storage cost of fragment Fj at sit k unit storage cost at sit k processing cost transmission cost access cost integrity enforcement cost for concurrency control update component retrieval component Cost Computation Access cost Transmission cost -- update Transmission cost -- retrieval Constraints Execution time for each query = maximum response time Storage constraint: The total size of all fragments allocated at a site must be less than the storage capacity at that site. Processing constraint: The total processing load because of all queries at a site must be less than the processing capacity at that site. Solution Methods NP-complete Heuristics approaches Reduce search space DHF: Correctness Completeness: primary horizontal fragmentation based on completeness of selection predicates. For derived horizontal fragmentation based on referential integrity Reconstruction: union Disjointedness: primary horizontal fragmentation based on mutually exclusive simple predicates. For derived horizontal fragmentation, based on whether the link between owner and member is 1:1 or 1:m relationship DHF: Issues Multiple owners for a member relation; how should we derived horizontally fragment a member relation. There can be a chain of derived horizontal fragmentation. L3.2.2: DDBS Design Introduction Fragmentation Horizontal fragmentation Vertical fragmentation Allocation Vertical Fragmentation (VF) Vertical partitioning of a relation R produces fragments R1, R2, ..., Rm, each of which contains a subset of Rs attributes as well as the primary key of R The object of vertical fragmentation is to reduce irrelevant attribute access, and thus irrelevant data access ``Optimal vertical fragmentation is one that minimizes the irrelevant data access for user applications For a relation with m non-primary key attributes, the number of possible fragments is approximately equal to mm VF - Approaches Grouping: eac