This paper defines XP models for classes of XPath queries stored in cache, as materialized views. After declaration of issue of the correspondence between the tree models used for classes of XPath queries stored in cache and the set of trees associated to a XML document, it follows the solutions for rewriting the XPath views by transformation of patterns of trees. Author's personal contribution consists in modelling the set of trees associated to XP queries, for a multitude of constraints of XPath expressions and description of functions of correspondence in XP{ /, //, *, [] } representation. Verification of possibility to return the result of a query using the views materialized in cache requires the analysis of compatibility of tree models associated to XPath queries and XPath views materialized in cache. Finding a morphism of XP models demonstrates the real possibility of rewriting the XP view. This paper describes a method for establishing a semantic cache of XPath views. Composition of queries using a semantic cache of XP views, assumes the existence of a query which, by composition with a view from cache, will return the result of query.
Abstract:A method to optimize the access at the objects of a relational database is through the optimization of the queries. This article presents an approach of the cost model used in optimization of Select-Project-Join (SPJ) queries with conjunction of predicates and proposes a join optimization algorithm named System RO-H (System Rank Ordering Heuristic). The System RO-H algorithm for optimizing SPJ queries with conjunction of predicates is a System R Dynamic Programming algorithm that extends optimal linear join subplans using a rank-ordering heuristic method as follows: choosing a predicate in ascending order according to the h-metric, where the h-metric depends on the selectivity and the cost per tuple of the predicate, using an expression with heuristic constants. The System Rank-Ordering Heuristic algorithm finds an optimal plan in the space of linear left deep join trees. The System RO-H algorithm saves not a single plan, but multiple optimal plans for every subset, one for each distinct such order, termed interesting order. In order to build an optimal execution plan for a set S of i relations, the optimal plan for each subset of S, consisting of i-1 relations is extended, using the Lemma based on a h-metric for predicates. Optimal plans for subsets are stored and reused. The optimization algorithm chooses a plan of least cost from the execution space.
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