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Abstract : |
Abstract 2. ODB-QOPTIMIZER: the tool ODB-QOPTIMIZER is a ODMG 93 compliant tool for the schema validation and semantic query optimization. The approach is based on two fundamental ingredients. The first one is the OCDL description logics (DLs) proposed as a common formalism to express class descriptions, a relevant set of integrity constraints rules (IC rules) and queries. The second one are DLs inference techniques, exploited to evaluate the logical implications expressed by IC rules and thus to produce the semantic expansion of a given query. 1. ODB-QOPTIMIZER: the approach OCDL: a DLs for database schema with IC rules OCDL (Object Constraints Description Language) [2] is an extension of the DLs proposed in [ll. It extends the expressiveness of traditional DLs languages in order to represent the semantics of complex object data models [3]. Its main characteristics are: a distinction between values and objects with identity and, thus, between value types and class types; type constructors, such as tuple, set and sequence recursively used to define complex objects; declarative formulation of IC rules: if then rules whose antecedent and consequent can be expressed as OCDL types. Query Optimization by DLs inference techniques The queries referred to a single target class, including the navigation through its composition hierarchy, can be expressed as OCDL types. DLs inference techniques, such as subsumption computation, incoherence detection and canonical form generation [l], can be used to produce the semantic expansion of an OCDL query, which incorporates any possible restriction not present in the original query but logically implied by the query and by the overall schema (classes + value types + IC rules). Following the approach of [5] for semantic query optimization, but exploiting subsumption computation to evaluate logical implication, we perform the semantic expansion of the types included at each nesting level in the query description., |
