Report 79-30.pdf

An approach to query optimization is described that draws on two sources of knowledge: real world constraints on the values for the application domain served by the database; and knowledge about the current structure of the database and the cost of available retrieval processes. Real world knowledge is embodied in rules that are much like semantic integrity rules. The approach, called "query rephrasing", is to generate semantic equivalents of user queries that cost less to process than the original queries. The operation of a prototype system based on this approach is discussed in the context 0. simple queries which restrict a single file. The need for heuristics to limit the generation of equivalent queries is also discussed, and a method ut g "constraint thresholds" derived from