Solving queries by tree projections

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Abstract
Suppose a database schema D is extended to D¯ by adding new relation schemas, and states for D are extended to states for D¯ by applying joins and projections to existing relations. It is shown that certain desirable properties that D¯ has with respect to D . These properties amount to the ability to compute efficiently the join of all relations in a state for D from an extension of this state over D¯ . The equivalence is proved for unrestricted (i.e., both finite and infinite) databases. If D¯ is obtained from D by adding a set of new relation schemas that form a tree schema, then the equivalence also holds for finite databases. In this case there is also a polynomial time algorithm for testing the existence of a tree projection of D¯ with respect to D .

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