Abstract
A formal model of denormalized relational schemas based on outer joins of base relations is presented. The composition of joins is modeled with a hyper-tree, called hyper-schema, with vertexes representing equi-joins, and edges representing joined base relations. The base relations are not the relations defined in the conceptual schema, but their “atoms.” The semantics of hyper-relations is defined in terms of join disjunctions. Three different denormalized outer join forms are also defined, in terms of constraints imposed to hyper-schemas that gradually make the maintenance of hyper-relations simpler and more efficient. The presented model can be used as a formal underpinning for 1) a special, but important case of materialized views, 2) an implementation of the physical, yet relational schema of a highly denormalized database, or 3) different optimization techniques based on denormalization.
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