A query optimizer is developed for a particular query processing environment. A model in Cascades describes the features specific to the particular DBMS corresponding to this query processing environment.
The structure of a Cascades model is influenced by the concept of data independence, discussed in section 2. Thus, Cascades models can be broken into two pieces, the logical model and the physical model. The logical model relates to the semantics of the database system -- what is to be computed. The physical model relates to such things as data structures, implementation algorithms and storage devices -- how it is to be computed. Together the logical and physical models form a particular Cascades model which represents both the logical and physical aspects of a particular database system.
1. Volcano, the predecessor of Cascades, and Exodus, the predecessor of Volcano, are discussed in Section 5.
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