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by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Data Warehouse DesignDealing With Recursive Data Relationships

Recursive many-to-many relationships contain a database object that has a many-to-many relationship with other occurrences of the same object. These relationships are often termed Bill-of-Materials (BOM) relationships, and the graphical representation of the recursive relationship is sometimes termed a Bill-of-Materials explosion. These relationships are termed recursive because a single query makes many sub-passes through the tables to arrive at the solution (see Figure 4.8).

Figure 4.8 An example of recursive many-to-many relationships.

Bill-of-materials relationships are very common in data warehouses, and they present a difficult problem with system performance. For example, a part may consist of other parts, but at the same time, it is a component in a larger assembly. A class at a university may have many prerequisites, but at the same time, the class is a prerequisite for another class. In the legal arena, a court case may cite other cases, but at the same time, it is being cited by later cases.

To illustrate, a BOM request for components of a Big Meal shows that it consists of a hamburger, fries, and soda. Yet a hamburger consists of a meat patty, bun, and pickles--and a meat patty consists of meat and filler, and so on. Another example of a BOM relationship would be the division of a carburetor into subparts, although the carburetor itself is a subpart in a larger unit (the engine, which is a subpart of the car).

Figure 4.9 describes the course-prerequisite hierarchy for a university. Note that the IS-A prerequisite relationships are relatively straightforward, indicating which courses are required to be completed before taking other courses. For example, the prerequisites for Linear Equations 445 are Business 400, Accounting 305, and Multivariate Statistics 450. These courses all have prerequisites of their own, which may also have prerequisites, and so on.

Figure 4.9 A recursive data relationship for course prerequisites.

Each occurrence of a course object has different topics, and a complete implementation must iterate through all courses until reaching terminus, the point where the course has no further prerequisites.

This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:


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