Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Data Warehouse Design
Massive De-normalization: STAR Schema Design
Remember, the rules of database design have changed. Ten years ago, normalization theory emphasized the need to control redundancy and touted the benefits of a structure that was free of redundant data. Today, with disk prices at an all-time low, the attitude toward redundancy has changed radically. The relational vendors are offering a plethora of tools to allow snapshots and other methods for replicating data. Other vendors, such as UniSQL, are offering database products that allow for non-first normal form implementations. Today, it is perfectly acceptable to create first normal form implementations of normalized databases, which means pre-joining tables to avoid the high performance costs of runtime SQL joins.

The basic principle behind the STAR query schema is to introduce highly redundant data for performance reasons. Let's evolve the 3NF database into a STAR schema by creating a fact table to hold the quantity for each item sold. Essentially, a fact table is a first normal form representation of the database, with a very high degree of redundancy being added into the tables. This de-normalized design (see Figure 4.13) greatly improves the simplicity of the design, but at the expense of redundant data.

Figure 4.13 The completed STAR schema.

At first glance, it is hard to believe that this representation contains the same data as the fully normalized database. The new fact table will contain one row for each item on each order, resulting in a tremendous amount of redundant key information. Of course, the STAR query schema is going to require far more disk space than the 3NF database. The STAR schema would most likely be a read-only database due to the widespread redundancy introduced into the model. Also, the widespread redundancy would make updating difficult, if not downright impossible.

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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:
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm