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Oracle Tips
by Burleson Consulting
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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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