1. Dimensional Model January 14, 2003
Ch 5
Dimensional Model
Composed of one fact table that participates in many one-to-many relationships with dimension tables.
Primary key of the fact table is the catenation of the primary keys of the dimension tables.
Alternatively, we can consider a dimensional model to comprise a many-to-many-to-…-to-many relationship amongst dimension tables. (The fact table is the implementation of the relationship.)
Jan 2003
Ron McFadyen

2. Normalized ER Models and the DM
Page
A single entity relationship diagram breaks down into multiple fact table diagrams.
Jan 2003
Ron McFadyen

3. Advantages of the dimensional approach
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Advantages of the dimensional approach:
DM yields predictable standard models. Report writers and query tools can make strong assumptions regarding the user interface.
SQL of typical queries are very symmetrical. Star joins can be recognized as query patterns that a database engine can optimize.
Jan 2003
Ron McFadyen

4. Advantages of the dimensional approach
3. Gracefully extensible
Add new facts to an existing fact table
Add new dimensions
Add new dimensional attributes
Standard approaches exist for DM situations
Utilities for managing aggregates
Jan 2003
Ron McFadyen

5. A data mart is a dimensional model Warehouse is built iteratively
Bus Architecture P
Bus Architecture
Assuming:
A data mart is a dimensional model
Warehouse is built iteratively
Warehouse is the collection of data marts
When the same dimension is used across all data marts, then that dimension is a conformed dimensions.
In the bus architecture for data warehousing, all dimensions are conformed dimensions.
Jan 2003
Ron McFadyen

6. The use of conformed dimensions means we avoid “stovepipe” data marts,
Bus Architecture
The use of conformed dimensions means
we avoid “stovepipe” data marts,
we can meaningfully join data across data marts
we can plug new stars into the bus
Conformed dimensions should use surrogate keys.
Protection from reused keys in legacy system
Slowly changing dimensions
Jan 2003
Ron McFadyen

7. we can meaningfully join data across data marts
Bus Architecture
Conformed facts
Want similar facts to be at the same grain, to be defined on the same units.
we can meaningfully join data across data marts
Jan 2003
Ron McFadyen

8. Data Warehouse Bus Matrix Rows are data marts Columns are dimensions
Bus Architecture
Data Warehouse Bus Matrix
Rows are data marts
Columns are dimensions
A useful planning tool
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Warehouse
Date
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Retail Inv X X X
Retail Del X X X
Warehouse Inv X X X
Jan 2003
Ron McFadyen