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 91.4904 Ron McFadyen

Normalized ER Models and the DM Page 146-147 A single entity relationship diagram breaks down into multiple fact table diagrams. Jan 2003 91.4904 Ron McFadyen

Advantages of the dimensional approach Page 147-150 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 91.4904 Ron McFadyen

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 91.4904 Ron McFadyen

A data mart is a dimensional model Warehouse is built iteratively Bus Architecture P 153-164 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 91.4904 Ron McFadyen

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 91.4904 Ron McFadyen

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 91.4904 Ron McFadyen

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 Promotion Store Warehouse Date Product Retail Sales X X X X Retail Inv X X X Retail Del X X X Warehouse Inv X X X Jan 2003 91.4904 Ron McFadyen