Junk Dimension
Ch.02: DWH | DWH Components | Data Modeling | Dimension Types
Lesson Notes
Garbage (Junk) Dimensions Lesson Notes
Video
Junk Dimension
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It used to reduce the number of dimensions (low-cardinality columns) in the dimensional model and reduce the number of columns in the fact table. It is a collection of random transnational codes, flags, or text attributes.
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It optimizes space as fact tables should not include low-cardinality or text fields. It mainly includes measures, foreign keys, and degenerate dimension keys.
Junk Dimension Example
Junk Dimension Table Size
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We must split the Junk dimension into more dimensions in case the size grows by the time.
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It is easy to calculate the expected number of rows as it is the total number of combinations between the low-cardinality attributes; 3 columns each have 3 values total = 3 * 3 = 9.
Further Reading
- Chapter 6.3.8 Identifying Garbage (Junk) Dimensions page 282 from Dimensional Modeling: In a Business Intelligence Environment. The book is free, and you can download it from this link.