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.
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
We must split the Junk dimension into more dimensions in case the size grows by the time.
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.
- 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.