Junk Dimension

Cardinality: Number of Unique Values.

A junk dimension is a dimension table created by grouping low cardinality and unrelated attributes.

The idea behind a junk dimension is to reduce the number of dimension tables in a data warehouse and simplify queries.

An example of a junk dimension could be a table that includes binary flags such as "is_promotion", "is_return", and "is_discount".

Possible Values for is_promotion as Y or N same for is_return and is_discount.

Example:

fact_sale table

Date
Product 
Store
OrderNumber

PaymentMode
StoreType
CustomerSupport

Quantity
UnitPrice

Possible values for these columns

PaymentMode - Cash/Credit/Check

StoreType - Warehouse/Marketplace

CustomerSupport - Yes/No

So how to handle the situation

Option 1: Add it to Fact Table 

The problem is, Fact table data is not that important and sometimes won't make sense
Option 2: Add it as Dimension Table

The problem is with more Dimension table adds more joins to your queries.

Do you know how to take care of this situation?

Let's create a new table with values from all possible combinations.

So, the revised Fact Table looks like this

  • Basically to group low cardinality columns.

  • If the values are too uncorrelated.

  • Helps to keep the DW simple by minimizing the dimensions.

  • Helps to improve the performance of SQL queries.

Note: If the resultant dimension has way too many rows, then don’t create a Junk dimension.

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