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Data & Analytics · Intermediate

Column Cardinality

The number of distinct values in a column; high cardinality means many unique values.

Definition

Cardinality affects index selection, compression and model size. Low-cardinality columns such as Gender suit bitmap indexes and compress well in columnstores. High-cardinality columns such as transaction IDs or timestamps inflate Power BI model size and are poor clustering keys when nearly unique.

Example

Splitting a DateTime column into Date and Time columns reduces cardinality in a Power BI model.

Exam tip

To shrink a Power BI or columnstore model, remove or reduce high-cardinality columns.

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