Definition: Cardinality means the number of distinct values in a column or dataset.
High-cardinality columns have many unique values; low-cardinality columns have relatively few unique values.
Examples:
| Column | Example values | Cardinality |
|---|---|---|
gender | M, F | Low |
country | ~200 countries | Low |
user_id | Millions of users | High |
In ClickHouse, understanding cardinality is important because it affects storage, indexing, grouping, and query performance.