Max cardinality: All the values are unique
Min cardinality: All the values are the same
Some of the columns are called the high-cardinality columns as they have constraints in place (like unique) preventing you from inserting the same value in each row.
Cardinality is a property which affects the ability to cluster, sort and search the data. It is, hence, an essential measurement for the query planners in databases, it is a heuristic which they can use to pick the best plans.
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