What is cardinality in MySQL?

Question

What is cardinality in MySQL? Please describe in simple, non-technical language.

If an index detail of any of the table shows the cardinality of a field say group_id as 11, then what does it actually mean?

Answer 1

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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