How Does Cardinality Impact Query Optimization in MySQL?

Understanding Cardinality in MySQL

Cardinality, a crucial aspect of MySQL database optimization, pertains to the uniqueness and distribution of values within a column. It ranges from a minimum cardinality, where all values are identical, to a maximum cardinality, where all values are distinct.

Cardinality in an Index

When examining an index's details, the cardinality of a field like "group_id" reflects the uniqueness of its values. A cardinality of 11 means that:

• Max Cardinality: Close to all values are unique.
• Min Cardinality: All values are identical.

Significance of Cardinality

Cardinality influences the efficiency of queries, clustering, sorting, and searching operations. High-cardinality columns, with constraints like uniqueness, prevent duplicate values in each row. Low-cardinality columns, on the other hand, indicate that values tend to repeat.

For query planners in MySQL, cardinality serves as a heuristic guide in choosing the optimal execution plans for queries, considering the degree of uniqueness and the distribution of values in the specified columns.

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