Max cardinality: All values are unique
Min cardinality: All values are the same
Some columns are called high-cardinality columns because they have constraints in place (like unique) prohibiting you from putting the same value in every row.
Cardinality is a property which affects the ability to cluster, sort and search data. It is therefore an important measurement for the query planners in DBs, it is a heuristic which they can use to choose the best plans.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
