MySQL中的基数是什么? [英] What is cardinality in MySQL?

问题描述

MySQL中的基数是什么?请用简单的非技术性语言进行解释.

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

如果任何表的索引明细显示字段的基数，例如group_id为11，那是什么意思?

If a index detail of any table displays the cardinality of a field say group_id as 11, then what does that mean?

推荐答案

最大基数:所有值都是唯一的

Max cardinality: All values are unique

最小基数:所有值都相同

Min cardinality: All values are the same

有些列称为高基数列，因为它们具有适当的约束(例如唯一性)，禁止您在每一行中都放置相同的值.

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.

基数是一个属性，会影响对数据进行聚类，排序和搜索的能力.因此，这对于数据库中的查询计划者而言是一项重要的衡量标准，是一种启发式方法，可供他们选择最佳计划.

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.

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