Index on Mysql column has lower cardinality than count distinct?

问题

I have a varchar(50) field named token. When I do

count(distinct token) from table 

I get ~ 400k. However, if I do

create index idx on table (token)

The cardinality is only 200. What could be going on here? Shouldn't cardinality be the same as the number of distinct tokens?

回答1:

The cardinality is an estimate of the number of unique values in the index. According to the documentation:

Cardinality is counted based on statistics stored as integers, so the value is not necessarily exact even for small tables.

来源：https://stackoverflow.com/questions/29441260/index-on-mysql-column-has-lower-cardinality-than-count-distinct