MySQL创建复合索引

在MySQL数据库中，创建复合索引的时候，不知道在创建过程中哪个列在前面，哪个列该在后面，用以下方式即可：

select count(distinct first_name)/count(*) as first_name_selectivity,
count(distinct last_name)/count(*) as last_name_selectivity,
count(*)
from actor\G

mysql> select count(distinct first_name)/count(*) as first_name_selectivity,
    -> count(distinct last_name)/count(*) as last_name_selectivity,
    -> count(*)
    -> from actor\G
*************************** 1. row ***************************
first_name_selectivity: 0.6400
 last_name_selectivity: 0.6050
              count(*): 200
1 row in set (0.01 sec)

first_name_selectivity: 0.6400，因此 first_name 列的选择性更高，所以答案将其作为索引列的第一列：

mysql> alter table yoon add key (first_name,last_name);