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);