-- 查看出現頻率
select count(*) as cnt,city
from sakila.city_demo group by city order by cnt desc limit 10;

1.select count(distinct city)/count(*) from sakila.city_demo;

*完整列的選擇性

2. select count(distinct left(city,3))/count(*) as sel3 ,
count(distinct left(city,4))/count(*) as sel4 ,
count(distinct left(city,5))/count(*) as sel5 ,
count(distinct left(city,6))/count(*) as sel6 ,
count(distinct left(city,7))/count(*) as sel7

from sakila.city_demo;

*前綴輸出越貼近 1 輸出，性能越高

3. alter table sakilal.city_demo add key (city(7));

使用場景：

16進制唯一ID使用前綴索引。 通常使用前8位，性能顯著提高

轉載于:https://www.cnblogs.com/MarchThree/p/4622197.html

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