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— 查看出现频率
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;
*完整列的选择性
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;
*前缀输出越贴近 1 输出,性能越高
3. alter table sakilal.city_demo add key (city(7));
使用场景:
16进制唯一ID使用前缀索引。 通常使用前8位,性能显著提高
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