mysql建立部分索引

-- 计算部分索引平均值

SELECT COUNT(DISTINCT t.show_sentence)/COUNT(1) from t_user_show t;

-- 找出部分索引最佳长度（得到结果与平均值相近）

SELECT COUNT(DISTINCT LEFT(t.show_sentence,10) )/COUNT(*) as sel10,

COUNT(DISTINCT LEFT(t.show_sentence,20) )/COUNT(*) as sel20,

COUNT(DISTINCT LEFT(t.show_sentence,30) )/COUNT(*) as sel30,

COUNT(DISTINCT LEFT(t.show_sentence,40) )/COUNT(*) as sel40 from t_user_show t;

平均值  == 得到结果与平均值相近

-- 建部分索引语句

ALTER TABLE t_user_show add key (show_sentence(40));