mysql建立前綴索引

ALTER table 表名 add index title_pre(列名(100))索引 列名後面的數字表明前綴的長度，前綴長度並非越長越好，這裏涉及到一個選擇性問題，it select count(distinct 列名)/count(*)as a,COUNT(DISTINCT left(列名,100)) as b, COUNT(DISTINCT left(列名,110)) as c from

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