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HomeDatabaseMysql TutorialMySQL索引之前缀索引和索引选择性

通常可以索引开始的几个字符,而不是全部值,以节约空间并得到好的性能。这使索引需要的空间变小,但是也会降低选择性。索引选择

有时需要索引很长的字符列,它会使索引变大而且变慢。一个策略就是模拟哈希索引。但是有时这也不够好,那?

通常可以索引开始的几个字符,而不是全部值,以节约空间并得到好的性能。这使索引需要的空间变小,但是也会降低选择性。索引选择性是不重复的索引值和表中所有行的比值。高选择性的索引有好处,因为它使mysql在查找匹配的时候可以过波掉更多的行。唯一索引的选择率为1,为最佳值。

如果索引BLOG和TEXT列,,或者很长的varchar列,就必须定义前缀索引,因为mysql不允许索引它们的全文化。

可以在同一个查询中针对许多不同的前缀长度进行计算,选择选择性好的。 【Linux公社 】

(用LEFT函数,left(city,4))

添加前缀索引

mysql>alter table test.test add key (city(7));

前缀索引能很好地减少索引的大小及提高速度,但是mysql不能在order by 和group by查询中使用前缀索引,也不能把它们用伯覆盖索引。

有时后缀索引也挺有用,例如查找某个域名的所有电子邮件地址。mysql不支持反向索引,但是可以把反向字符串保存起来,并且索引它的前缀。可以用触发器维护这种索引。

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