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Merge pull request #459 from hanandjun/patch-2

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修改错别字
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docs/database/MySQL Index.md
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@ -70,7 +70,7 @@ select * from user where city=xx ; // 无法命中索引
冗余索引指的是索引的功能相同，能够命中 就肯定能命中 ，那么 就是冗余索引如（name,city ）和（name ）这两个索引就是冗余索引，能够命中后者的查询肯定是能够命中前者的 在大多数情况下，都应该尽量扩展已有的索引而不是创建新索引。
MySQLS.7 版本后，可以通过查询 sys 库的 `schema_redundant_indexes` 表来查看冗余索引
MySQL 5.7 版本后，可以通过查询 sys 库的 `schema_redundant_indexes` 表来查看冗余索引
### Mysql如何为表字段添加索引？？？