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fix issue#758 #760

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Snailclimb merged 1 commit into from
May 1, 2020
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fix issue#758 #760

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2 changes: 1 addition & 1 deletion docs/essential-content-for-interview/PreparingForInterview/美团面试常见问题总结.md
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Expand Up @@ -229,7 +229,7 @@ HTTP 响应报文主要由状态行、响应头部、响应正文 3 部分组成

1. 避免 where 子句中对字段施加函数，这会造成无法命中索引。
2. 在使用 InnoDB 时使用与业务无关的自增主键作为主键，即使用逻辑主键，而不要使用业务主键。
3. 将打算加索引的列设置为 NOT NULL ，否则将导致引擎放弃使用索引而进行全表扫描
3. 将打算加索引的列建议设置为 NOT NULL ，因为 NULL 比空字符串需要更多的存储空间（不仅仅是索引列，普通的列如果业务允许都建议设置为 NOT NULL）
4. 删除长期未使用的索引，不用的索引的存在会造成不必要的性能损耗 MySQL 5.7 可以通过查询 sys 库的 schema_unused_indexes 视图来查询哪些索引从未被使用
5. 在使用 limit offset 查询缓慢时，可以借助索引来提高性能

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