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<>1. 如何做sql优化
explain分析sql的执行计划
<>1.1 insert语句的优化
在大批量插入数据时，对于InnoDB的表，有以下几种提高导入效率方式
* 主键顺序插入（主键有序的话，插入效率高）
* 关闭唯一性校验
* 手动提交事务
对一张表插入多行数据时，可以使用多个值表的insert语句；这种方式可以缩减客户端与数据库之间的连接、关闭等消耗
insert into tb_test values(1,'Tom'); insert into tb_test values(2,'Cat');
insert into tb_test values(3,'Jerry');
优化后
insert into tb_test values(1,'Tom'),(2,'Cat'),(3,'Jerry');
<>1.2 order by语句优化
filesort排序：先返回数据再排序
index排序：直接返回有序数据（需要select后面跟索引字段）
尽量添加索引，让MySQL使用index排序
而且order by的字段要么都是升序，要么都是降序
否则会造成filesort 排序
<>1.3 group by语句优化
group by后最好跟上有索引的字段
如果没有索引，那么加上order by null
可以避免效率低的filesort
因为group by也会进行排序操作
<>1.4 or查询优化
因为使用or查询时，or两边字段必须都有索引，才能利用索引，容易造成索引失效。
所以我们用union关键字代替or，union可以连接两个select语句，代替or的作用。
<>1.5 优化嵌套查询
用两张表连接查询代替嵌套查询
因为嵌套查询需要在内存中创建临时表，步骤更加繁琐
<>1.6 优化分页查询limit
* 思路一：分页之前得排序，可以选择在索引上完成分页操作，最后根据主键关联回原表查询的其他内容（先把对应页的主键表查出来，再根据主键查出整表）
* 思路二：可以把limit查询转换成某个位置的查询：select * from 表名 where id > 1000000 limit 10;
该方法适用于主键自增的表，且主键不能断层
例如：日志信息表，只有插入没有修改删除。
