MySQL combat: how to optimize the group by statement?

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CREATE TABLE `staff` (
  `id` bigint(11) NOT NULL AUTO_INCREMENT COMMENT '主键id',
  `name` varchar(64) NOT NULL COMMENT '姓名',
  `age` int(4) NOT NULL COMMENT '年龄',
  `city` varchar(64) NOT NULL COMMENT '城市',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8 COMMENT='员工表';

explain select city ,count(*) as num from staff group by city;

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group by optimization

order by null does not sort

explain select city ,count(*) as num from staff group by city order by null

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There is only Using temporary, there is no Using filesort, indicating that the sorting is not performed.

Group field plus index

alter table staff add index idx_city(city)

explain select city ,count(*) as num from staff group by city

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Using index, indicating that the index is covered, the type is inde, indicating that the index idx_city has been traversed

Using temporary and Using filesort are all gone

DROP INDEX idx_city ON staff

Use in-memory temporary tables as much as possible

Use SQL_BIG_RESULT

explain select SQL_BIG_RESULT city ,count(*) as num from staff group by city

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Reference blog

[1]https://mp.weixin.qq.com/s/EybebcYX4i13r5nDRG14fg
[2]https://juejin.cn/post/7053966777088213005