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带你搞定MySQL实战，轻松对应海量业务处理及高并发需求，从容应对大场面试
DB Version
mysql> select version(); +-----------+ | version() | +-----------+ | 5.7.28 | +-----------+ 1 row in set mysql>
Table
CREATE TABLE `employees` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名', `age` int(11) NOT NULL DEFAULT '0' COMMENT '年龄', `position` varchar(20) NOT NULL DEFAULT '' COMMENT '职位', `hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间', PRIMARY KEY (`id`), KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='员工记录表';
两个索引
- 主键索引
- 二级索引 KEY
idx_name_age_position(name,age,position) USING BTREE
重点就是这个二级索引 ，记号了哈。
数据量
mysql> select count(1) from employees ; +----------+ | count(1) | +----------+ | 100002 | +----------+ 1 row in set mysql>
案例一 ：explain select * from employees where name = ‘LiLei’ and position = ‘dev’ order by age
explain select * from employees where name = 'LiLei' and position = 'dev' order by age ;
先想一下这个order by 会不会走索引 ？
会走索引
原因呢 ？
脑海中要有这个联合索引在MySQL底层的B+Tree的数据结构 ， 索引 排好序的数据结构。
name = ‘LiLei’ and position = ‘dev’ order by age
name 为 LiLei , name 确定的情况下， age 肯定是有序的 ，age 有序不能保证position 有序
所以 这个order by age 是可以走索引的
继续分析下这个explain
mysql> explain select * from employees where name = 'LiLei' and position = 'dev' order by age ; +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | ref | idx_name_age_position | idx_name_age_position | 74 | const | 1 | 10 | Using index condition | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ 1 row in set
order by 走的索引 是不会体现在key_len上的, 这个74 = 3 * 24 + 2 ， 是计算的name 。 最左匹配原则 ，中间字段不能断，因此查询用到了name索引。
但是 Extra直接里面可以看出来 Using index condition ，说明age索引列用在了排序过程中 。 如果没有走索引的话，那就是 Using FileSort 了
接下来继续看几个例子，加深理解，重点是脑海中的 索引B+Tree结构
案例二： explain select * from employees where name = ‘LiLei’ order by position
mysql> explain select * from employees where name = 'LiLei' order by position ;
想一想，这个order by 会走索引吗？
我们来看下索引 KEY idx_name_age_position (name,age,position) USING BTREE
再来看下查询SQL
where name = 'LiLei' order by position ;
name = LiLei , name 值能确定下来， 符合最左匹配原则 所以查询会走索引 ， 用了联合索引中的name字段， key len = 74 . 所以 Using index condition
order by position , 在索引中 中间缺失了age , 用position ，跳过了age , 那索引树能是有序的吗？ 肯定不是。。。所以 position肯定不是排好序的 ， 无法走索引排序，因此 Extra信息 有 Using filesort
来看下执行计划
mysql> explain select * from employees where name = 'LiLei' order by position ; +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ | 1 | SIMPLE | employees | NULL | ref | idx_name_age_position | idx_name_age_position | 74 | const | 1 | 100 | Using index condition; Using filesort | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ 1 row in set mysql>
正如分析~
有感觉了吗？ 再来看一个
案例三：explain select * from employees where name = ‘LiLei’ order by age , position
这个SQL和案例二的很相似 ， 仅仅在排序的时候在前面多了一个age字段参与排序 ， 那分析分析 order by 会走索引吗
mysql> explain select * from employees where name = 'LiLei' order by age , position ;
时刻不要那个索引树 ，来分析一下
name = LiLei , name 固定，结合 建立的索引， 最左原则，所以查询肯定会走联合索引中的部分索引 name .
在name都是LiLei 的情况下 ， order by age , position 结合索引树 ，age和position用于排序 也是有序的，应该不会走using filesort
我们来看下执行计划
mysql> explain select * from employees where name = 'LiLei' order by age , position ; +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | ref | idx_name_age_position | idx_name_age_position | 74 | const | 1 | 100 | Using index condition | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-----------------------+ 1 row in set mysql>
案例四：explain select * from employees where name = ‘LiLei’ order by position , age
再分析一个，和案例上也很像。 把 order by的排序顺序 调整一下，我们来分析一下 order by会不会走索引
explain select * from employees where name = 'LiLei' order by position , age ;
mysql> explain select * from employees where name = 'LiLei' order by position , age ; +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ | 1 | SIMPLE | employees | NULL | ref | idx_name_age_position | idx_name_age_position | 74 | const | 1 | 100 | Using index condition; Using filesort | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ 1 row in set mysql>
咦 ， 执行计划中有 using filesort
为什么呢？
看看我们二级索引的建立的字段顺序 ， 创建顺序为name,age,position，但是排序的时候age和position颠倒位置了， 那排好序的特性肯定就无法满足了，那你让MySQL怎么走索引？
案例五：explain select * from employees where name = ‘LiLei’ and age = 18 order by position , age ;
这个order by 会走索引吗？
mysql> explain select * from employees where name = 'LiLei' and age = 18 order by position , age ; +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | employees | NULL | ref | idx_name_age_position | idx_name_age_position | 78 | const,const | 1 | 100 | Using index condition | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-----------------------+ 1 row in set mysql>
走了dx_name_age_position 索引中的 name 和 age , order by 其实也走了索引，你看extra中并没有 using filesort ,因为age为常量，在排序中被MySQL优化了，所以索引未颠倒，不会出现Using filesort
案例六：explain select * from employees where name = ‘LiLei’ order by age asc , position desc ;
mysql> explain select * from employees where name = 'LiLei' order by age asc , position desc ; +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ | 1 | SIMPLE | employees | NULL | ref | idx_name_age_position | idx_name_age_position | 74 | const | 1 | 100 | Using index condition; Using filesort | +----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+---------------------------------------+ 1 row in set
我们可以看到虽然排序的字段列与建立索引的顺序一样， order by默认升序排列，而SQL中的 position desc变成了降序排列，导致与索引的排序方式不同，从而产生Using filesort。
Note: Mysql8以上版本有降序索引可以支持该种查询方式。
案例七：explain select * from employees where name in (‘HanMeiMei’ , ‘LiLei’) order by age , position ;
mysql> explain select * from employees where name in ('HanMeiMei' , 'LiLei') order by age , position ; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+---------------------------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 74 | NULL | 2 | 100 | Using index condition; Using filesort | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+---------------------------------------+ 1 row in set mysql>
对order by 来讲 ，多个相等的条件也是 范围查询。 既然是范围查询， 可能对于每个值在索引中是有序的，但多个合并在一起，就不是有序的了，所以 using filesort .
案例八： explain select * from employees where name > ‘HanMeiMei’ order by name ;
mysql> explain select * from employees where name > 'HanMeiMei' order by name ; +----+-------------+-----------+------------+------+-----------------------+------+---------+------+-------+----------+-----------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+------+-----------------------+------+---------+------+-------+----------+-----------------------------+ | 1 | SIMPLE | employees | NULL | ALL | idx_name_age_position | NULL | NULL | NULL | 96845 | 50 | Using where; Using filesort | +----+-------------+-----------+------------+------+-----------------------+------+---------+------+-------+----------+-----------------------------+ 1 row in set mysql>
MySQL自己内部有一套优化机制，且数据量不同、版本不一样，结果也可能有差异
一般情况下， 联合索引第一个字段用范围不一定会走索引 ， 可以采用 覆盖索引进行优化，避免回表带来的性能开销 。
mysql> explain select name from employees where name > 'a' order by name ; +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+--------------------------+ | 1 | SIMPLE | employees | NULL | range | idx_name_age_position | idx_name_age_position | 74 | NULL | 48422 | 100 | Using where; Using index | +----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+-------+----------+--------------------------+ 1 row in set mysql>
group by 优化
- group by与order by类似，其实质是先排序后分组，遵照索引创建顺序的最左前缀法则。
- 对于group by的优化如果不需要排序的可以加上order by null禁止排序。
- where高于having，能写在where中的限定条件就不要去having限定了。
小结
MySQL支持两种方式的排序filesort和index，Using index是指MySQL扫描索引本身完成排序
order by满足两种情况会使用Using index
A: order by语句使用索引最左前列。
B: 使用where子句与order by子句条件列组合满足索引最左前列
尽量在索引列上完成排序，遵循索引建立（索引创建的顺序）时的最左前缀法则
如果order by的条件不在索引列上，就会产生Using filesort
能用覆盖索引尽量用覆盖索引
搞定MySQL
https://artisan.blog.csdn.net/article/details/107767649?spm=1001.2014.3001.5502
