explain 基本作用
1、explain 作用和 describe 一致；
2、explian tbl_name 的作用同 show columns from tbl_name 是一致的；
3、如果在 select 语句之前加入该关键字，则可以模拟优化器执行 sql，从而得知 Mysql 如何处理这条 sql 语句的，帮助我们分析查询语句提高性能；
mysql> explain `order`;
+---------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| orderNo | varchar(45) | YES | UNI | NULL | |
| price | int(11) | YES | | NULL | |
| status | tinyint(4) | YES | | NULL | |
| date | datetime | YES | | NULL | |
| method | tinyint(4) | YES | | NULL | |
+---------+-------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)
mysql> describe `order`;
+---------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| orderNo | varchar(45) | YES | UNI | NULL | |
| price | int(11) | YES | | NULL | |
| status | tinyint(4) | YES | | NULL | |
| date | datetime | YES | | NULL | |
| method | tinyint(4) | YES | | NULL | |
+---------+-------------+------+-----+---------+----------------+
explain 分析 select 语句结果
mysql> explain select * from `order`;
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+
| 1 | SIMPLE | order | NULL | ALL | NULL | NULL | NULL | NULL | 4854583 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------+
explain 结果分析
- 1、id
id 是一组数字，表示查询过程中操作表的顺序，共分为三种情况：
1）id 相同，执行顺序自上而下；
mysql> explain select t1.*
-> from orders t1, order_detail t2, seller t3
-> where t1.orderNo = t2.orderNo and t2.sellerId = t3.sellerId;
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+----------------------------------------------------+
| 1 | SIMPLE | t3 | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | NULL |
| 1 | SIMPLE | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 20.00 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 4854583 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+----------------------------------------------------+
3 rows in set, 1 warning (0.02 sec)
2）id 不同，如果是子查询，id的序号会递增，id值越大优先级越高，越先被执行；
mysql> explain select * from orders where orderNo = ( select orderNo from order_detail where sellerId = (select sellerId from seller where id = 1));
+----+-------------+--------------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
| 1 | PRIMARY | orders | NULL | ALL | NULL | NULL | NULL | NULL | 4854583 | 100.00 | Using where |
| 2 | SUBQUERY | order_detail | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 20.00 | Using where |
| 3 | SUBQUERY | seller | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+--------------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
3 rows in set, 1 warning (0.01 sec)
3）id相同不同，同时存在。id 相同的认为一组自上而下顺序执行，id 不同，id 值越大越先执行；
mysql> explain select t1.* from (
-> select orderNo
-> from order_detail
-> where id = 1) t2, orders t1
-> where t1.orderNo = t2.orderNo;
+----+-------------+--------------+--------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+--------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 50 | Using where |
| 2 | DERIVED | order_detail | const | PRIMARY | PRIMARY | 4 | | 1 | |
+----+-------------+--------------+--------+---------------+---------+---------+------+------+-------------+
3 rows in set (0.00 sec)
- 2、select_type
1） simple 简单 sql 查询，不包含子查询或者 union
mysql> explain select * from orders;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | orders | ALL | NULL | NULL | NULL | NULL | 50 | |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
2）primary 查询过程中包含其他子部分
mysql> explain select * from (select orderNo from order_detail where id = 1) t;
+----+-------------+--------------+--------+---------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+--------+---------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | order_detail | const | PRIMARY | PRIMARY | 4 | | 1 | |
+----+-------------+--------------+--------+---------------+---------+---------+------+------+-------+
2 rows in set (0.00 sec)
3） subquery 子查询的第一个 select
mysql> explain select * from orders where orderNo = (select orderNo from order_detail where id = 1);
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | orders | ALL | NULL | NULL | NULL | NULL | 50 | Using where |
| 2 | SUBQUERY | order_detail | const | PRIMARY | PRIMARY | 4 | const | 1 | |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
2 rows in set (0.01 sec)
4）union union中的从第二个之后的 select；union result union的结果
mysql> explain select * from orders where id = 1
-> union
-> select * from orders where id = 2;
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | orders | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 2 | UNION | orders | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| NULL | UNION RESULT | | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+
3 rows in set (0.00 sec)
5）derived（衍生） 在 from 包含的子查询，MySQL会递归执行这些子查询，把结果放在临时表里；（3中有derived）
- 3、table
查询结果数据引用的表
- 4、type
1）system 表只有一行记录（等于系统表），平时很少出现，时 const 的一个特例；
2）const 表中最多只有一行匹配行，用于查询条件中的列是 primary key 或者 unique 索引的列；
mysql> show index from orders;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| orders | 0 | PRIMARY | 1 | id | A | 50 | NULL | NULL | | BTREE | | |
| orders | 0 | un_index | 1 | orderNo | A | 50 | NULL | NULL | YES | BTREE | | |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql> explain select * from orders where orderNo = 'dac4c327-9810-11eb-af55-000c296fcfe0';
+----+-------------+--------+-------+---------------+----------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+----------+---------+-------+------+-------+
| 1 | SIMPLE | orders | const | un_index | un_index | 48 | const | 1 | |
+----+-------------+--------+-------+---------------+----------+---------+-------+------+-------+
1 row in set (0.00 sec)
3）eq_ref 唯一性索引，对于每个索引键，表中只有一条记录与之匹配，常见于主键或唯一索引扫描;
mysql> explain select * from orders t1, order_detail t2 where t1.id = t2.id;
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------+
| 1 | SIMPLE | t2 | ALL | PRIMARY | NULL | NULL | NULL | 4 | |
| 1 | SIMPLE | t1 | eq_ref | PRIMARY | PRIMARY | 4 | test.t2.id | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------+
2 rows in set (0.00 sec)
4）ref 非唯一索引，返回匹配结果的所有行；
mysql> create index in_or on orders(orderNo);
Query OK, 0 rows affected (0.45 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from orders where orderNo = 'dac4c327-9810-11eb-af55-000c296fcfe0';
+----+-------------+--------+------+---------------+-------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-------+---------+-------+------+-------------+
| 1 | SIMPLE | orders | ref | in_or | in_or | 48 | const | 1 | Using where |
+----+-------------+--------+------+---------------+-------+---------+-------+------+-------------+
1 row in set (0.05 sec)
5）range 只检索给定范围的行，使用一个索引来选择行。key列显示使用了哪个索引
一般就是在你的where语句中出现了between、<、>、in等的查询
这种范围扫描索引扫描比全表扫描要好，因为他只需要开始索引的某一点，而结束语另一点，不用扫描全部索引
mysql> explain select * from orders where id > 34;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | orders | range | PRIMARY | PRIMARY | 4 | NULL | 4 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
6）index 从索引文件中读取数据，因为索引文件一般情况下比数据文件要下因此，速度更快；
mysql> explain select orderNo from orders;
+----+-------------+--------+-------+---------------+-------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+-------+---------+------+------+-------------+
| 1 | SIMPLE | orders | index | NULL | un_or | 48 | NULL | 4 | Using index |
+----+-------------+--------+-------+---------------+-------+---------+------+------+-------------+
1 row in set (0.03 sec)
7）all 遍历全表
mysql> explain select * from orders;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | orders | ALL | NULL | NULL | NULL | NULL | 4 | |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
- 5、possible_keys
查询语句涉及的字段上若存在索引，则会被列出，但不一定在查询时使用到，如果为 null 则表示没有用到索引；
mysql> explain select * from orders where orderNo = 'dac4c327-9810-11eb-af55-000c296fcfe0' and id =51;
+----+-------------+--------+-------+------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+------------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | orders | const | PRIMARY,un_index | PRIMARY | 4 | const | 1 | |
+----+-------------+--------+-------+------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
- 6、key
查询过程中实际用到的索引，为 null 时表示没有用到索引；
- 7、key_len
表示索引的字节数，key_len 显示的长度表示索引的最大长度，并不是实际使用长度，在不损失精度的情况下，长度越小越好；
- 8、ref
显示索引那一列被使用了，如果可能的话，是一个常数。那些列或常量被用于查找索引列上的值
- 9、rows
大致估算执行查询语句找到结果所需要读取的行数
- 10、extra
重要的额外信息
1） Using filesort 表示 mysql 会用外部的一个索引排序，而不是按照表内的索引顺序进行读取；
mysql> explain select * from orders order by orderNo;
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
| 1 | SIMPLE | orders | ALL | NULL | NULL | NULL | NULL | 4 | Using filesort |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)
2） Using temporary 使用临时表存储中加数据，常用于 order by 和 group by；
mysql> explain select * from orders group by orderNo order by id desc;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-----------------+
| 1 | SIMPLE | orders | index | NULL | PRIMARY | 4 | NULL | 4 | Using temporary |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-----------------+
1 row in set (0.00 sec)
- Using index 使用覆盖索引
mysql> explain select orderNo from orders;
+----+-------------+--------+-------+---------------+-------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+-------+---------+------+------+-------------+
| 1 | SIMPLE | orders | index | NULL | un_or | 48 | NULL | 4 | Using index |
+----+-------------+--------+-------+---------------+-------+---------+------+------+-------------+
1 row in set (0.00 sec)