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Explain 执行计划
概述
示例
mysql> select * from user ;
+----+----------+----------+---------------------+
| id | username | password | date |
+----+----------+----------+---------------------+
| 1 | guo | 111 | 2018-05-20 09:55:47 |
| 2 | root | aaa | 2018-05-20 09:55:50 |
| 3 | 11111 | 22222 | 2018-05-20 09:55:51 |
| 4 | 1 | 1 | 2018-05-20 09:55:56 |
| 5 | a | b | 2018-05-20 09:55:54 |
| 7 | a | b | 2018-05-20 09:56:58 |
+----+----------+----------+---------------------+
6 rows in set
mysql> explain select * from user where id = 1 ;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | user | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set说明
- id ：DML 语句的执行顺序，id 值越大优先级别越高；若包含子查询，则子查询的优先级别要高于主查询
- key_len : 索引长度，用户判断是否使用到了索引，使用了多少列
- 如果是单列索引，长度是一样的；如果是组合索引，长度为该查询中使用到了多少列
- key_len只计算where条件用到的索引长度，而排序和分组就算用到了索引，也不会计算到key_len中
| 类型分类 | 数据类型 | 计算公式 | 备注说明 |
|---|---|---|---|
| 可空 | null | +1 | 如果字段可空索引长度 + 1 |
| 字符 | char | length * 编码格式 | UTF-8:3 ; GBK：2 ；latin:1 |
| 字符 | varchar | length * 编码格式 + 2 | UTF-8:3 ; GBK：2 ；latin:1 |
| 数值 | int | 4 | - |
| 数值 | tinyint | 1 | - |
| 时间 | date | 3 | - |
| 时间 | timestamp | 4 | - |
mysql> desc test ;
+-------------+-------------+------+-----+-------------------+-----------------------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+-------------------+-----------------------------+
| id | int(10) | NO | PRI | NULL | auto_increment |
| name | varchar(10) | NO | | NULL | |
| sex | char(1) | YES | | NULL | |
| age | tinyint(3) | NO | | NULL | |
| create_time | timestamp | NO | | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
| update_time | date | NO | | NULL | |
+-------------+-------------+------+-----+-------------------+-----------------------------+
6 rows in set
mysql> alter table test add index idx_name
(`name`,`sex`,`age`,`create_time`,`update_time`);
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select id from test where id = 1 ;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set
mysql> explain select * from test where name = "1" ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | idx_name | idx_name | 32 | const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set
-- key_len : 10*3 +2
mysql> explain select * from test where name = "1" and sex = "1" ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | idx_name | idx_name | 36 | const,const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------+------+----------+-------------+
1 row in set
-- key_len : 10 * 3 + 3 + 1
mysql> explain select * from test where name = "1" and sex = "1" and age = 1 ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | idx_name | idx_name | 37 | const,const,const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------+
1 row in set
mysql> explain select * from test where name = "1" and sex = "1" and age = 1 and create_time = "2018-07-29 00:00:00" ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | idx_name | idx_name | 41 | const,const,const,const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------------+------+----------+-------------+
1 row in set
mysql> explain select * from test where name = "1" and sex = "1" and age = 1 and create_time = "2018-07-29 00:00:00" and update_time = "2018-07-29" ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------------------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | idx_name | idx_name | 44 | const,const,const,const,const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------------------+------+----------+-------------+
1 row in set
- filtered : 返回结果的行占需要读到的行(rows列的值)的百分比
- filtered值只对index和all的扫描有效
- filtered的值真正有效的场合要where条件中出现的列上建有索引，又要执行计划不走索引的range或ref扫描，而走全表扫描(all)或覆盖索引(index)扫描，所以可以说这个filtered列几乎无用