MySQL联合索引最左匹配范例

MySQL联合索引最左匹配范例
参考文章：http://blog.jobbole.com/24006/

创建示例表。
示例表来自MySQL官方文档： https://dev.mysql.com/doc/employee/en/
CREATE TABLE titles (
emp_no      INT             NOT NULL,
title       VARCHAR(50)     NOT NULL,
from_date   DATE            NOT NULL,
to_date     DATE,
PRIMARY KEY (emp_no,title,from_date)
) ;

导入测试数据  load_titles.dump 
https://github.com/datacharmer/test_db

索引情况
mysql> show index from titles;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+------
| Table  | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comme
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+------
| titles |          0 | PRIMARY  |            1 | emp_no      | A         |           0 |     NULL | NULL   |      | BTREE      |      
| titles |          0 | PRIMARY  |            2 | title       | A         |           0 |     NULL | NULL   |      | BTREE      |      
| titles |          0 | PRIMARY  |            3 | from_date   | A         |           0 |     NULL | NULL   |      | BTREE      |      
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+------
3 rows in set (0.00 sec)


情况一.WHERE条件中全部列都在索引中 
mysql> desc SELECT * FROM titles WHERE emp_no='10001' AND title='Senior Engineer' AND from_date='1986-06-26';
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref               | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | titles | NULL       | const | PRIMARY       | PRIMARY | 159     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)


mysql> desc SELECT * FROM titles WHERE title='Senior Engineer' AND emp_no='10001' AND from_date='1986-06-26';
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref               | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | titles | NULL       | const | PRIMARY       | PRIMARY | 159     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)


mysql> desc SELECT * FROM titles WHERE from_date='1986-06-26' AND title='Senior Engineer' AND emp_no='10001';
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref               | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | titles | NULL       | const | PRIMARY       | PRIMARY | 159     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

 
mysql> desc SELECT * FROM titles WHERE emp_no='10001' AND title in('Senior Engineer','Staff','Assistant Engineer') AND from_date='1986-06-26';
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | titles | NULL       | range | PRIMARY       | PRIMARY | 159     | NULL |    3 |   100.00 | Using where |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)


mysql>  desc SELECT * FROM titles WHERE emp_no in('10061','10062','10063','10064') AND title in('Senior Engineer','Staff','Assistant Engineer') AND from_date='1986-06-26';
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | titles | NULL       | range | PRIMARY       | PRIMARY | 159     | NULL |   12 |   100.00 | Using where |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

 
mysql> desc SELECT * FROM titles WHERE emp_no='10001' AND title='Senior Engineer' AND from_date>='1986-06-26';
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | titles | NULL       | range | PRIMARY       | PRIMARY | 159     | NULL |    1 |   100.00 | Using where |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)


情况二.WHERE条件和ORDER BY中全部列都在索引中  
mysql> desc SELECT * FROM titles WHERE emp_no='10001' AND title='Senior Engineer' order by from_date;
+----+-------------+--------+------------+------+---------------+---------+---------+-------------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key     | key_len | ref         | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+---------+---------+-------------+------+----------+-------------+
|  1 | SIMPLE      | titles | NULL       | ref  | PRIMARY       | PRIMARY | 156     | const,const |    1 |   100.00 | Using where |
+----+-------------+--------+------------+------+---------------+---------+---------+-------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec) 
 

mysql> desc SELECT * FROM titles WHERE emp_no='10001' order by title,from_date;
+----+-------------+--------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | titles | NULL       | ref  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using where |
+----+-------------+--------+------------+------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)


mysql> desc SELECT * FROM titles order by emp_no,title,from_date;
+----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref  | rows   | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------+
|  1 | SIMPLE      | titles | NULL       | index | NULL          | PRIMARY | 159     | NULL | 441772 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------+
1 row in set, 1 warning (0.01 sec)