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 不管WHERE的顺序如何执行计划都可以完整用到索引。 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，索引最左列在WHERE条件中， 不管ORDER BY是ASC还是DESC，执行计划都可以完整用到索引。 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' AND title='Senior Engineer' order by from_date desc; +----+-------------+--------+------------+------+---------------+---------+---------+-------------+------+----------+-------------+ | 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 WHERE emp_no='10001' order by title,from_date desc; +----+-------------+--------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------------+ | 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; Using filesort | +----+-------------+--------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE emp_no='10001' order by title desc,from_date desc; +----+-------------+--------+------------+------+---------------+---------+---------+-------+------+----------+-------------+ | 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 (0.00 sec) 情况三.当WHERE条件和ORDER BY中全部列都在索引中，索引最左列不在WHERE条件中而是在ORDER BY中 执行计划会走index。 mysql> desc SELECT * FROM titles WHERE title='Senior Engineer' order by emp_no,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 | 10.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE title='Senior Engineer' order by emp_no desc,from_date desc; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 10.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE from_date='1986-06-26' order by emp_no,title; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 10.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE from_date='1986-06-26' order by emp_no desc,title desc; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 10.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE title='Senior Engineer' AND from_date='1986-06-26' order by emp_no; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 1.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE from_date='1986-06-26' AND title='Senior Engineer' order by emp_no; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 1.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE title='Senior Engineer' AND from_date='1986-06-26' order by emp_no desc; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 1.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles WHERE from_date='1986-06-26' AND title='Senior Engineer' order by emp_no desc; +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ | 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 | 1.00 | Using where | +----+-------------+--------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) 情况四.当WHERE条件无过滤，索引列全在ORDER BY中 1.当order by 中ASC和DESC顺序和索引一致执行计划会走INDEX，如果不一致则会走全表扫描。 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) mysql> desc SELECT * FROM titles order by emp_no,title desc,from_date desc; +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | SIMPLE | titles | NULL | ALL | NULL | NULL | NULL | NULL | 441772 | 100.00 | Using filesort | +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles order by emp_no desc,title,from_date; +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | SIMPLE | titles | NULL | ALL | NULL | NULL | NULL | NULL | 441772 | 100.00 | Using filesort | +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc SELECT * FROM titles order by emp_no,title desc,from_date desc; +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ | 1 | SIMPLE | titles | NULL | ALL | NULL | NULL | NULL | NULL | 441772 | 100.00 | Using filesort | +----+-------------+--------+------------+------+---------------+------+---------+------+--------+----------+----------------+ 1 row in set, 1 warning (0.00 sec)