mysql> select * from test;
+----+------+------+
| id | a | b |
+----+------+------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 2 |
| 4 | 2 | 2 |
| 5 | 2 | 2 |
| 6 | 3 | 3 |
| 7 | 4 | 4 |
| 8 | 4 | 4 |
+----+------+------+
69 rows in set (0.00 sec)mysql> show index from test;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test | 0 | PRIMARY | 1 | id | A | 69 | NULL | NULL | | BTREE | | |
| test | 1 | a | 1 | a | A | 2 | NULL | NULL | YES | BTREE | | |
| test | 1 | b | 1 | b | A | 2 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
EXPLAIN select * from test where b=31 and a=31;mysql> EXPLAIN select * from test where b=31 and a=31;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | a,b | a | 5 | const | 1 | 5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
结果说明：a与b同时不满足条件，使用了最左索引 a
mysql> EXPLAIN select * from test where b=11 and a=1;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | a,b | b | 5 | const | 1 | 5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)结果说明：a满足条件,b不满足条件，使用了索引 b
mysql> EXPLAIN select * from test where b=1 and a=11;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | a,b | a | 5 | const | 1 | 5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)结果说明：a不满足条件,b满足条件，使用了索引 a
mysql> EXPLAIN select * from test where b=1 and a=1;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | a,b | b | 5 | const | 1 | 5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)结果说明：a满足条件,b满足条件，使用了索引符合该条件的行数少的索引b
explain select * from test where b=2 and a=1;
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
| 1 | SIMPLE | test | NULL | index_merge | a,b | a,b | 5,5 | NULL | 2 | 100.00 | Using intersect(a,b); Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
1 row in set, 1 warning (0.00 sec)结果说明：a满足条件,b满足条件，最左索引a符合条件少于b，使用了索引 a,b
mysql> explain select * from test where b=3 and a=3;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | a,b | a | 5 | const | 1 | 16.67 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)结果说明：a满足条件,b满足条件，最左索引a符合条件等于于b，使用了最左索引 a
mysql> explain select * from test where b=4 and a=4;
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
| 1 | SIMPLE | test | NULL | index_merge | a,b | a,b | 5,5 | NULL | 1 | 100.00 | Using intersect(a,b); Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
1 row in set, 1 warning (0.00 sec结果说明：a满足条件,b满足条件，最左索引a符合条件等于b(大于2)，使用了索引 a,b
总结：根据实验大概得知结果
- and查询会根据关联性高(符合该条件的行数少)选择具体走哪个索引，
- 如果关联性相等(符合该条件的行数相同)，满足一条，会走最左索引，大于一条，会走两个索引
- and查询会根据关联性高(符合该条件的行数少)，最左索引最小，会选择两个索引，
- 两个都不满足，走最左索引
以上测试结果仅供参考，如有错误，欢迎指正