mysql> select count(*) from tf_user_index where  age = 30;
+----------+
| count(*) |
+----------+
|   196477 |
+----------+
1 row in set (1.66 sec)

mysql> select count(*) from tf_user_index where  score = 30;
+----------+
| count(*) |
+----------+
|   126306 |
+----------+
1 row in set (0.03 sec)

mysql> explain select count(*) from tf_user_index where  age = 30;
+----+-------------+---------------+-------+---------------+---------------+---------+------+---------+--------------------------+
| id | select_type | table         | type  | possible_keys | key           | key_len | ref  | rows    | Extra                    |
+----+-------------+---------------+-------+---------------+---------------+---------+------+---------+--------------------------+
|  1 | SIMPLE      | tf_user_index | index | NULL          | score_sex_age | 6       | NULL | 8962455 | Using where; Using index |
+----+-------------+---------------+-------+---------------+---------------+---------+------+---------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select count(*) from tf_user_index where  score = 30;
+----+-------------+---------------+------+---------------+---------------+---------+-------+--------+-------------+
| id | select_type | table         | type | possible_keys | key           | key_len | ref   | rows   | Extra       |
+----+-------------+---------------+------+---------------+---------------+---------+-------+--------+-------------+
|  1 | SIMPLE      | tf_user_index | ref  | score_sex_age | score_sex_age | 4       | const | 241442 | Using index |
+----+-------------+---------------+------+---------------+---------------+---------+-------+--------+-------------+
1 row in set (0.01 sec)
 

score排第一，查詢時間很快。age排第三，查詢要1秒多時間。

mysql> select count(*) from tf_user where  score = 30;
+----------+
| count(*) |
+----------+
|   126306 |
+----------+
1 row in set (3.37 sec)

mysql> select count(*) from tf_user where  age = 30;
+----------+
| count(*) |
+----------+
|   196477 |
+----------+
1 row in set (2.44 sec)
 

沒有複合索引，都需要好幾秒時間。複合索引有一定的效果，但是單獨查詢時，對排第一的更有效。

調換一下順序，現在將age排第一。

mysql> select count(*) from tf_user_index where  age = 30;
+----------+
| count(*) |
+----------+
|   196477 |
+----------+
1 row in set (0.05 sec)

mysql> select count(*) from tf_user_index where  score = 30;
+----------+
| count(*) |
+----------+
|   126306 |
+----------+
1 row in set (1.82 sec)

mysql> explain select count(*) from tf_user_index where  score = 30;
+----+-------------+---------------+-------+---------------+---------------+---------+------+---------+--------------------------+
| id | select_type | table         | type  | possible_keys | key           | key_len | ref  | rows    | Extra                    |
+----+-------------+---------------+-------+---------------+---------------+---------+------+---------+--------------------------+
|  1 | SIMPLE      | tf_user_index | index | NULL          | age_score_sex | 6       | NULL | 8962455 | Using where; Using index |
+----+-------------+---------------+-------+---------------+---------------+---------+------+---------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select count(*) from tf_user_index where  age = 30;
+----+-------------+---------------+------+---------------+---------------+---------+-------+--------+-------------+
| id | select_type | table         | type | possible_keys | key           | key_len | ref   | rows   | Extra       |
+----+-------------+---------------+------+---------------+---------------+---------+-------+--------+-------------+
|  1 | SIMPLE      | tf_user_index | ref  | age_score_sex | age_score_sex | 1       | const | 408456 | Using index |
+----+-------------+---------------+------+---------------+---------------+---------+-------+--------+-------------+
1 row in set (0.00 sec)
 

排第一很重要。

mysql> explain select count(*) from tf_user_index where  age = 30 and score =30;
+----+-------------+---------------+------+---------------+---------------+---------+-------------+------+-------------+
| id | select_type | table         | type | possible_keys | key           | key_len | ref         | rows | Extra       |
+----+-------------+---------------+------+---------------+---------------+---------+-------------+------+-------------+
|  1 | SIMPLE      | tf_user_index | ref  | age_score_sex | age_score_sex | 5       | const,const | 2467 | Using index |
+----+-------------+---------------+------+---------------+---------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

mysql> select count(*) from tf_user_index where  age = 30 and score =30;
+----------+
| count(*) |
+----------+
|     2468 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from tf_user_index where  score =30 and age = 30;
+----------+
| count(*) |
+----------+
|     2468 |
+----------+
1 row in set (0.01 sec)

mysql> select count(*) from tf_user_index where  score =30 and sex = 2;
+----------+
| count(*) |
+----------+
|    63031 |
+----------+
1 row in set (2.06 sec)

只要帶到age，查詢就很快。

聯合索引中，第一個位置很重要。