复合索引，排第一的效果
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，查询时间很快。age排第三，查询要1秒多时间。
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)
没有复合索引，都需要好几秒时间。复合索引有一定的效果，但是单独查询时，对排第一的更有效。
调换一下顺序，现在将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)
只要带到age，查询就很快。
