4
我有一個問題，如何分析查詢以瞭解其性能（好或壞）。 我搜索了很多，得到的東西象下面這樣：分析查詢mysql
SELECT count(*) FROM users; => Many experts said it's bad.
SELECT count(id) FROM users; => Many experts said it's good.
請見下表：
+---------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+----------------+
| userId | int(11) | NO | PRI | NULL | auto_increment |
| f_name | varchar(50) | YES | | NULL | |
| l_name | varchar(50) | YES | | NULL | |
| user_name | varchar(50) | NO | | NULL | |
| password | varchar(50) | YES | | NULL | |
| email | varchar(50) | YES | | NULL | |
| active | char(1) | NO | | Y | |
| groupId | smallint(4) | YES | MUL | NULL | |
| created_date | datetime | YES | | NULL | |
| modified_date | datetime | YES | | NULL | |
+---------------+-------------+------+-----+---------+----------------+
但是，當我嘗試使用EXPLAIN命令，我得到了結果：
EXPLAIN SELECT count(*) FROM `user`;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | user | index | NULL | groupId | 3 | NULL | 83 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
EXPLAIN SELECT count（userId）FROM user;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | user | index | NULL | groupId | 3 | NULL | 83 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
所以，我的第一件事就是： 我能理解這是相同的性能呢？
P/S：MySQL版本是5.5.8。