Question

I have a question about, how to analyze a query to know performance of its (good or bad).
I searched a lot and got something like below:

SELECT count(*) FROM users; => Many experts said it's bad.

SELECT count(id) FROM users; => Many experts said it's good.

Please see the table:

+---------------+-------------+------+-----+---------+----------------+
| 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    |                |
+---------------+-------------+------+-----+---------+----------------+

But when I try to using EXPLAIN command for that, I got the results:

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)

So, the first thing for me:
Can I understand it’s the same performance?

---

P/S: MySQL version is 5.5.8.

Answer 1

No, you cannot. Explain doesn’t reflect all the work done by mysql, it just gives you a plan of how it will be performed.

What about specifically count(*) vs count(id). The first one is always not slower than the second, and in some cases it is faster.

count(col) semantic is amount of not null values, while count(*) is – amount of rows.

Probably mysql can optimize count(col) by rewriting into count(*) as well as id is a PK thus cannot be NULL (if not – it looks up for NULLS, which is not fast), but I still propose you to use COUNT(*) in such cases.

Also – the internall processes depend on used storage engine. For myisam the precalculated number of rows returned in both cases (as long as you don’t use WHERE).