一，在InnoDB引擎上测试
1，检查表格引擎，这里是InnoDB

mysql>  show table status from gd where name = 'app_channel_code_info';
+-----------------------+--------+---------+------------+--------+----------------+-------------+
| Name                  | Engine | Version | Row_format | Rows   | Avg_row_length | Data_length | 
+-----------------------+--------+---------+------------+--------+----------------+-------------+
| app_channel_code_info | InnoDB |      10 | Compact    | 163371 |            343 |    56180736 |               
+-----------------------+--------+---------+------------+--------+----------------+-------------+
1 row in set (0.19 sec)

2，不加条件进行全表count(*)，用时0.93秒

mysql> SELECT count(*) FROM `app_channel_code_info`;
+----------+
| count(*) |
+----------+
|   169666 |
+----------+
1 row in set (0.93 sec)

3，添加条件进行count(*)，用时0.20秒

mysql> SELECT count(*) FROM `app_channel_code_info` where update_time > '20120427172452';
+----------+
| count(*) |
+----------+
|   169628 |
+----------+
1 row in set (0.20 sec)

二，在MyISAM上测试
1，更改表格引擎，并检查，这里已经改为MyISAM

mysql> alter table `app_channel_code_info` engine=MyISAM;
Query OK, 169666 rows affected (15.49 sec)
Records: 169666  Duplicates: 0  Warnings: 0

mysql> show table status from gd where name = 'app_channel_code_info';
+-----------------------+--------+---------+------------+--------+----------------+-------------+
| Name                  | Engine | Version | Row_format | Rows   | Avg_row_length | Data_length | 
+-----------------------+--------+---------+------------+--------+----------------+-------------+
| app_channel_code_info | MyISAM |      10 | Dynamic    | 169666 |            171 |    29058532 | 
+-----------------------+--------+---------+------------+--------+----------------+-------------+
1 row in set (0.01 sec)

2，不加条件进行count(*)，用时0.01秒

mysql> SELECT count(*) FROM `app_channel_code_info`;
+----------+
| count(*) |
+----------+
|   169666 |
+----------+
1 row in set (0.01 sec)

3，添加条件进行count(*)，用时0.22秒；

mysql> SELECT count(*) FROM `app_channel_code_info` where update_time > '20120427172452';
+----------+
| count(*) |
+----------+
|   169628 |
+----------+
1 row in set (0.22 sec)

三，结论：
1，不加条件进行全表count(*)，MyISAM引擎用时0.01秒，而InnoDB用时0.93秒，前者明显优于后者；这是因为InnoDB会进行全表扫描；
2，添加条件进行count(*)，MyISAM用时0.22秒，InnoDB用时0.20，效率相当；