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优化总结：

1.任何情况下SELECT COUNT(*) FROM xxx 是最优选择；
2.尽量减少SELECT COUNT(*) FROM xxx WHERE COL = ‘xxx’ 这种查询；
3.杜绝SELECT COUNT(COL) FROM tablename WHERE COL = ‘xxx’ 的出现。(其中COL非主键)

环境：
MySQL版本：5.0.45
OS：Windows XP SP3

数据表一：sphinx
+—-+——+–+–+—+——+
| Field | Type | Null | Key | Default | Extra |
+—-+——+–+–+—+——+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| til | varchar(100) | NO | | | |
| content | text | NO | | | |
| dataline | int(11) | NO | | | |
+—-+——+–+–+—+——+

记录数：1120100

查询一：

mysql> select count(*) as totalnum from sphinx;
+—-+
| totalnum |
+—-+
| 1120100 |
+—-+
1 row in set (0.00 sec)

查询二：

mysql> select count(*) as totalnum from sphinx where id>1000;
+—-+
| totalnum |
+—-+
| 1119100 |
+—-+
1 row in set (2.17 sec)

查询三：

mysql> select count(*) as totalnum from sphinx where id>1000;
+—-+
| totalnum |
+—-+
| 1119100 |
+—-+
1 row in set (0.61 sec)

查询四：

mysql> select count(*) as totalnum from sphinx where id>1000;
+—-+
| totalnum |
+—-+
| 1119100 |
+—-+
1 row in set (0.61 sec)

查询五：

mysql> select count(id) as totalnum from sphinx;
+—-+
| totalnum |
+—-+
| 1120100 |
+—-+
1 row in set (0.00 sec)

查询六：

mysql> select count(til) as totalnum from sphinx where id>1000;
+—-+
| totalnum |
+—-+
| 1119100 |
+—-+
1 row in set (1 min 38.61 sec)

查询七：

mysql> select count(id) as totalnum from sphinx where id>11000;
+—-+
| totalnum |
+—-+
| 1109100 |
+—-+
1 row in set (0.61 sec)

查询八：

mysql> select count(id) as totalnum from sphinx;
+—-+
| totalnum |
+—-+
| 1120100 |
+—-+
1 row in set (0.03 sec)

结论：

在 select count() 没有 where 条件的时候 select count(*) 和 select count(col) 所消耗的查询时间相差无几。
在 select count() 有 where 条件的时候 select count(col) 所消耗的查询时间 比 select count(*) 明显多出数量级的时间。