关于分页的优化。
我们知道，在MySQL中分页很简单，直接LIMIT page_no,page_total 就可以了。
可是当记录数慢慢增大时，她就不那么好使了。
这里我们创建摘要表来记录页码和原表之间的关联。
下面为测试数据。
原表：
CREATE TABLE `t_group` (
`id` int(11) NOT NULL auto_increment,
`money` decimal(10,2) NOT NULL,
`user_name` varchar(20) NOT NULL,
`create_time` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
KEY `idx_combination1` (`user_name`,`money`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
原表总记录数：
mysql> select count(*) from t_group;
+----------+
| count(*) |
+----------+
| 10485760 |
+----------+
1 row in set (0.00 sec)
分页表：
CREATE TABLE `t_group_ids` (
`id` int(11) NOT NULL,
`group_id` int(11) NOT NULL,
PRIMARY KEY (`id`,`group_id`),
KEY `idx_id` (`id`),
KEY `idx_group_id` (`group_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
插入分页表数据。当然这里如果你的表主键不是ID,那你得自己想办法搞这个分页表的数据了。这个好实现，就不说了。
mysql> insert into t_group_ids select ceil(id/20),id from t_group;
Query OK, 10485760 rows affected (2 min 56.19 sec)
Records: 10485760 Duplicates: 0 Warnings: 0
现在来看看对比数据。
用普通LIMIT来实现分页。
mysql> select * from t_group where 1 limit 20;
+----+--------+-----------+---------------------+
| id | money | user_name | create_time |
+----+--------+-----------+---------------------+
| 1 | 50.23 | david | 2008-10-23 12:55:49 |
| 2 | 55.23 | livia | 2008-10-23 10:02:09 |
| 3 | 100.83 | leo | 2008-10-23 10:02:22 |
| 4 | 99.99 | lucy | 2008-10-23 10:02:39 |
| 5 | 299.99 | simon | 2008-10-23 10:02:52 |
| 6 | 599.99 | sony | 2008-10-23 10:03:03 |
| 7 | 599.99 | rick | 2008-10-23 10:03:12 |
| 8 | 9.99 | anne | 2008-10-23 10:03:47 |
| 9 | 9.99 | sarah | 2008-10-23 10:04:31 |
| 10 | 900.99 | john | 2008-10-23 10:04:50 |
| 11 | 0.23 | david | 2008-10-23 10:05:31 |
| 12 | 5.23 | livia | 2008-10-23 10:05:31 |
| 13 | 50.83 | leo | 2008-10-23 10:05:31 |
| 14 | 49.99 | lucy | 2008-10-23 10:05:31 |
| 15 | 249.99 | simon | 2008-10-23 10:05:31 |
| 16 | 549.99 | sony | 2008-10-23 10:05:31 |
| 17 | 549.99 | rick | 2008-10-23 10:05:31 |
| 18 | -40.01 | anne | 2008-10-23 10:05:31 |
| 19 | -40.01 | sarah | 2008-10-23 10:05:31 |
| 20 | 850.99 | john | 2008-10-23 10:05:31 |
+----+--------+-----------+---------------------+
20 rows in set (0.01 sec)
用分页表来实现分页。
mysql> select a.* from t_group as a inner join t_group_ids as b where a.id = b.g
roup_id and b.id = 1;
+----+--------+-----------+---------------------+
| id | money | user_name | create_time |
+----+--------+-----------+---------------------+
| 1 | 50.23 | david | 2008-10-23 12:55:49 |
| 2 | 55.23 | livia | 2008-10-23 10:02:09 |
| 3 | 100.83 | leo | 2008-10-23 10:02:22 |
| 4 | 99.99 | lucy | 2008-10-23 10:02:39 |
| 5 | 299.99 | simon | 2008-10-23 10:02:52 |
| 6 | 599.99 | sony | 2008-10-23 10:03:03 |
| 7 | 599.99 | rick | 2008-10-23 10:03:12 |
| 8 | 9.99 | anne | 2008-10-23 10:03:47 |
| 9 | 9.99 | sarah | 2008-10-23 10:04:31 |
| 10 | 900.99 | john | 2008-10-23 10:04:50 |
| 11 | 0.23 | david | 2008-10-23 10:05:31 |
| 12 | 5.23 | livia | 2008-10-23 10:05:31 |
| 13 | 50.83 | leo | 2008-10-23 10:05:31 |
| 14 | 49.99 | lucy | 2008-10-23 10:05:31 |
| 15 | 249.99 | simon | 2008-10-23 10:05:31 |
| 16 | 549.99 | sony | 2008-10-23 10:05:31 |
| 17 | 549.99 | rick | 2008-10-23 10:05:31 |
| 18 | -40.01 | anne | 2008-10-23 10:05:31 |
| 19 | -40.01 | sarah | 2008-10-23 10:05:31 |
| 20 | 850.99 | john | 2008-10-23 10:05:31 |
+----+--------+-----------+---------------------+
20 rows in set (0.00 sec)
取第50W页的数据。
原来表：
mysql> select * from t_group where 1 limit 9999980,20;
+----------+---------+-----------+---------------------+
| id | money | user_name | create_time |
+----------+---------+-----------+---------------------+
| 9999981 | 810.13 | david | 2008-10-23 10:09:24 |
| 9999982 | 815.13 | livia | 2008-10-23 10:09:24 |
| 9999983 | 860.73 | leo | 2008-10-23 10:09:24 |
| 9999984 | 859.89 | lucy | 2008-10-23 10:09:24 |
| 9999985 | 1059.89 | simon | 2008-10-23 10:09:24 |
| 9999986 | 1359.89 | sony | 2008-10-23 10:09:24 |
| 9999987 | 1359.89 | rick | 2008-10-23 10:09:24 |
| 9999988 | 769.89 | anne | 2008-10-23 10:09:24 |
| 9999989 | 769.89 | sarah | 2008-10-23 10:09:24 |
| 9999990 | 1660.89 | john | 2008-10-23 10:09:24 |
| 9999991 | 760.13 | david | 2008-10-23 10:09:24 |
| 9999992 | 765.13 | livia | 2008-10-23 10:09:24 |
| 9999993 | 810.73 | leo | 2008-10-23 10:09:24 |
| 9999994 | 809.89 | lucy | 2008-10-23 10:09:24 |
| 9999995 | 1009.89 | simon | 2008-10-23 10:09:24 |
| 9999996 | 1309.89 | sony | 2008-10-23 10:09:24 |
| 9999997 | 1309.89 | rick | 2008-10-23 10:09:24 |
| 9999998 | 719.89 | anne | 2008-10-23 10:09:24 |
| 9999999 | 719.89 | sarah | 2008-10-23 10:09:24 |
| 10000000 | 1610.89 | john | 2008-10-23 10:09:24 |
+----------+---------+-----------+---------------------+
20 rows in set (4.21 sec)
分页表：
mysql> select a.* from t_group as a inner join t_group_ids as b where a.id = b.g
roup_id and b.id = 500000;
+----------+---------+-----------+---------------------+
| id | money | user_name | create_time |
+----------+---------+-----------+---------------------+
| 9999981 | 810.13 | david | 2008-10-23 10:09:24 |
| 9999982 | 815.13 | livia | 2008-10-23 10:09:24 |
| 9999983 | 860.73 | leo | 2008-10-23 10:09:24 |
| 9999984 | 859.89 | lucy | 2008-10-23 10:09:24 |
| 9999985 | 1059.89 | simon | 2008-10-23 10:09:24 |
| 9999986 | 1359.89 | sony | 2008-10-23 10:09:24 |
| 9999987 | 1359.89 | rick | 2008-10-23 10:09:24 |
| 9999988 | 769.89 | anne | 2008-10-23 10:09:24 |
| 9999989 | 769.89 | sarah | 2008-10-23 10:09:24 |
| 9999990 | 1660.89 | john | 2008-10-23 10:09:24 |
| 9999991 | 760.13 | david | 2008-10-23 10:09:24 |
| 9999992 | 765.13 | livia | 2008-10-23 10:09:24 |
| 9999993 | 810.73 | leo | 2008-10-23 10:09:24 |
| 9999994 | 809.89 | lucy | 2008-10-23 10:09:24 |
| 9999995 | 1009.89 | simon | 2008-10-23 10:09:24 |
| 9999996 | 1309.89 | sony | 2008-10-23 10:09:24 |
| 9999997 | 1309.89 | rick | 2008-10-23 10:09:24 |
| 9999998 | 719.89 | anne | 2008-10-23 10:09:24 |
| 9999999 | 719.89 | sarah | 2008-10-23 10:09:24 |
| 10000000 | 1610.89 | john | 2008-10-23 10:09:24 |
+----------+---------+-----------+---------------------+
20 rows in set (0.03 sec)
我们来取最后一页的数据。
原表：
mysql> select * from t_group where 1 limit 10485740,20;
+----------+---------+-----------+---------------------+
| id | money | user_name | create_time |
+----------+---------+-----------+---------------------+
| 10485741 | 1935.42 | david | 2008-10-23 10:09:24 |
| 10485742 | 1955.42 | livia | 2008-10-23 10:09:24 |
| 10485743 | 2137.82 | leo | 2008-10-23 10:09:24 |
| 10485744 | 2134.46 | lucy | 2008-10-23 10:09:24 |
| 10485745 | 2934.46 | simon | 2008-10-23 10:09:24 |
| 10485746 | 4134.46 | sony | 2008-10-23 10:09:24 |
| 10485747 | 4134.46 | rick | 2008-10-23 10:09:24 |
| 10485748 | 1774.46 | anne | 2008-10-23 10:09:24 |
| 10485749 | 1774.46 | sarah | 2008-10-23 10:09:24 |
| 10485750 | 5338.46 | john | 2008-10-23 10:09:24 |
| 10485751 | 1735.42 | david | 2008-10-23 10:09:24 |
| 10485752 | 1755.42 | livia | 2008-10-23 10:09:24 |
| 10485753 | 1937.82 | leo | 2008-10-23 10:09:24 |
| 10485754 | 1934.46 | lucy | 2008-10-23 10:09:24 |
| 10485755 | 2734.46 | simon | 2008-10-23 10:09:24 |
| 10485756 | 3934.46 | sony | 2008-10-23 10:09:24 |
| 10485757 | 3934.46 | rick | 2008-10-23 10:09:24 |
| 10485758 | 1574.46 | anne | 2008-10-23 10:09:24 |
| 10485759 | 1574.46 | sarah | 2008-10-23 10:09:24 |
| 10485760 | 5138.46 | john | 2008-10-23 10:09:24 |
+----------+---------+-----------+---------------------+
20 rows in set (4.88 sec)
分页表：
mysql> select a.* from t_group as a inner join t_group_ids as b where a.id = b.g
roup_id and b.id = 524288;
+----------+---------+-----------+---------------------+
| id | money | user_name | create_time |
+----------+---------+-----------+---------------------+
| 10485741 | 1935.42 | david | 2008-10-23 10:09:24 |
| 10485742 | 1955.42 | livia | 2008-10-23 10:09:24 |
| 10485743 | 2137.82 | leo | 2008-10-23 10:09:24 |
| 10485744 | 2134.46 | lucy | 2008-10-23 10:09:24 |
| 10485745 | 2934.46 | simon | 2008-10-23 10:09:24 |
| 10485746 | 4134.46 | sony | 2008-10-23 10:09:24 |
| 10485747 | 4134.46 | rick | 2008-10-23 10:09:24 |
| 10485748 | 1774.46 | anne | 2008-10-23 10:09:24 |
| 10485749 | 1774.46 | sarah | 2008-10-23 10:09:24 |
| 10485750 | 5338.46 | john | 2008-10-23 10:09:24 |
| 10485751 | 1735.42 | david | 2008-10-23 10:09:24 |
| 10485752 | 1755.42 | livia | 2008-10-23 10:09:24 |
| 10485753 | 1937.82 | leo | 2008-10-23 10:09:24 |
| 10485754 | 1934.46 | lucy | 2008-10-23 10:09:24 |
| 10485755 | 2734.46 | simon | 2008-10-23 10:09:24 |
| 10485756 | 3934.46 | sony | 2008-10-23 10:09:24 |
| 10485757 | 3934.46 | rick | 2008-10-23 10:09:24 |
| 10485758 | 1574.46 | anne | 2008-10-23 10:09:24 |
| 10485759 | 1574.46 | sarah | 2008-10-23 10:09:24 |
| 10485760 | 5138.46 | john | 2008-10-23 10:09:24 |
+----------+---------+-----------+---------------------+
20 rows in set (0.01 sec)
总结：我们看到，当表记录数增加时，LIMIT的性能随着线性增长。而当我们存放了页码与主键的关联后，性能大增。