經(jīng)常碰到的一個(gè)問題是limit的offset太高，如：limit 100000,20，這樣系統(tǒng)會(huì)查詢100020條，然后把前面的100000條都扔掉，這是開銷很大的操作，導(dǎo)致查詢很慢。假設(shè)所有分頁的頁面訪問頻率一樣，這樣的查詢平均掃描表的一半數(shù)據(jù)。優(yōu)化的方法，要么限制訪問后面的頁數(shù)，要么提升高偏移的查詢效率。

一個(gè)簡單的優(yōu)化辦法是使用覆蓋查詢（covering index）查詢，然后再跟全行的做join操作。如：

復(fù)制代碼 代碼如下:

SQL>select * from user_order_info limit 1000000,5;

復(fù)制代碼 代碼如下:

select * from user_order_info inner join (select pin from user_order_info limit 1000000,5) as lim using(pin);
SQL>explain select * from user_order_info limit 1000000,5;
+----+-------------+-----------------+------+---------------+------+---------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+------+---------------+------+---------+------+----------+-------+
| 1 | SIMPLE | user_order_info | ALL | NULL | NULL | NULL | NULL | 23131886 | |
+----+-------------+-----------------+------+---------------+------+---------+------+----------+-------+
1 row in set (0.00 sec)
SQL>explain extended select * from user_order_info inner join (select pin from user_order_info limit 1000000,5) as lim using(pin);
+----+-------------+-----------------+--------+---------------+---------+---------+---------+----------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------+--------+---------------+---------+---------+---------+----------+----------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | |
| 1 | PRIMARY | user_order_info | eq_ref | PRIMARY | PRIMARY | 42 | lim.pin | 1 | 100.00 | |
| 2 | DERIVED | user_order_info | index | NULL | PRIMARY | 42 | NULL | 23131886 | 100.00 | Using index |
+----+-------------+-----------------+--------+---------------+---------+---------+---------+----------+----------+-------------+
3 rows in set, 1 warning (0.66 sec)