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正文
数据库有中1千万条数据，如下所示：
mysql> select count(*) from score;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (1.89 sec)
mysql> select * from score limit 0,5;
+----+------+
| id | num |
+----+------+
| 1 | 695 |
| 2 | 3635 |
| 3 | 7766 |
| 4 | 4574 |
| 5 | 134 |
+----+------+
5 rows in set (0.00 sec)
mysql> select * from score order by id desc limit 0,5;
+----------+------+
| id | num |
+----------+------+
| 10000000 | 1230 |
| 9999999 | 4717 |
| 9999998 | 4719 |
| 9999997 | 3636 |
| 9999996 | 9254 |
+----------+------+
5 rows in set (0.00 sec)做分页查询时，当offset较小时，查询效率很高；但是当offset较大时，查询效率很容易达到秒级。如下所示：
mysql> select * from score limit 0,5;
+----+------+
| id | num |
+----+------+
| 1 | 695 |
| 2 | 3635 |
| 3 | 7766 |
| 4 | 4574 |
| 5 | 134 |
+----+------+
5 rows in set (0.00 sec)
mysql> select * from score limit 9000000,5;
+---------+------+
| id | num |
+---------+------+
| 9000001 | 507 |
| 9000002 | 5379 |
| 9000003 | 3095 |
| 9000004 | 1925 |
| 9000005 | 1308 |
+---------+------+
5 rows in set (2.68 sec)网上能找到的优化方法主要有两种：
一种是，将select * from score limit 9000000,5改写为select * from score as a inner join (select id from score limit 9000000,5) as b on a.id=b.id;，先查询出偏移后的id，然后再取出对应的数据，个人感觉这种优化程度还是不够好。不过当表中的字段较多时，应该会有较大的提升。
第二种是，将select * from score limit 9000000,5改写为select * from score where id>=9000000 limit 0,5;，这样能做到和查询前5条数据基本一样的效果。但是不足是需要记录上一页最后一个id的值。
以上两种情况可以根据实际场景选择。