记录当前页最大id和最小id进行优化
原理是，记录住当前页id的最大值和最小值，计算跳转页面和当前页相对偏移，由于页面相近，这个偏移量不会很大，这样的话m值相对较小，大大 减少扫描的行数。
假设每一页只有10条数据，当前页码数是5，那么最大id是50，最小id是40，只要上一页和下一页
查找下一页
select * from score where sid > 50 LIMIT 10;
查找上一页
select * from score where sid > 50 LIMIT 10;
使用 between and 查找数量不一定够10条，因为id不连续
假设每一页只有10条数据，当前页码数是5，那么最大id是50，最小id是40，然后跳到第8页或者第2页
跳到第8页
以最大值为基准
select * from tb1 where
nid < ( select nid from (select nid from tb1 where nid > 50 limit 30) A order by A.nid desc limit 1 ) # 通过当前页取第8页最大id
order by tb1.nid desc limit 10;
以最小值为基准
select * from tb1 where
nid > ( select nid from (select nid from tb1 where nid > 40 limit 30) A order by A.nid desc limit 1 ) # 通过当前页取第8页最小id
order by tb1.nid asc limit 10;
代码分解：
select nid from tb1 where nid > 50 limit 30
# 通过主键id获取当前页到跳转页最大值id的行数t1，
select nid from t1 order by A.nid desc limit 1
# 然后倒序取t1中的最大值，max_id or min_id
select * from tb1 where nid < max_id order by tb1.nid desc limit 10;
或者
select * from tb1 where nid > min_id order by tb1.nid asc limit 10;
跳到第2页
以最大页为基准
select * from tb1 as t where nid <
(select nid from (select nid from tb1 as t1 where t.nid < 50 limit 30 order by t1.nid desc) A order by A.nid asc limit 1) # 通过当前页取第2页最大id
order by nid desc limit 10;
以最小页为基准
select * from tb1 as t where nid >
( select nid from (select nid from tb1 as t1 where nid < 40 limit 30 order by t1.nid desc) A order by A.nid asc limit 1) # 通过当前页取第2页最小id
limit 10;
代码分解
select nid from tb1 as t1 where nid < 40 limit 30 order by t1.nid desc
# 结果为t1，因为倒序，结果是从大到小，
select nid from t1 A order by A.nid asc limit 1
# 因为是倒序，想取最小值，还得再排序，然后取t1中的max_id or min_id
select * from tb1 where nid < max_id order by tb1.nid desc limit 10;
或者
select * from tb1 where nid > min_id order by tb1.nid asc limit 10;