mysql分页优化

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;