如何优化limit

limit 分页优化方法

   1.子查询优化法
   先找出第一条数据，然后大于等于这条数据的id就是要获取的数据
   缺点：数据必须是连续的，可以说不能有where条件，where条件会筛选数据，导致数据失去连续性
   实验下
    mysql> set profi=1;
   Query OK, 0 rows affected (0.00 sec)
   mysql> select count(*) from Member;
   +———-+
   | count(*) |
   +———-+
   |   169566 |
   +———-+
   1 row in set (0.00 sec)
   mysql> pager grep !~-
   PAGER set to ‘grep !~-‘
   mysql> select * from Member limit 10, 100;
   100 rows in set (0.00 sec)
   mysql> select * from Member where MemberID >= (select MemberID from Member limit 10,1) limit 100;
   100 rows in set (0.00 sec)
   mysql> select * from Member limit 1000, 100;
   100 rows in set (0.01 sec)
   mysql> select * from Member where MemberID >= (select MemberID from Member limit 1000,1) limit 100;
   100 rows in set (0.00 sec)
   mysql> select * from Member limit 100000, 100;
   100 rows in set (0.10 sec)
   mysql> select * from Member where MemberID >= (select MemberID from Member limit 100000,1) limit 100;
   100 rows in set (0.02 sec)
   mysql> nopager
   PAGER set to stdout
   mysql> show profilesG
   *************************** 1. row ***************************
   Query_ID: 1
   Duration: 0.00003300
      Query: select count(*) from Member
   *************************** 2. row ***************************
   Query_ID: 2
   Duration: 0.00167000
      Query: select * from Member limit 10, 100
   *************************** 3. row ***************************
   Query_ID: 3
   Duration: 0.00112400
      Query: select * from Member where MemberID >= (select MemberID from Member limit 10,1) limit 100
   *************************** 4. row ***************************
   Query_ID: 4
   Duration: 0.00263200
      Query: select * from Member limit 1000, 100
   *************************** 5. row ***************************
   Query_ID: 5
   Duration: 0.00134000
      Query: select * from Member where MemberID >= (select MemberID from Member limit 1000,1) limit 100
   *************************** 6. row ***************************
   Query_ID: 6
   Duration: 0.09956700
      Query: select * from Member limit 100000, 100
   *************************** 7. row ***************************
   Query_ID: 7
   Duration: 0.02447700
      Query: select * from Member where MemberID >= (select MemberID from Member limit 100000,1) limit 100
    从结果中可以得知，当偏移1000以上使用子查询法可以有效的提高性能。
   2.倒排表优化法
   倒排表法类似建立索引，用一张表来维护页数，然后通过高效的连接得到数据
   缺点：只适合数据数固定的情况，数据不能删除，维护页表困难
   3.反向查找优化法
   当偏移超过一半记录数的时候，先用排序，这样偏移就反转了
   缺点：order by优化比较麻烦，要增加索引，索引影响数据的修改效率，并且要知道总记录数
   ，偏移大于数据的一半
   引用
   limit偏移算法：
   正向查找： (当前页 – 1) * 页长度
   反向查找：总记录 – 当前页 * 页长度
   做下实验，看看性能如何
   总记录数：1,628,775
   每页记录数： 40
   总页数：1,628,775 / 40 = 40720
   中间页数：40720 / 2 = 20360
   第21000页
   正向查找SQL:
   Sql代码
   SELECT * FROM `abc` WHERE `BatchID` = 123 LIMIT 839960, 40
   时间：1.8696 秒
   反向查找sql:
   Sql代码
   SELECT * FROM `abc` WHERE `BatchID` = 123 ORDER BY InputDate DESC LIMIT 788775, 40
   时间：1.8336 秒
   第30000页
   正向查找SQL:
   Sql代码
   1.SELECT * FROM `abc` WHERE `BatchID` = 123 LIMIT 1199960, 40
   SELECT * FROM `abc` WHERE `BatchID` = 123 LIMIT 1199960, 40
   时间：2.6493 秒
   反向查找sql:
   Sql代码
   1.SELECT * FROM `abc` WHERE `BatchID` = 123 ORDER BY InputDate DESC LIMIT 428775, 40
   SELECT * FROM `abc` WHERE `BatchID` = 123 ORDER BY InputDate DESC LIMIT 428775, 40
    时间：1.0035 秒
   注意，反向查找的结果是是降序desc的，并且InputDate是记录的插入时间，也可以用主键联合索引，但是不方便。
   4.limit限制优化法
   把limit偏移量限制低于某个数。。超过这个数等于没数据，我记得alibaba的dba说过他们是这样做的

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