数据库-事务-乐观锁-调优

关系型数据库

基于关系代数理论

缺点：表结构不直观，实现复杂，速度慢
优点：健壮性高，社区庞大

示例：
   product表
           productId   productName   categoryId   price
       1   4           toyota       2           100000
       2   3           porsche       2           1000000
       3   2           addidas       1           500
       4   1           nike       null       600

   category
           categoryId   categoryName
       1   2           automobile
       2   1            shoe

select * from 'product' join 'category' 没有条件会发生笛卡尔积

select * from 'product' p join 'category' c
on p.'categoryId' = c.'categoryId'

   左外连接，左表都有，
   select * from 'product'   p left join 'category' c
   on p.'categoryId' = c.'categoryId'

   右外连接
   select * from 'product'   p left join 'category' c
   on p.'categoryId' = c.'categoryId'

   select * from 'product'   p left join 'category' c
   on p.'categoryId' = c.'categoryId'
   group by p.'categoryId'

   select * from 'producet' p left join (
       select p.'categoryId', 'categoryName', MIN(p.'price') as min_price from 'product' p left join 'category' c
       on p.'categoryId' = c.'categoryId'
       group by p.'categoryId','categoryName') as cat_min
   on p.'categoryId' = cat_min.categoryId
   where p.'price' = cat_min.price

事务
   ACID
       Atomicity
       Consistency
       Isolation
       Durability

   事务的隔离级别
       Read uncommitted
       Read Committed
       Repeatable Reads
       Serializable

   product表
           productId   productName   categoryId   price        count
       1   4           toyota       2           100000
       2   3           porsche       2           1000000
       3   2           addidas       1           500        50
       4   1           nike       null       600

   set session transaction isolation level read committed;
   begin;
   set autocommit = 0;
   select count from 'product' where 'productId'=2;

update 'product' set 'count'=49 where 'productId' = 2;

select count from 'product' where 'productId'=2 for update; //会被锁掉，事务做完才会运行
commit;

update 'product' where set count = count-1 where 'productId' = 2;

乐观锁

   演示：
       读取数据，记录Timestamp
       修改数据
       检查和提交数据

select count from 'product' where 'productId' = 2;
update 'product' set 'count' = 46 where 'productId' =2 and 'count'=47; //and后这个就是乐观锁，设置46之前需要读到count为47

程序调优：
   改善数据访问方式以提升缓存命中率
   利用数据库连接池替代直接的数据库访问
   使用迭代替代递归
   合并多个远程调用批量发送
   共享冗余数据提高访问效率

数据库-事务-乐观锁-调优

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