sybase锁机制以及死锁避免

在数据库实现中，通过锁定机制控制数据库的并发访问，保证数据库访问的正确性。根据定义：

锁定是一种并发控制机制：它可以确保数据在同一事务中和不同事务之间保持一致。在多用户环境中，由于几个用户可能会在同一时间使用同一数据，因此需要锁定功能。

 sybase锁分类

按照锁性质可以分为共享锁，排他锁。当在数据库事务中，读取信息时，会对数据库添加共享锁。当修改信息时，会添加排他锁。

按照锁的粒度，可以分为行锁，页锁，表锁等。

 sybase隔离级别

sybase分为0,1,2,3四个隔离级别。

0 读取未提交的，允许事务读取未提交的数据更改，排他锁在对数据库进行写操作后立即释放，不会持有到事务提交或回滚。
1 读取已提交的，仅允许事务读取已提交的数据更改，排他锁持有到事务提交或回滚，但共享锁在加载数据到内存后立即释放。
2 可重复读取事务可重复同一查询，事务读取过的任何行都不会被更新或删除，排它锁和共享锁都会持有到事务结束，查询结果集不可以删除和修改，但是可以插入。
3 可串行化读取事务可重复同一查询，且得到完全相同的结果。不能插入任何将出现在结果集中的行，排它锁和共享锁都会持有到事务结束，查询结果集不可以增删改。

可以使用select @@isolation语句查看数据库的隔离级别。

sybase数据库通常隔离界别设置为1，值得注意的是使用WAS通过jdbc连接数据库上，经常会将隔离级别提升为2。在使用E-SQL编译时，通常将隔离级别提升为3。

 sybase死锁

sybase数据库出现死锁，即处于死锁中的各事务，都持有锁，但又都在等待其他锁，从而组成一个环，造成死锁。

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