sql-server - 锁问题 - 'U' 锁与 'X' 锁

Question

我有几个关于更新 (U) 锁和排他 (X) 锁的问题。

1)当资源即将更新时，我是否正确地将“X”锁放在资源上？

2)我对U锁有点模糊。当读取资源并且 SQL Server 认为它可能需要稍后更新资源时应用 U 锁，我是否正确？如果这是正确的，是否只有在事务上下文中进行读取时才会应用“U”锁？我想我试图了解在什么情况下 SQL Server 认为它可能需要稍后更新它现在刚刚读取的行。

谢谢 - 兰迪

Answer 1

1) Am I correct that an 'X' lock is put on a resource when the resource is about to get updated?

是的。

2) I'm a little fuzzy on U locks. Am I correct that a U lock is applied when a resource is read and SQL Server thinks it might need to update the resource later? If this is correct, would a 'U' lock only get applied when a read is being done within the context of a transaction? I guess I'm trying to understand under what circumstances SQL Server thinks it might need to update later a row it just read now.

U锁与读锁兼容，但彼此不兼容， X锁与读锁不兼容。
U锁由 DML 放置查询 ( UPDATE , DELETE , MERGE ) 而 扫描 表行(尚未决定更新)，而 X当决定更新行时，就会放置锁。

在 READ COMMITTED隔离模式，更新锁在记录被评估为保持原样后解除，在更高的隔离模式下，它们会一直保留到事务结束。