问题描述
我想知道哪种并发控制更有效，是悲观还是乐观并发控制？ SQL Server 2005使用悲观并发控制作为默认值是否有特殊原因？是出于与性能相关的原因吗？
I would like to know which concurrency control is more efficient, pessimistic or optimistic concurrency control? Is there a special reason why SQL Server 2005 uses pessimistic concurrency control as default? Is it for performance related reasons?
谢谢期待
推荐答案
我不确定 SQL Server 2005默认使用悲观并发控制是什么意思。 IMO SQL Server 2005为我们提供了一些工具，使我们能够自己实现乐观或悲观。我在简单对话中写了一些示例：
I am not sure what do you mean by "SQL Server 2005 uses pessimistic concurrency control as default". IMO SQL Server 2005 provides us with tools that allow us to implement optimistic or pessimistic ourselves. I wrote a few examples on simple-talk: Developing Modifications that Survive Concurrency
编辑：我不认为SQL Server的默认行为完全是悲观并发控制。让我们考虑以下简单示例，该示例在默认隔离级别READ COMMITTED下运行：
I do not think that the default behavior of SQL Server is exactly "pessimistic concurrency control". Let us consider the following simple example, which runs under default isolation level, READ COMMITTED:
-- Connection one
BEGIN TRANSACTION;
SELECT * FROM Schedule
WHERE ScheduledTime BETWEEN '20110624 06:30:00'
AND '20110624 11:30' ;
-- Connection two
UPDATE Schedule
SET Priority = 'High'
WHERE ScheduledTime ='20110624 08:45:00'
-- nothing prevent this update from completing,
-- so this is not exactly pessimistic
-- Connection one
DELETE FROM Schedule
WHERE ScheduledTime ='20110624 08:45:00' ;
COMMIT ;
-- nothing prevents us from deleting
-- the modified row
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