Question

我有一个存储过程来更新表，并且我将隔离级别设置为READ UNCOMMITTED。在此示例中，我设置了注释计数（CommentCount + = 1）。如果同时有一个以上的用户调用此sp，则注释计数的增加量是否可能少于添加注释的用户数？

Answer 1

SQL Server仍以隔离级别READ UNCOMMITTED获取对更新行的锁定。像这样的UPDATE语句在同时由多个READ UNCOMMITTED会话执行时不会丢失增量：

UPDATE dbo.Post
SET CommentCount += 1
WHERE PostID = @PostID;

这是此语句的锁痕，由PostID上的群集主键更新。排他锁将阻止该行的其他并发更新。

+---------------+----+--------------+----------+--+
| Lock:Acquired | 58 | 5 - OBJECT   | 8 - IX   |  |
| Lock:Acquired | 58 | 6 - PAGE     | 8 - IX   |  |
| Lock:Acquired | 58 | 7 - KEY      | 5 - X    |  |
| Lock:Released | 58 | 7 - KEY      | 0 - NULL |  |
| Lock:Released | 58 | 6 - PAGE     | 0 - NULL |  |
| Lock:Released | 58 | 7 - KEY      | 5 - X    |  |
| Lock:Released | 58 | 6 - PAGE     | 8 - IX   |  |
| Lock:Released | 58 | 5 - OBJECT   | 8 - IX   |  |
+---------------+----+--------------+----------+--+

这是使用非聚集主键索引的行所在位置的跟踪信息。非群集键上的更新锁将序列化同一键的其他更新语句，而群集键上的排他锁将阻止其他数据修改。

+---------------+----+------------+----------+--+
| Lock:Acquired | 58 | 5 - OBJECT | 8 - IX   |  |
| Lock:Acquired | 58 | 6 - PAGE   | 7 - IU   |  |
| Lock:Acquired | 58 | 7 - KEY    | 4 - U    |  |
| Lock:Acquired | 58 | 6 - PAGE   | 7 - IU   |  |
| Lock:Acquired | 58 | 7 - KEY    | 4 - U    |  |
| Lock:Acquired | 58 | 6 - PAGE   | 8 - IX   |  |
| Lock:Acquired | 58 | 7 - KEY    | 5 - X    |  |
| Lock:Released | 58 | 7 - KEY    | 0 - NULL |  |
| Lock:Released | 58 | 6 - PAGE   | 0 - NULL |  |
| Lock:Released | 58 | 7 - KEY    | 4 - U    |  |
| Lock:Released | 58 | 6 - PAGE   | 7 - IU   |  |
| Lock:Released | 58 | 7 - KEY    | 5 - X    |  |
| Lock:Released | 58 | 6 - PAGE   | 8 - IX   |  |
| Lock:Released | 58 | 5 - OBJECT | 8 - IX   |  |
+---------------+----+------------+----------+--+

读取多个事务上的未提交隔离级别行为

1 个答案: