两个指数陷入僵局

Question

我在SQL Server(Azure SQL)中有一个表:

CREATE TABLE Commands
(
    Id int NOT NULL PRIMARY KEY,
    Body nvarchar(1000) NOT NULL,
    Priority int NOT NULL,
    DeliveryDate datetime NOT NULL,
    VisibleFrom datetime NULL,
)

该表还有一个索引:

CREATE NONCLUSTERED INDEX IX_PriorityAndDate ON Commands (Priority DESC, DeliveryDate ASC)

然后我有两个会话.

第1节

WITH command AS
(
    SELECT TOP(1) *
    FROM Commands q
    WHERE q.DeliveryDate <= @CurrentDate
      AND (q.VisibleFrom IS NULL OR q.VisibleFrom <= @CurrentDate)
    ORDER BY q.Priority DESC, q.DeliveryDate
)
UPDATE command SET command.VisibleFrom = DATEADD(SECOND, @LeaseTimeout, @CurrentDate)
OUTPUT inserted.Id,
       inserted.Body

第二节

DELETE FROM Commands WHERE Id = @Id

在某些情况下会发生死锁:

1. 会话1锁定IX_PriorityAndDate索引(U锁定).
2. 会话2锁定PK_Commands索引(X锁定).
3. 会话1阻止等待PK_Commands(获取U锁).
4. 会话2阻止等待IX_PriorityAndDate(获取X锁).

如何解决这个僵局？

Answer 1

在我看来，你遇到了密钥查找僵局

尝试使用以下索引：

CREATE NONCLUSTERED INDEX IX_PriorityAndDate
  ON Commands (Priority DESC, DeliveryDate ASC) INCLUDE(VisibleFrom)

或者尝试创建一个新索引：

CREATE NONCLUSTERED INDEX IX_PriorityAndDate
  ON Commands(DeliveryDate, VisibleFrom) INCLUDE(Priority)