"set deadlock_priority" not effective in SQL Server 2012?

Question

I am trying to control which of two sessions is chosen as the deadlock victim in SQL Server 2012. I have set up a test that induces a deadlock by updating two tables in opposite order with a "waitfor" in between the updates. I can consistently
repro the deadlock and have code that will catch it and re-try the transaction.
However, when I run "set deadlock_priority low" for one of the sessions, it does not change which victim is chosen. SQL Server consistently chooses the session that starts last as the deadlock victim regardless of this setting. I have verified
that the setting is correct by selecting deadlock_priority from sys.dm_exec_sessions. I have tried setting the session that starts first to "low" and the one that starts second to "high". In spite of that, the second one is always
the one that gets chosen as the victim.
Is this the expected behavior?
Thanks,
Ron Rice
Ron Rice

Answer 1

I didn't try extremely hard, but I was not able to reproduce the issue. Running in the Northwind database, I had this in one window:
SET DEADLOCK_PRIORITY LOW
go
BEGIN TRANSACTION
go
SELECT * FROM Orders WITH (REPEATABLEREAD) WHERE OrderID = 11000
WAITFOR DELAY '00:00:02'
UPDATE Orders SET CustomerID = 'ALFKI' WHERE OrderID = 11000
go
ROLLBACK TRANSACTION
and in the other window I had:
SET DEADLOCK_PRIORITY HIGH
go
BEGIN TRANSACTION
go
SELECT * FROM Orders WITH (REPEATABLEREAD) WHERE OrderID = 11000
WAITFOR DELAY '00:00:02'
UPDATE Orders SET CustomerID = 'ALFKI' WHERE OrderID = 11000
go
ROLLBACK TRANSACTION
And every time I tried, it was the first window that fell on the floor.
Erland Sommarskog, SQL Server MVP, [email protected]