sqlindian – On Locks and Deadlocks

Random thoughts on performance and concurrency

DEADLOCK on SELECT due to UNORDERED PREFETCH

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In a previous post, I have provided links to multiple articles that demonstrate how a SELECT statement involving a key lookup running under READ COMMITTED isolation level can deadlock with another session running an UPDATE statement.

In this post we will discuss about another case where a SELECT statement running under READ COMMITTED isolation level  can deadlock with another session due to the locking behavior.

Setup

Run the below script to create the test table and populate it.

IF DB_ID(‘DeadLockTest’) IS NULL

CREATE Database DeadLockTest

GO

USE DeadLockTest

GO

create table dbo.rcdl (id int identity(1,1) primary key clustered, id2 int, filler char(1000) default ‘abc’)

GO

set nocount on

declare @i int

set @i = 0

while @i < 1000

begin

insert dbo.rcdl(id2) values ( @i)

set @i = @i + 1

end

Go

create index ix_rcdl on dbo.rcdl(id2)

go

Now start another session (Session 1) and run the below command.

BEGIN TRAN

UPDATE dbo.rcdl

SET filler = ‘xyz’,id2=id

WHERE id = 24

WAITFOR DELAY ’00:00:05′

UPDATE dbo.rcdl

SET filler = ‘xyz’,id2=id

WHERE id = 1

ROLLBACK TRAN

Start one more session (session 2) and run the below query

SELECT filler FROM dbo.rcdl WHERE id2 < 25

In five seconds, the SELECT statement running in session 2 will fail with the below error.

Msg 1205, Level 13, State 51, Line 1

Transaction (Process ID 74) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.

Analyze

Let’s go ahead and populate the deadlock analysis table from the deadlock trace.

Process Resource Mode State Command
process593048 KEY: 26:72057594042056704 (1edefaa29392) S WAIT SELECT
process593048 KEY: 26:72057594042056704 (bb0d06c12baa) S GRANT SELECT
process13ee08 KEY: 26:72057594042056704 (bb0d06c12baa) X GRANT UPDATE
process13ee08 KEY: 26:72057594042056704 (1edefaa29392) X WAIT UPDATE

You can see that the update command is holding an X lock on one row and waiting for another X lock. The SELECT statement is also holding a shared (S) lock and waiting for another shared lock. But why the SELECT statement is holding a shared lock under READ COMMITTED isolation level? As stated in a previous post, SELECT statements should only hold the shared lock for the duration of processing the row. For a visual representation of the cocept of lock duration under different isolation levels, see Micheal J Swart’s (b |t) post “Visualizing Transaction Isolations For SQL Server”

But there are few exceptions to this rule. In certain cases, SQL server will keep the shared locks until the end of the transaction to ensure consistency. One of them involves “UNORDERED PREFETCH”, which happens to be the case here. If you look at the execution plan xml of the SELECT statement, you can see that our plan uses a Nested Loop Join with UNORDERED PREFETCH. Craig Freedman blogged about the scenarios where the shared locks are held until the end of the transaction here. He also wrote about UNORDERED PREFETCH in detail here.

Fix

You can use any of the below methods to resolve this issue.

  1. Combine the UPDATE statements to a single UPDATE statement.
  2. Cover the index by including the filler column
  3. Acquire non-compatible locks using a locking hint.

This post is part of the series SQL Server deadlocks and live locks – Common Patterns. If you have questions, please post a comment here or reach me at @sqlindian

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Written by sqlindian

July 13, 2012 at 11:34 PM

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