Question

SQL Server 2005 - Table locks! Do you recommend SNAPSHOT ISOLATION?

Asked by: Endelm

Hi,

I have a system that periodically updates static tables that users use (to reduce load on database).

However, when the system is updating the static tables, nobody can select from them! :(

Somebody recommended that I look into the isolation level:
SNAPSHOT ISOLATION

Could that help me and is it recommended?

Thanks! :)

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Asked On
2009-10-30 at 06:36:21ID24857974
Topics

SQL Query Syntax

,

SQL Server 2005

,

SQL Server 2008

Participating Experts
1
Points
500
Comments
9

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Answers

 

by: EndelmPosted on 2009-10-30 at 07:03:34ID: 25703028

While both of these new isolation levels have pros and cons, READ_COMMITTED_SNAPSHOT is generally recommended over ALLOW_SNAPSHOT_ISOLATION for the following reasons:

    * It uses less tempdb space.
    * It works with distributed transactions.
    * It works with most pre-existing applications.
    * It is not vulnerable to update conflicts.

http://www.sql-server-performance.com/articles/per/new_isolation_levels_p2.aspx


U agree?

 

by: carter-greenanPosted on 2009-10-30 at 07:44:34ID: 25703426

The prob with read committed snapshot, is that if you turn it on, you are automatically turning off read committed, which is the default behavior. This means that all transactions where you do not specify an isolation level will use this, thus killing your server. That is why it is deprecated and removed in 2008...

Instead, I suggest that you use a locking hint, to force row locks, rather than table lock. This will not perform aswell, but will reduce the contention. Alternativly, just select a lower isolation level for the transaction. Make sure it is not set at a higher level than read committed (unless there is a compelling reason)

 

by: EndelmPosted on 2009-10-30 at 07:54:41ID: 25703551


I cannot change the sql code. That's the problem :)

Should I turn on:
- READ_COMMITTED_SNAPSHOT
- ALLOW_SNAPSHOT_ISOLATION

?

 

by: carter-greenanPosted on 2009-10-30 at 08:13:37ID: 25703759

You really should NOT do that, unless all transactions are specifying isolation levels explicitly. Your server will grind to a halt.

If you can not change the sql code, could you use a plan guide? sp_create_plan_guide, and use that to change the isolation level for the code, or specify a locking hint for the code.

That should be a way around the issue

 

by: carter-greenanPosted on 2009-10-30 at 08:32:32ID: 25703956

I think maybe I did not explain myself well in the last post...

If you turn on READ_COMMITTED_SNAPSHOT - your server will die

If you turn on ALLOW_SNAPSHOT_ISOLATION - You still have to modify the code, to specify you want SNAPSHOT isolation. If you applied SNAPSHOT isolation to the whole database, you would have the same issue as with READ_COMMITTED_SNAPSHOT...

 

by: EndelmPosted on 2009-10-30 at 08:46:05ID: 25704102

Thanks a lot carter-greenan

"Instead, I suggest that you use a locking hint, to force row locks, rather than table lock. This will not perform aswell, but will reduce the contention."

Can you please explain or give me an example?

Thank you :)

 

by: carter-greenanPosted on 2009-10-30 at 09:29:15ID: 25704568

Hi Endelm,
An example of how to use a locking hint is attached. Depending on your exact requirements, the ones that may be of interest to you are:
ROWLOCK  - Specifies that row locks are taken when page or table locks are ordinarily taken
READUNCOMMITTED  - Specifies that dirty reads are allowed. No shared locks are issued to prevent other transactions from modifying data read by the current transaction
NOLOCK - Is equivalent to READUNCOMMITTED
If you are unable to modify the code however, you could implement it with a plan guide. Example also attached...
This will force query optimizer to use the hint, without changing the underlying code. You can also specify it just for a specific PROC by using the  @module_or_batch parameter

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
GO
BEGIN TRANSACTION;
GO
SELECT Title
FROM HumanResources.Employee WITH (NOLOCK);
GO
.....
.....
EXEC sp_create_plan_guide 
    @name = N'Guide1', 
    @stmt = N'SELECT Title
              FROM HumanResources.Employee WITH (NOLOCK);, 
    @type = N'SQL',
    @module_or_batch = NULL, 
    @params = NULL, 
    @hints = NULL';

                                              
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by: EndelmPosted on 2009-11-02 at 03:00:45ID: 31648000

Thanks a lot carter-greenan. You helped me a lot! :)

 

by: EndelmPosted on 2009-11-02 at 05:51:51ID: 25719222

Why do people praise READ_COMMITTED_SNAPSHOT if it is so bad?


http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=132825

20120131-EE-VQP-002

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