SQL locking in an UPDATE...FROM statement

Use Standard SQL instead of dialect, and you will also catch
cardinality problems.

UPDATE TblOne
SET TblOne.value
= (SELECT TblTwo.value
    FROM TblTwo
    WHERE TblTwo.unique_id = TblOne.unique_id
    AND TblTwo.value <> TblOne.value)
WHERE EXISTS
(SELECT *
    FROM TblTwo
WHERE TblTwo.unique_id = TblOne.unique_id
    AND TblTwo.value <> TblOne.value);

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Andrew J. Kelly - 24 Jul 2008 20:06 GMT

You can ignore Joe on this one since the Update lock will be there
regardless of how you do it. It is there to prevent someone from taking a
table level lock out when you have a row or page in that same table. And if
you don't have a proper index it may attempt a table level lock to begin
with.

Signature

Andrew J. Kelly SQL MVP
Solid Quality Mentors

> Use Standard SQL instead of dialect, and you will also catch
> cardinality problems.
[quoted text clipped - 10 lines]
> WHERE TblTwo.unique_id = TblOne.unique_id
> AND TblTwo.value <> TblOne.value);

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Alex Kuznetsov - 25 Jul 2008 15:33 GMT

> Use Standard SQL instead of dialect, and you will also catch
> cardinality problems.
[quoted text clipped - 10 lines]
> WHERE TblTwo.unique_id = TblOne.unique_id
> AND TblTwo.value <> TblOne.value);

Because it usually runs about two times slower, most likely it will
make the matters much worse.

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Hi Goran,
Your UPDATE statement might lead to a full table scan, therefore a table lock was acquired in
this case. Also there might be Lock escalation happened. However it is hard to avoid a table
lock for such a scenario. If there are a large number of rows, it also does not make sense to
lock each row since it is a huge cost for SQL Server to maintain so many row locks.

Anyway I would like to recommend the following KB article for your reference. You may check
if it could give you some ideas to optimize your query:
How to resolve blocking problems that are caused by lock escalation in SQL Server
http://support.microsoft.com/kb/323630/en-us
Lock Escalation (Database Engine)
http://msdn.microsoft.com/en-us/library/ms184286.aspx

If you have any other questions or concerns, please feel free to let us know.

Best regards,
Charles Wang
Microsoft Online Community Support
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Hi Goran,
Your UPDATE statement might lead to a full table scan, therefore a table lock was acquired in
this case. Also there might be Lock escalation happened. However it is hard to avoid a table
lock for such a scenario. If there are a large number of rows, it also does not make sense to
lock each row since it is a huge cost for SQL Server to maintain so many row locks.

Anyway I would like to recommend the following KB article for your reference. You may check
if it could give you some ideas to optimize your query:
How to resolve blocking problems that are caused by lock escalation in SQL Server
http://support.microsoft.com/kb/323630/en-us
Lock Escalation (Database Engine)
http://msdn.microsoft.com/en-us/library/ms184286.aspx

If you have any other questions or concerns, please feel free to let us know.

Best regards,
Charles Wang
Microsoft Online Community Support
===========================================================
Delighting our customers is our #1 priority. We welcome your
comments and suggestions about how we can improve the
support we provide to you. Please feel free to let my manager
know what you think of the level of service provided. You can
send feedback directly to my manager at: msdnmg@microsoft.com.
===========================================================
Get notification to my posts through email? Please refer to
http://msdn.microsoft.com/subscriptions/managednewsgroups/default.aspx#notifications.

Note: The MSDN Managed Newsgroup support offering is for
non-urgent issues where an initial response from the community
or a Microsoft Support Engineer within 1 business day is acceptable.
Please note that each follow up response may take approximately
2 business days as the support professional working with you may
need further investigation to reach the most efficient resolution.
The offering is not appropriate for situations
that require urgent, real-time or phone-based interactions or complex
project analysis and dump analysis issues. Issues of this nature are best
handled working with a dedicated Microsoft Support Engineer by
contacting Microsoft Customer Support Services (CSS) at
http://msdn.microsoft.com/subscriptions/support/default.aspx.
============================================================

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This posting is provided "AS IS" with no warranties, and confers no rights.

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Thx guys. That was very helpful.
Goran

> Hi all,
> In an UPDATE...FROM statement, how do I instruct SQL server to
[quoted text clipped - 14 lines]
> Thanks
> Goran Djuranovic

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