LOCK

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LOCK

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Saurabh Dwivedy SSCommitted Points: 1887 More actions · Answer 1

CirquedeSQLeil (2/18/2010)
Saurabh Dwivedy (2/18/2010)
Paul White (2/17/2010)
With an exclusive lock, no other transactions can modify data (except for operations with NOLOCK hint or read uncommitted isolation level)
What???!!! :w00t:
I want my point back!
Unless someone can post code or a reference to show that NOLOCK or READ UNCOMMITTED allows modification of data protected by an exclusive lock!
Grrr!
With an exclusive lock, no other transactions can modify data (read operations with NOLOCK hint or read uncommitted isolation level can take place),
The above choice states clearly that no other transaction can MODIFY data - which is correct; that being said, Read operations with NOLOCK hint or under read uncommitted isolation level can certain take place - REFER to the following link ----- http://www.sqlteam.com/article/introduction-to-locking-in-sql-server (exclusive lock section)
Exclusive locks prevent write access to a resource by concurrent transactions ,
I don't think there is any ambiguity in this choice - it is clear enough and correct - X Locks prevents any concurrent write access
Data modification statements typically request both shared locks and exclusive locks
I must admit that I was momentarily confused by the wording of this choice - I only answered it from gut feel; Typically DML operations should take an UPDATE lock; While Shared locks can be escalated to an exclusive lock when the DML kicks in, it can end up in a deadlock if another transaction (under shared lock mode) attempts to update the same data. To prevent this situation Update locks are used. Saying that Data Modification statements typically REQUEST both shared and exclusive locks does lead to an ambiguous situation. It could have been more appropriately worded.
The question originally stated (in essence) that a nolock hint could be used to modify while an exclusive lock was held. It has been changed since. Paul wasn't disputing that a read could happen while using the nolock hint if an exclusive lock was held - just that the wording was wrong and thus made that option incorrect.

Thanks Cirque... it sounds like I joined the party late when the question had already been modified...

Saurabh Dwivedy
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My Blog: http://tinyurl.com/dwivedys

For better, quicker answers, click on the following...
http://www.sqlservercentral.com/articles/Best+Practices/61537

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SQLRNNR SSC Guru Points: 281334 More actions · Answer 2

Saurabh Dwivedy (2/18/2010)
Thanks Cirque... it sounds like I joined the party late when the question had already been modified...

You're welcome and NP.

Jason...AKA CirqueDeSQLeil
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I have given a name to my pain...MCM SQL Server, MVP
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Paul White SSC Guru Points: 150468 More actions · Answer 3

Thanks for having my back there Jason 😎

Paul White
All articles available on SQL.kiwi
@SQL_Kiwi

TomThomson SSC Guru Points: 104773 More actions · Answer 4

I'm glad it had been edited (twice) by the time I saw it - at least the answer is now right. The explanation is still slighly wrong, though: the first sentence suggests that all concurrent access is prevented, but in fact read access is allowed if it uses the right "isolation" level (quote marks because I would call READ UNCOMITTED an "unisolation level").

Tom

Malcolm Daughtree SSCrazy Points: 2382 More actions · Answer 5

Data modification statements typically request both shared locks and exclusive locks !!

from MSDN

When lock escalation occurs, the lock selected for the heap or index is strong enough to meet the requirements of the most restrictive lower level lock.

Not at the same time and what about the issuing of IXWhen lock escalation occurs, the lock selected for the heap or index is strong enough to meet the requirements of the most restrictive lower level lock.

For example, assume a session:

Begins a transaction.

Updates a table containing a clustered index.

Issues a SELECT statement that references the same table.

The UPDATE statement acquires these locks:

Exclusive (X) locks on the updated data rows.

Intent exclusive (IX) locks on the clustered index pages containing those rows.

An IX lock on the clustered index and another on the table.

The SELECT statement acquires these locks:

Shared (S) locks on all data rows it reads, unless the row is already protected by an X lock from the UPDATE statement.

Intent Share locks on all clustered index pages containing those rows, unless the page is already protected by an IX lock.

No lock on the clustered index or table because they are already protected by IX locks.

There are more answers than those provided.

CodeOn

😛

Peter Trast SSCarpal Tunnel Points: 4392 More actions · Answer 6

I will have to remember to answer questions 2 weeks late all the time to make sure I have a well-worded question 😉

Thanks for the edits! I can't afford any more confusion than I already possess...

Peter Trast
Microsoft Certified ...(insert many literal strings here)
Microsoft Design Architect with Alexander Open Systems

Paul White SSC Guru Points: 150468 More actions · Answer 7

Sometimes, it would just be better to delete the question completely.

Paul White
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