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DIFFERENCE BETWEEN NOLOCK VS WITH(NOLOCK) IN SQL SERVER 2008


DIFFERENCE BETWEEN NOLOCK VS WITH(NOLOCK) IN SQL SERVER 2008

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kbhanu15
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Hi All,

can any one clarify the difference between NOLOCK and WITH(NOLOCK) in sql server 2008 ?
which one is the best practice to use ?

Thanks
Bhanu
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kbhanu15 (1/9/2014)
Hi All,

can any one clarify the difference between NOLOCK and WITH(NOLOCK) in sql server 2008 ?

Thanks
Bhanu


There is no diference between NOLOCK & WITH (NOLOCK). They both execute the query in "Read Uncommited" Isolation level.


which one is the best practice to use ?


No one, as they both may provide the result set with dirty reads.


Sujeet Singh
GilaMonster
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The first is deprecated, the second is not, both can result in incorrect results, duplicate rows and missing rows, so neither is the best to use. Best is to write the SQL properly without hints.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
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kbhanu15
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Thanks for you support.

it is mandatory in my query ti use(NOLOCK or WITH(NOLOCK), so i can prefer to use WITH (NOLOCK).

it is your conclusion.
GilaMonster
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Why is a hint that will sooner or later result in incorrect results 'mandatory'? Do the people you're writing that query for realise that from time to time their results will be incorrect?

Gail Shaw
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NOLOCK is not a good habit at all. In fact query hints in general are a last resort and should only be used when you REALLY understand what they are doing. If you are unsure, avoid them.

Before you claim that NOLOCK is mandatory for your query you need to fully understand what that hint does and the serious issues it can and will bring to your system. Here are 3 very excellent articles describing what happens when you use that hint.

http://sqlblogcasts.com/blogs/tonyrogerson/archive/2006/11/10/1280.aspx

http://blogs.msdn.com/b/davidlean/archive/2009/04/06/sql-server-nolock-hint-other-poor-ideas.aspx

http://www.jasonstrate.com/2012/06/the-side-effect-of-nolock/

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We have some semi-static reporting databases that run long queries. Some of them use nolock to avoid one report blocking another. In this case, the data was refreshed over night from production, and is static during the day. There is not really a concern about users getting up to the minute accurate information. Does this qualify as an acceptable use ?



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No, it's an unnecessary use. If the data is static during the day then there aren't going to be any X locks around to block the reports and so adding nolock is a waste of typing

Gail Shaw
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kbhanu15 (1/9/2014)
Hi All,

can any one clarify the difference between NOLOCK and WITH(NOLOCK) in sql server 2008 ?
which one is the best practice to use ?

I've been in the situation where nolock use was mandatory. I'm pretty sure that the current supported syntax is WITH (NOLOCK).

Make sure you realise what actually happens and the ramifications of using it. In particular, knowledge of isolation modes is highly recommended when programming databases, I like the wikipedia articles on it. Nolock looks to be equal to read uncommitted.

http://en.wikipedia.org/wiki/Isolation_(database_systems)

to properly post on a forum:
http://www.sqlservercentral.com/articles/61537/
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GilaMonster (1/10/2014)
No, it's an unnecessary use. If the data is static during the day then there aren't going to be any X locks around to block the reports and so adding nolock is a waste of typing


So am I mis-remembering situations in which long running read-only queries have caused blocking of other read-only queries ?



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