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Comments posted to this topic are about the item Isolation Levels
Always nice to brush up on the basic concepts of sql:)
Nice question, thanks.
Explanation didn' mention snapshot though.
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MCSE Business Intelligence - Microsoft Data Platform MVP
Wondering what I would do next , when I am done with this one :ermm:
(..actually came to know these levels 6 years ago, I used (NOLOCK) in the select statements on DWH queries and was wondering rather than typing nolock for each table join.. is there any simple open/close of nolock for all the tables in that query? and thats where I came to know the isolation levels and started to use the read uncommited and read committed when needed, apart from these two I have not used any other level.)
The first and the hardest SQL statement I have wrote- "select * from customers" - and I was happy and felt smart.
Thanks for the question
-------------------------------Posting Data Etiquette - Jeff Moden [/url]Smart way to ask a question
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I loved this question so much I wanna hug you... *ahem*
Andre Guerreiro Neto
Mr or Mrs. 500
Nice question, thanks!
Very good question. I am surprised that more than 60% got this wrong!
This is certainly a good example of a totally unambiguous question as far as I can see - I am surprised at only 34% right so far.
Nice back to basics question.
Unfortunately, knowing what the isolation levels are has never seemed to make people understand when to use which of them. Using read committed when what is needed is repeated read or serializable is the most common error, but using red uncommitted when it's totally inappropriate is almost as common. Also, most people seem to think that snapshot implies serializable, although it clearly doesn't.. Perhaps because they think that snapshot is a version of "optimistic locking" and know that there is a way of delivering serializable and think that that means everything which might be described as optimistic locking delivers serializable. In fact the MS snapshot isolation level is somewhere in between ISO repeatable read and ISO serializable, not the same as either or them. Maybe the fact that read committed snapshot is not a separate isolation level, but a mechanism for providing read committed isolation makes people think snapshot is just a mschanism too, not a separate isolation level.
Incidentally, I find it slightly disappointing that MS haven't provided an optimistic locking mechanism to provide serializable isolation, although how to do this has been known for a couple of decades or more; maybe they've found it doesn't enough performance over the current serializable method to be worth doing.
A nice explanation to the Locking and Isolation Level article can be viewed at :-
Can someone explain in what way SNAPSHOT doesn't meet the stated requirements? That's the answer I selected and I always like to know exactly how I've been a helpless newb... 😛
Thanks for the great question and thanks Tom for the explanation.
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