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Transaction Isolation Levels


Transaction Isolation Levels

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WayneS
WayneS
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Comments posted to this topic are about the item Transaction Isolation Levels

Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes
If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!
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CROSS-TABS and PIVOT tables Part 1 & Part 2, Using APPLY Part 1 & Part 2, Splitting Delimited Strings

jemerson 16199
jemerson 16199
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As you point out Read Committed Snapshot provides a transactionally consistent set of data at the time the Statement began. Therefore, if T1 reads a row; T2 later commits changes to that row; and T1 reads that row again it will get a different result on the second read. So, Repeatable Read is not guaranteed by Read Committed Snapshot.
Anuj Kumar Singh
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Great article, Well done! thank you!
Misha_SQL
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Great explanation and examples!



SQLRNNR
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Nice article Wayne.



Jason AKA CirqueDeSQLeil
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WayneS
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Thank you Anuj, Misha and Jason.

Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes
If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!
Links: For better assistance in answering your questions, How to ask a question, Performance Problems, Common date/time routines,
CROSS-TABS and PIVOT tables Part 1 & Part 2, Using APPLY Part 1 & Part 2, Splitting Delimited Strings

WayneS
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jemerson 16199 (2/13/2014)
As you point out Read Committed Snapshot provides a transactionally consistent set of data at the time the Statement began. Therefore, if T1 reads a row; T2 later commits changes to that row; and T1 reads that row again it will get a different result on the second read. So, Repeatable Read is not guaranteed by Read Committed Snapshot.


Thanks for reading the article and responding!

If we take a closer look at the snapshot example in the article, "T1" is reading the same set of rows twice, and during that time "T2" is adding a row. Since "T1" is doing both reads in a transaction, both of the reads are returning the same data, without the modifications that "T2" is making.

Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes
If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!
Links: For better assistance in answering your questions, How to ask a question, Performance Problems, Common date/time routines,
CROSS-TABS and PIVOT tables Part 1 & Part 2, Using APPLY Part 1 & Part 2, Splitting Delimited Strings

jemerson 16199
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That's true for snapshot isolation mode. However,the chart shows that Read Committed Snapshot also ensures Repeatable Read, no phantom reads, and no missing/double reads. Since for Read Committed Snapshot, consistency is guaranteed only for the Statement, I believe you can still have repeatable read (and the other) problems. When the same statement is run a second time, it will get a (possibly different) consistent set of rows as they existed when the second statement started.
Shcherbunov Neil
Shcherbunov Neil
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Since for Read Committed Snapshot, consistency is guaranteed only for the Statement, I believe you can still have repeatable read (and the other) problems.


Of course, and anyone need no more but 5 min. only to check this fact on the practice. So, the second table from the start of the article is TOTALLY incorrect.
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