Setting Snapshot Isolation Level

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Setting Snapshot Isolation Level

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james marriot

Posted Wednesday, January 02, 2013 8:24 AM

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Do i need to explicitly say SET TRANSACTION ISOLATION LEVEL SNAPSHOT; in each batch that i want to operate under Snapshot isolation?

I altered database
Set Allow_Snapshot_Isolation ON, but even when i disconnect all sessions and reconnect, DBCC USEROPTIONS, and system behaviour say im using Read Committed.

Im obviously missing something stupid here but just cant see what it is.

Post #1401870

GSquared

Posted Wednesday, January 02, 2013 8:58 AM

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If you want snapshot by default, you need to set Read Committed Snapshot Isolation. http://msdn.microsoft.com/en-us/library/ms175095(v=SQL.105).aspx

Otherwise, allowing snapshot isolation just allows it. It doesn't set it as the default, and you will need to set it for each connection.

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Post #1401887

opc.three

Posted Thursday, January 03, 2013 2:21 PM

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james marriot (1/2/2013)

Not to muddy the waters but yes, you must issue the SET every time if you want SNAPSHOT isolation. SNAPSHOT is its own isolation level, separate from READ COMMITTED which is the unsettable default.

The other option containing "snapshot", READ_COMMITTED_SNAPSHOT, is not an isolation level in and of itself. Rather it is an alternate "mode" or "implementation" of the READ COMMITTED isolation level. When READ_COMMITTED_SNAPSHOT is enabled row versioning occurs when in the READ COMMITTED isolation level, but that is very different from SNAPSHOT.

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Post #1402565

GilaMonster

Posted Thursday, January 03, 2013 3:12 PM

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james marriot (1/2/2013)

Do i need to explicitly say SET TRANSACTION ISOLATION LEVEL SNAPSHOT; in each batch that i want to operate under Snapshot isolation?

Yes.

I altered database
Set Allow_Snapshot_Isolation ON, but even when i disconnect all sessions and reconnect, DBCC USEROPTIONS, and system behaviour say im using Read Committed.

The option is Allow_Snapshot_Isolation. Buy turning it on, you allow sessions to request snapshot isolation level. That's all.

Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
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Post #1402587

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