10777 - ISOLATION LEVEL

hi experts

in this code
-- Retrieve the changes between the last extracted and current versions
SET TRANSACTION ISOLATION LEVEL SNAPSHOT

DECLARE @previous_version BigInt
SELECT @previous_version = MAX(LastExtractedVersion)
FROM stg.ExtractLog
WHERE SourceName = 'Salespeople'

DECLARE @current_version BigInt
SET @current_version = CHANGE_TRACKING_CURRENT_VERSION();

SELECT  @previous_version 'Previously retrieved Version',
          @current_version 'Current version',
            CT.SalesPersonID,
        r.SalespersonName,
            r.StoreName,
            r.PostalCode,
            r.City,
            r.Region,
            r.Country
FROM
CHANGETABLE(CHANGES src.Salespeople, @previous_version) CT
INNER JOIN src.Salespeople r ON CT.SalesPersonID = r.SalesPersonID

UPDATE stg.ExtractLog
SET LastExtractedVersion = @current_version
WHERE SourceName = 'Salespeople'

SET TRANSACTION ISOLATION LEVEL READ COMMITTED
GO

why use:
SET TRANSACTION ISOLATION LEVEL SNAPSHOT
and
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
enrique_aeoAsked:
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Racim BOUDJAKDJIDatabase Architect - Dba - Data ScientistCommented:
As Row Versionning physically separates within a transaction or a database the data that is used in a SELECT from the version that is used in the INSERT/UPDATE/DELETE statements.  

The instruction makes sure that all values used in the SELECT statement are necessarily committed values ignoring all uncommitted values (at the point in time of the transaction) but adding to them an on-the-fly value coming from CDC session.  Since this feature creates overhead, the transaction put back the normal isolation mode once the UPDATE transaction is complete.

Hope this helps.
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Vitor MontalvãoMSSQL Senior EngineerCommented:
By the code you posted, there's two SELECT statement followed by an UPDATE, so I think the idea is to assure that you are working with the same set of data during all process, so the need for SET TRANSACTION ISOLATION LEVEL SNAPSHOT. Also, this isolation level doesn't cause locks, since it's working with a snapshot and not with the table.

At the end SET TRANSACTION ISOLATION LEVEL READ COMMITTED to going back to the default isolation level that avoids dirty reads ( cannot read data that has been modified but not committed by other transactions).
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enrique_aeoAuthor Commented:
Dear Victor, your answer is very good, unfortunately I had already qualifed question, I will have a more careful next time. Thank you
0
Vitor MontalvãoMSSQL Senior EngineerCommented:
No worries Enrique. The idea is to help you. Points are secondary.
Cheers
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Microsoft SQL Server 2008

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