October 6, 2008 at 9:03 am
Hi,
Is there any relationship between isolation level and indexes ?
How indexes affect the use of isolation level ?
Is there any performance boost-up obtained during data retrieval using where clause when isloation level is implemented on a table with indexes ?
:alien: :alien: :alien:
Can any one explain in detail ? Links and resources would be of great help :hehe:
Thanks,
Sree
October 6, 2008 at 9:23 am
No relationship.
Isolation levels affect how long locks are kept within a transaction and how isolated changes made by one transaction are from another transaction.
Indexes speed up data retrieval by giving the query processor a fast way to find rows affected by queries.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
October 6, 2008 at 8:36 pm
Hi GilaMonster,
Thanks for the reply 🙂
Can u plz go thro the below paragraphs and offer me your valuable guidance/suggestions.
When i tried to explore isolation levels i found the below stuff.
The following statements are executed to test READCOMMITTED isolation level
CREATE TABLE Consultants (
ConsultantID INT IDENTITY,
ConsultantName VARCHAR(20),
Technology VARCHAR(15),
HireDate DATETIME,
HourlyRate MONEY)
---------------------------------------------------------------------
-- Fill some data
---------------------------------------------------------------------
INSERT INTO Consultants (ConsultantName, Technology, HireDate, HourlyRate)
SELECT 'Jacob', 'SQL Server', '2000-10-03', 120
INSERT INTO Consultants (ConsultantName, Technology, HireDate, HourlyRate)
SELECT 'Smith', 'ASP.NET', '2005-01-01', 140000
INSERT INTO Consultants (ConsultantName, Technology, HireDate, HourlyRate)
SELECT 'Michael', 'C#.NET', '2002-10-30', 80000
I executed the following statements in Query Window 1
BEGIN TRAN
UPDATE c SET
HourlyRate = 250
FROM Consultants c
WHERE ConsultantName = 'Smith'
When execcuted the following statements from Query Window 2
select * from Consultants
the query blocks and does not return. This query blocks with the transaction running on
the other session. Now move back to Query Window 1 and COMMIT the transaction.
COMMIT TRAN
Now switch back again to Query Window 2. You will notice that the query returned the following.
/*
ConsultantID ConsultantName Technology HireDate HourlyRate
------------ -------------------- --------------- ----------------------- ---------------------
1 Jacob SQL Server 2000-10-03 00:00:00.000 120.00
2 Smith ASP.NET 2005-01-01 00:00:00.000 250.00
3 Michael C#.NET 2002-10-30 00:00:00.000 100.00
*/
I then created a clustered index on the ConsultantID column.
ALTER TABLE Consultants
ADD CONSTRAINT PK_Consultants PRIMARY KEY (ConsultantID)
Now move to Query Window 1 and update the record of Smith.
BEGIN TRAN
UPDATE c SET
HourlyRate = 250
FROM Consultants c
WHERE ConsultantName = 'Smith'
Now, Move to Query Window 2 and run the following queries.
SELECT *
FROM Consultants
WHERE ConsultantID = 3
/*
ConsultantID ConsultantName Technology HireDate HourlyRate
------------ -------------------- --------------- ----------------------- ---------------------
3 Michael C#.NET 2002-10-30 00:00:00.000 100.00
*/
Now, the thing i want to know is that "is still something
there to know more about isolation levels in connection with indexes ?"
N.B: the above code and observations were obtained from the following URL
http://www.sqlserverandxml.com/2008/08/sql-server-transaction-isolation-level.html
Thanks and Regards,
Sree
October 7, 2008 at 12:54 am
Without indexes SQL has to do table scans and hence will take table-level locks. With appropriate indexes, SQL can take more granular locks and hence the two connections don't block each other.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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