I have an oledb-connection from clickview to a sql2005-server and I would like this connection to use transaction isolation level of read uncommitted. My seconde choice would be to set it on the user. How can I accomplish this?
http://msdn.microsoft.com/en-us/library/ms709786(VS.85).aspx
When you starts transaction by ITransactionLocal::StartTransaction you explicitly need specify isolation level.
Related
I can set isolation level with a transaction using sql.TxOptions. So I would like to know any way I can configure the isolation for Query, Exec.
Update: To be more precise, I am asking how to set the isolation level for connections like jdbc. The answer I have now is adding params to the database url.
I have to get data from a table which is excessively updated. Dirty read is not a problem for me. I decided to use read uncommitted in my stored procedure.
Then I added this line before select:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
I learned that this code scope is connection, unlike nolock. I heard you should change it to default after your work is done is that right?
Do I have to just add
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
end of the line? I could not find any example on the web where isolation level is changed back after work is done. Is there any example?
Only one of the isolation level options can be set at a time, and it remains set for that connection until it is explicitly changed. All read operations performed within the transaction operate under the rules for the specified isolation level unless a table hint in the FROM clause of a statement specifies different locking or versioning behavior for a table.
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If you issue SET TRANSACTION ISOLATION LEVEL in a stored procedure or trigger, when the object returns control the isolation level is reset to the level in effect when the object was invoked. For example, if you set REPEATABLE READ in a batch, and the batch then calls a stored procedure that sets the isolation level to SERIALIZABLE, the isolation level setting reverts to REPEATABLE READ when the stored procedure returns control to the batch.
https://msdn.microsoft.com/en-us/library/ms173763.aspx
I want to update a status in a DB table row and I want this to be immediately visible for other processes/transactions reading the same row. Do I have to put this statement in a separate transaction which will be committed right after the update?
I suppose this depends on the DB engine and isolation level, but as this can change I want my code to handle whatever the DB engine and isolation level is.
You have two solutions:
Use the appropriate transaction isolation level SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (details)
Other queries can use NOLOCK (details)
Check the links from more details.
If right after BEGIN I have SET TRANSACTION ISOLATION LEVEL ... statement, will the given transaction level in force for the entire scope of the stored procedure regardless if I use BEGIN TRANSACTION or not? Namely if I have simple SELECT statements, which are atomic/transacted by definition, will the default transaction level for them set to the given one?
BEGIN
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
-- will a transaction level for a atomic transaction created by SQL Server for this statement be READ COMMITTED
SELECT * FROM T
END
First, the default isolation level in SQL Server is Read Committed, so that statement doesn't really do anything unless you've changed the default isolation level.
But, in general, yes, SET Transaction Isolation Level will change the isolation level for the whole procedure (the duration of the connection, in fact)
Keep in mind that all SQL statements are implicit transactions meaning that if, for example, an update fails 99% through, it will rollback automatically; no BEGIN TRAN/COMMIT is necessary.
To answer your question, yes, your SELECT statements will inherit the isolation level you set (or the default if you do not set one) unless you override the behavior with a query hint like WITH NOLOCK which will make the individual query behave as though you did SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
Once you alter database to use Read Commited Snapshot isolation level, OR snapshot isolation level
Do all transaction now uses that isolation as default or Read Commited is stil the default isolation level and you have to change all stored proc to use by specifying
SET TRANSACTION ISOLATION LEVEL Read_committed_snapshot to use Read Committed Snapshot
The entire database and code and transaction all use whatever your setting us
Unless you set SET TRANSACTION ISOLATION LEVEL in the code somewhere separately...