If I'm using
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
Do I need to wrap the query in a transaction
e.g.
BEGIN TRAN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT * FROM
T1
COMMIT
Or can I just have a normal query?
Also, is there any benefit to including SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED and WITH (NOLOCK). I understand that one is for the table level and one is for the whole connection level. But is there any benefit to having both?
Such as:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT * FROM
T1 WITH (NOLOCK)
Always we executes procedure of INSERT, UPDATE and DELETE transaction with READ COMMITTED ISOLATION LEVEL. If we need dirty data from any table then it helps. Because Transaction is defined with READ COMMITTED and table return dirty reads using WITH(NOLOCK) in JOIN Query.
You don't need to wrap the query below in a transaction:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
Because even if wrapping the query above in a transaction as shown below, isolation level is still set on session(connection) level but not on transaction level:
BEGIN TRAN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
COMMIT
So, even if rollbacking isolation level as shown below, isolation level is not rollbacked so it's still READ UNCOMMITTED but not READ COMMITTED.
BEGIN TRAN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
ROLLBACK -- Isolation level is still "READ UNCOMMITTED" but not "READ COMMITTED"
Related
When using snapshot isolation, why do I often see IF ##TRANCOUNT = 0 before setting the transaction level?
I.e., in a stored proc:
IF ##TRANCOUNT = 0
BEGIN
SET TRANSACTION ISOLATION LEVEL SNAPSHOT;
END
one reason is this from Microsoft Documentations:
A transaction cannot be set to SNAPSHOT isolation level that started with another isolation level; doing so will cause the transaction to abort. If a transaction starts in the SNAPSHOT isolation level, you can change it to another isolation level and then back to SNAPSHOT. A transaction starts the first time it accesses data.
I am trying to update table, which controlls application (application performs some select statements). I would like to update the table in transaction with isolation level set to read uncommited, so if application doesn't work as expected I can rollback transactions.
But following code doesn't work:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
go
begin transaction
go
update [DB].[dbo].[Table]
set ID = ID - 281
where ID > 2
When I open another query window, I cannot query this table... I thought, that with such transaction level I would be able to query the table without rolling back/commiting transaction.
Isolation level works in another way as you suppose.
You can only read uncommitted data, but others still cannot see what you done within transaction until you commit.
If you want to see uncommitted data from this transaction in your select you need to set
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
to this select
You need to use SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED from a session which reads data.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT *
FROM [DB].[dbo].[Table]
This query will execute immediately without lock. And you'll see the dirty data.
Consider the following SQL statement:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
BEGIN TRANSACTION
insert into Customer(name)
values('CustomerName')
COMMIT
Am I right in saying this Isolation Level is not needed since we are doing a simple insert? Not actually READing anything
Can we set isolation level for plain SQL statements in a stored procedure in SQL Server 2005/2008?
Case 1: (This will work fine)
CREATE PROCEDURE MySP
AS
BEGIN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
BEGIN TRAN
SELECT * FROM MyTable
COMMIT TRAN
END
Case 2: (does this isolation stuff work here?)
CREATE PROCEDURE MySP
AS
BEGIN
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT * FROM MyTable
END
If case 2 is correct, would it work for several select statements as well?
Yes it will work for multiple select statements.
If you are worried about the lack if a transaction in the second stored procedure you should know that the query is executed under an implicit transaction as opposed to an explicit transaction that you have in the first stored procedure.
If you use SET TRANSACTION ISOLATION LEVEL in a stored procedure, this transaction isolation level is used for the duration of the stored proc. According to MSDN:
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.
Setting transaction isolation level is not the same as starting a transaction. Transaction isolation level tells SQL Server how to control locking.
If you only have multiple SELECT queries with READ UNCOMMITTED transaction isolation level, putting them all into a transaction won't make practical difference.
Isolation level can be set either at the session level with a session option or at the query level with a table hint. To set isolation level of the whole session we use command:
SET TRANSACTION ISOLATION LEVEL <isolation name>;
You can use a table hint to set the isolation level of a query as:
SELECT ... FROM <table> WITH (<isolationname>);
so in your case it would be like:
SELECT *
FROM MyTable WITH (READCOMMITTEDLOCK);
One thing here to note is: with the session option a space is specified between the words in case the name of the isolation level is made of more than one word, such as REPEATABLE READ. With the query hint,we don’t specify a space between the words—for example, WITH (REPEATABLEREAD).
If you set SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
on a stored procedure that selects from view which has a complex select from multiple tables, will this place READ UNCOMMITTED transaction isolation level on all the tables in that view?
The session will be set to be READ UNCOMMITTED - this will apply to all tables queried in the session (whether through a view or not), as long as this isolation level is set.