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
Related
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"
I am trying to dirty check a SQL Server table, but I am not sure whether or not the settings are saved to the table and will effect other reads, or automatically reset after the read. I would like the uncommitted flag to only be used each time the procedure is called, and not effect other reads to the table.
Case 1:
set transaction isolation level read uncommitted
select * from table
Case 2:
set transaction isolation level read uncommitted
select * from table
set transaction isolation level read committed
Is the last line in Case 2 necessary?
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.
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 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