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.
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.
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.
...
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
The title may be hard to read.
Scenario is like this: I have a view my_view (by default, it will be read committed). Then I have just created a new stored procedure:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
select *
from table1
inner join my_view
Now, of course, table1 will be read uncommitted, but what about my_view?
Is it read committed or read uncommitted?
Thanks
I have a view: my_view (read committed)
No, the view does not have an associated isolation level, as you seem to imply. The view is just a stored query. And that query will execute in whatever isolation level is in effect at the time you run it.
So if your view is queried from a stored procedure that sets the isolation level to read uncommitted (your example), then the query will be executed in that isolation level, simple as that.