SQL Server – Update Statements with SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

lockingsql server

If I have a stored procedure with set transaction isolation level read uncommitted, will it affect update statements?

I know that you should not use with (nolock) on update/delete statements, and this does pretty much the same thing but not sure if SQL ignores it on update statements in the procedure or if I should be careful not to use it if there are update statements.

EDIT:

Sorry for the confusion. I'm not trying to figure out what the effect of using this type of locking on manipulation statements would be or whether it's a good idea. In fact I do NOT want to use this kind of locking on manipulation statements, and so my question is whether putting "set transaction …" at the top of my stored proc is ever going to be honored by update/delete statements or whether it will be ignored. My hope is that it is just ignored.

I am fully aware of the effects (with its pros and cons) it will have on select statements.

Best Answer

"Transaction isolation level" mainly affects (in my understanding) the behavior of the read operation, i.e. whether a read operation will issue some locks. In the case of "read uncommitted", here is a quote from MSDN

Transactions running at the READ UNCOMMITTED level do not issue shared locks to prevent other transactions from modifying data read by the current transaction. READ UNCOMMITTED transactions are also not blocked by exclusive locks that would prevent the current transaction from reading rows that have been modified but not committed by other transactions

So to your question, the answer is NO, the update will not be affected by "read uncommitted" transaction isolation level inside the same stored procedure.

-- Update (a sample to prove this logic) In SSMS, we open two windows, and in Window 1 (W1 hereafter), run the following

use tempdb
create table dbo.t (a int)
go
insert into dbo.t (a) values (10)
go
begin tran
update dbo.t 
set a = 20
where a = 10

-- commit tran

In another Window (W2), run the following (see the comments for the behavior)

use TempdB
set transaction isolation level read uncommitted
select * from dbo.t -- you can have dirty read, showing [a]=20, which is an uncommitted UPDATE in W1

go
-- the following update will wait (before proper locks are granted)
update dbo.t 
set a = 30
where a= 10

This means the UPDATE statement in W2 (with READ UNCOMMITTED) is not impacted by the transaction isolation level (i.e. still behaves as expected) as the SELECT statement.

-- UPDATE 2: According to MSDN UPDATE t-sql,

WITH ( Table_Hint_Limited )

Specifies one or more table hints that are allowed for a target table. The WITH keyword and the parentheses are required. NOLOCK and READUNCOMMITTED are not allowed. For information about table hints, see Table Hints (Transact-SQL).

So my understanding is that when you run UPDATE statement, in SQL Server, there is no way that you can update on dirty data (i.e. uncommitted data) even if you can read the dirty data in your session.

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