I can't seem to find a straight answer on what should be a simple question. If I create a transaction in T-SQL and set the ISOLATION LEVEL to SERIALIZABLE, does this create a READ lock on the tables that I am modifying?
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
GO
BEGIN TRANSACTION;
GO
TRUNCATE TABLE TBL_PRODUCTS;
GO
**INSERT RECORDS HERE**
GO
COMMIT TRANSACTION;
GO
TRUNCATE TABLE will acquire a exclusive shema modify lock on the table preventing all users from reading from the table (unless they use TRANSACTION ISOLATION LEVEL READ UNCOMMITTED or WITH(NOLOCK)) and writing to the table (no exceptions for writing). The exclusive lock will be released at COMMIT TRANSACTION.
EDIT: As Martin Smith pointed out in his comment below the truncate table will acquire a schema modify lock. Meaning there are no other user will be able to read or modify the table whatsoever until a commit or rollback has taken place.
Yes, it will lock the table, and these are the rules for serializable:
Statements cannot read data that has been modified but not yet committed by other transactions.
No other transactions can modify data that has been read by the current transaction until the current transaction completes.
Other transactions cannot insert new rows with key values that would fall in the range of keys read by any statements in the current transaction until the current transaction completes.
https://msdn.microsoft.com/en-us/library/ms173763.aspx
Related
I have read that serializable isolation level blocks only: insert, update, delete but NOT read.
I have run in one window:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
UPDATE [dbo].[Categories]
SET name = 'aaa'
WHERE categoryid = 4;
-- without commit
And in the second window:
SELECT TOP (1000) [CategoryId]
,[Name]
FROM [dbo].[Categories]
And the above query is waiting for the end of the first query. So does serialization isolation level block also read?
Yes it blocks reads. Your transaction is not committed, so you can still do a rollback. That means the data you have updated should not be read by any other process before you commit.
"does serialization isolation level block also read" : No.
Serializable acquires some lock (in Ms Sql its shared lock + range lock) which prevents writes from other transactions.
Its the UPDATE statement's exclusive lock (the above shared lock now converts to exclusive lock) that prevents second transaction's SELECT (Read Commited or Repeatable Read by deafult depending on db) to wait.
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 am having an SP with transaction isolation level set as Read Uncommitted.
For Example
Create Procedure TrailSP
AS
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
BEGIN TRY
UPDATE TrialTable
SET TrailColumn ='Update'
WHERE TrailID=1
--this is followed by more updates and selects
END TRY
BEGIN CATCH
RETURN -1;
END CATCH
RETURN 0;
what I want to know is that the first update I have given in the SP will it get committed instantly as it executes or will it get committed along with the rest of the logic at the end of Sp.
It will get committed, as any update under any transaction isolation level, when the transaction commits. This has nothing to do with the stored procedure ending.
If the call to your procedure has a transaction, then the commit will occur when that transaction commits.
If the call to your procedure does not have a transaction but the session has enabled implicit transactions then it will commit when the application explicitly commits.
If the call to your procedure does not have a transaction and session has the auto commit transaction behavior (ie. the most common case) then transaction will commit when the UPDATE statement completes.
Enabling READ UNCOMMITTED for an UPDATE is a no-op.
-Any data read inside a READ UNCOMMITTED is data that could disappear because the transaction that wrote it rollback.
-Its also possible to not see some row that have been committed because a transaction that have not yet committed and might never commit deleted it.
-Its also possible for row to be missing or be duplicated because of PageSplit.
Basically anything is possible, so the data read should never be used to compute anything that should be written to the database or you risk corrupting your database.
TLDR: never use READ UNCOMMITTED
I have the following SP:
CREATE PROCEDURE [dbo].[sp_LockReader]
AS
BEGIN
SET NOCOUNT ON;
begin try
set transaction isolation level serializable
begin tran
select * from teste
commit tran
end try
begin catch
rollback tran
set transaction isolation level READ COMMITTED
end catch
set transaction isolation level READ COMMITTED
END
The table "test" has many values, so "select * from teste" takes several seconds. I run the sp_LockReader at same time in two diferent query windows and the second one starts showing test table contents without the first one terminates.
Shouldn't serializeble level forces the second query to wait?
How do i get the described behaviour?
Thanks
SERIALIZABLE at the most basic means "hold locks for longer". When you SELECT, the held lock is a shared lock which allows other readers.
If you want to block readers, use WITH (TABLOCKX) hint to take an exclusive lock where you don't need SERIALIZABLE. Or XLOCK with SERIALIZABLE
In other words:
SERIALIZABLE = Isolation Level = lock duration, concurrency
XLOCK = mode= sharing/exclusivity
TABLOCK = Granularity = what is locked
TABLOCKX = combined
See this question/answer for more info
A serializable transaction whose output is not affected by other concurrent transactions. In your case, you are SELECTing twice from the table; neither of those transactions changes the result set of the other, so they may both run simultaneously.
Even if one transaction did update the table, this would not necessarily prevent the other from executing, as the database may work from snapshots.
Have a look here for a better explanation than I can provide... http://en.wikipedia.org/wiki/Isolation_%28database_systems%29
Another note here. If you're using XLOCK under a SERIALIZABLE isolation, other transactions with READ COMMITTED isolation will still be able to read XLOCK'ed rows.
To prevent that, use PAGLOCK along with XLOCK.
See here for details
http://support.microsoft.com/kb/324417
How do I select all rows for a table, their isn't part of any transaction that hasn't committed yet?
Example:
Let's say,
Table T has 10 rows.
User A is doing a transaction with some queries:
INSERT INTO T (...)
SELECT ...
FROM T
// doing other queries
Now, here comes the tricky part:
What if User B, in the time between User A inserted the row and the transaction was committed, was updating a list in the system with a select on Table T.
I only want that the SELECT User B is using returned the 10 rows(all rows from the table, that can't later be rolled back). How do I do this, if it's even possible?
I have tried setting the isolationlevel on the transaction and adding "WITH(NOLOCK)" "WITH(READUNCOMMITTED)" to the query without any luck.
The query either return all 11 records or it's waiting for the transaction to commit, and that's not what I need.
Any tips is much appriciated, thanks.
You need to use (default) read committed isolation level and the READPAST hint to skip rows locked as they are not committed (rather than being blocked waiting for the locks to be released)
This does rely on the INSERT taking out rowlocks though. If it takes out page locks you will be back to being blocked. Example follows
Connection 1
IF OBJECT_ID('test_readpast') IS NULL
BEGIN
CREATE TABLE test_readpast(i INT PRIMARY KEY CLUSTERED)
INSERT INTO test_readpast VALUES (1)
END
BEGIN TRAN
INSERT INTO test_readpast
WITH(ROWLOCK)
--WITH(PAGLOCK)
VALUES (2)
SELECT * FROM sys.dm_tran_locks WHERE request_session_id=##SPID
WAITFOR DELAY '00:01';
ROLLBACK
Connection 2
SELECT i
FROM test_readpast WITH (readpast)
Snapshot isolation ?
Either I or else the three people who have answered early have misread/ misinterpreted your question, so I have given a link so you can determine for yourself.
Actually, read uncommitted and nolock are the same. They mean you get to see rows that have not been committed yet.
If you run at the default isolation level, read committed, you will not see new rows that have not been committed. This should work by default, but if you want to be sure, prefix your select with set transaction isolation level read committed.