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.
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.
These days I improved my knowledge about Isolation Level of transactions.
In particular, i had problem to understand well the difference between Read Committed and Repeatable Read.
I've read this fantastic article, and I understand all about dirty-reads, non-repeatable read and phantoms read but i don't understand what happen with multiple transactions in update.
Example:
Table: "Test" (Field: ID_REC - Data)
Transaction A:
set transaction isolation level read committed;
begin transaction
update test
set DATA = 't1'
where ID_REC = 1
waitfor delay '00:00:20'
commit transaction
Transaction B: (the same but with another record)
set transaction isolation level read committed;
begin transaction
update test
set DATA = 't2'
where ID_REC = 2
commit transaction
I execute both transactions in two seconds.
Second transaction doesn't start until first one is finished.
And also i can't execute query (select * from test).
So: why happen this? Transactions works on different rows... and about this case, what is the difference between Read Committed and Repeatable Read?
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