I have a question concerning transaction isolation in SQL Server. The default isolation level is set to 2 (READ_COMMITED). In the first transaction, I insert some data in table users; in the second, I try, unsuccessfully, to select all data from the same table, it seems that the second transaction waits for the first one to commit/rollback.
Does anyone have an explanation?
That is exactly what the read committed means, the second transaction will wait until it is able to read all the data. There are ways to either read the uncommitted data or skip it, but that is not something you would normally want to do.
There is quite a lot of material available for this, for example this one in TechNet
Related
I just learned that in Postgresql the default transaction isolation level is "Read committed". I'm very used to MySQLs "REPEATABLE READ" isolation level. In postgresql by my understanding this means in a default transaction "two successive SELECT commands can see different data". With that in mind, is there any benefit to transactions when only the last statement in the transaction is writing?
The transaction does not prevent you from data changing between statements, the only benefit I see is rolling the transaction back on failure. But if only one writing statement exists at the end, then that would happen anyway.
To make a bit more clear what I'm referring to, lets take a generic simple sequence of (pseudo) queries to a table:
BEGIN TRANSACTION
SELECT userId FROM users WHERE username = "the provided username"
INSERT INTO activites (activity, user_fk) VALUES ("posted on SO", userId)
COMMIT
In this sequence and any general sequence of statments where only the last statement is writing, is there a benefit in postgresql to using a transaction with the default isolation level?
Bonus question, is there any overhead from it?
The difference between READ COMMITTED and REPEATABLE READ is that the former takes a new database snapshot for each statement, while the latter takes a snapshot only for the first SQL statement and uses that snapshot for the whole transaction. This implies that REPEATABLE READ actually performs better that READ COMMITTED, since it takes fewer snapshots.
The disadvantage of REPEATABLE READ is that you can get serialization errors. That does not affect your example, but if you had an UPDATE instead of an INSERT, it could be that the row you are trying to update has been modified by a concurrent transaction since the snapshot was taken. The serialization error that causes would mean that you have to repeat the transaction. Another disadvantage of REPEATABLE READ transactions is that a long-running read-only transaction can hinder the progress of VACUUM, which it wouldn't do in READ COMMITTED mode.
For read-only transactions or transactions like the one you are showing, REPEATABLE READ is often the better isolation level. The nice thing about READ COMMITTED is that you can get no serialization errors apart from deadlocks.
To explicitly answer your question: there is no advantage to running the statement from your example in a single transaction. You may as well use the default autocommit mode to run them in separate transactions.
Incidentally, the SQL standard decrees that the default transaction isolation level be SERIALIZABLE, but I don't know any database that implements that.
If you have the following sql, is it possible that if it is run multiple times by many different processes at exactly the same time, that two or more processes may update the table?
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
UPDATE table
SET Column1 = 1
WHERE Column1 = 0
No other locks etc are specified in the sql, other that Read Uncommitted.
I'm trying to track down an issue, and I'm now clutching at straws...
Got this 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. When this option is set, it is possible to read uncommitted modifications, which are called dirty reads. Values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction. This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction. This is the least restrictive of the isolation levels.
So basically, this is equivalent to SQL Server , NOLOCK hint. This might result in dirty reads, i.e. if some process in updated 1000 records and updated 500 till now, and other process read that data, then data might be in inconsistent form. This also helps in executing update without getting blocked (shared lock) by multiple select queries.
Hope this make some sense to your question. For reference -- MSDN
Currently, we have a lot of background services running. These services are inserting/updating bulk amount of data to the server. This is why our SELECT queries are blocking. So, for temporary I need to make all all tables and all views isolation level to read uncommitted. Is it possible?
First of all, I would like to tell you, reading uncommited data is nothing but Dirty Read.
for eg, of you are working with AdventureWorks Database, then you just fire this query
use AdventureWorks
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITED.
This will help you to view all the records created / updated by some transaction but not yet commited.
By default, in SQL Server, Transaction Isolation Level is set to READ COMMITED.
so untill there is exclusive lock locked by some query, other transaction is not able to view the data of other transaction which is in uncommited state.
Feel free to reply about the result.
Kind Regards,
Ashay (India)
in your select query use hint NoLock
select * from userTbl with(NOLOCK)
I remember an example where reads in a transaction then writing back the data is not safe because another transaction may read/write to it in the time between. So i would like to check the date and prevent the row from being modified or read until my transaction is finish. Would this do the trick? and are there any sql variants that this will not work on?
update tbl set id=id where date>expire_date and id=#id
Note: date>expire_date happens to be my condition. It could be anything. Would this prevent other transaction from reading the row until i commit or rollback?
In a lot of cases, your UPDATE statement will not prevent other transactions from reading the row.
ziang mentioned transaction isolation levels.
Depending on the isolation level, databases use different types of locking. At the highest level, locking can be divided into two categories:
- pessimistic,
- optimistic
MS SQL 2008, for example, has 6 isolation levels, 4 of them are pessimistic, 2 are optimistic. By default , it uses READ COMMITTED isolation level, which falls into the pessimistic category.
Oracle, on another note, uses optimistic locking by default.
The statement that will lock your record for writing is
SELECT * FROM TBL WITH UPDLOCK WHERE id=#id
From that point on, no other transaction will be able to update your record with id=#id
And only transactions running in isolation level READ UNCOMMITTED will be able to read it.
With the default transaction level, READ COMMITTED, no other thansaction will be able to read or write into this record until you either commit or roll back your entire transaction.
It depends on the transaction isolation level you set on your transaction control. There are 4 types of read
READ UNCOMMITTED: this allows the dirty read
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
for more info, you can check msdn.
You should be able to do this in a normal select using a combination of
HOLDLOCK/ROWLOCK
It very well may work. Different platforms offer different services. For instance, in T-SQL, you can simply set the isolation level of the transaction and, as a result, force a lock to be obtained. I don't know what platform you are using so I cannot answer your question definitively.
My work has a financial application, written in VB.NET with SQL, that several users can be working on at the same time.
At some point, one user might decide to Post the batch of entries that they (and possibly other people) are currently working on.
Obviously, I no longer want any other users to add, edit, or delete entries in that batch after the Post process has been initiated.
I have already seen that I can lock all data by opening the SQL transaction the moment the Post process starts, but the process can be fairly lengthy and I would prefer not to have the Transaction open for the several minutes it might take to complete the function.
Is there a way to lock just the records that I know need to be operated on from VB.NET code?
If you are using Oracle you would Select for update on the rows you are locking.
here is an example
SELECT address1 , city, country
FROM location
FOR UPDATE;
You probably want to set an isolation level for the entire transaction rather than using with (rowlock) on specific tables.
Look at this page:
http://msdn.microsoft.com/en-us/library/ms173763.aspx
Specifically, search within it for 'row lock', and I think you'll find that READ COMMITTED or REPEATABLE READ are what you want. READ COMMITTED is the SQL Server default. If READ COMMITTED doesn't seem strong enough to you, then go for REPEATABLE READ.
Update: After reading one of your follow up posts, you definitely want repeatable read. That will hold the lock until you either commit or rollback the transaction.
add
with (rowlock)
to your SQL query
SQL Server Performance article
EDIT: ok, I misunderstood the question. What you want is transaction isolation. +1 to Joel :)
wrap it in a tran use an holdlock + updlock in the select
example
begin tran
select * from
SomeTable (holdlock,updlock)
where ....
processing here
commit