i am interesting to know know how the isolation level"READ COMMITTED" is provided in Oracle DB implementation. I already know that DB makes records in REDO log, but for now i think that that REDO log is only used to repeat the transaction in case when some unpredictable crash will happen during the transaction. Also i know that DBWR writes the "dirty blocks" every time the REDO log file is filled. But my question is: if DBWR writes "dirty"(changed blocks) to the disk, how isolation level"READ COMMITTED" is provided. I mean during writing DBWR writes data directly to data files or in some special "place" on disk that is visible from current transaction and invisible from other transaction? So after the COMMIT this "place" becomes visible and that's all ? How this works in reality? Sorry for bad English.
In addition to the REDO log, you also have the UNDO tablespace.
When updating data, the old value is stored in the UNDO tablespace. When Oracle sees that you would be reading uncommitted data for a record, it reconstructs the old value from there.
UNDO is also used during database recovery: In addition to re-applying writes that have been committed but not made it to the database files before the crash, the opposite can also take place: rolling back uncommitted changes to database files that happened before the crash.
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i am currently studying transaction management in dbms .... from database systems elmasri and navathe 6th edition link: http://mathcomp.uokufa.edu.iq/staff/kbs/file/2/Fundamentals%20of%20Database%20Systems%20-%20Ramez%20Elmasri%20&%20Navathe.pdf
can someone please tell (in short) the transaction commit process i.e. without going into too much detail .... i also read some portion from oracle forum link :
https://docs.oracle.com/cd/B19306_01/server.102/b14220/transact.htm
what i could understand is that actual writing can take place before or after committing ... but if changes made have to be visible to all users then it must take before commit not after commit , right ?
can someone please help me clear the confusion ??
As the forum post indicates, writes to the data files are completely independent of transaction control. Changes might be written before a transaction commits, they might be written after the transaction commits.
When changes are made, those changes are made to a version of the data in memory. In order for a transaction to commit successfully (assuming default commit settings), the change must be written to the redo logs. That allows the database to re-create the change if it has not been written to data files and the database crashes. Conversely, if a change is written to a data file before the transaction is committed, information on how to reverse the change will be in the undo logs so that if the transaction will be able to be rolled back if the database fails before the transaction commits (or if the application issues a rollback).
I need to write a sql select statement without creating any locks in tables but read committed records.
Can someone help please...
Reading/Selecting data under default transaction isolation level doesn't lock the table, but it obtains something called Shared Locks on the resources, It means multiple users can read the same rows all obtaining Shared Locks on the resources.
And when a user modifies a row it obtains an Exclusive Lock on the resources. Exclusive lock means no one else can access the data while its being modified. It is exclusively locked by that user.
Therefore moral of the story is stick to Default Transaction Isolation Level Read Committed and it will obtain a lock (shared lock) on the row before retrieving it, to avoid Dirty reads.
Otherwise less strict isolation level read uncommitted does not obtain any locks and will result in dirty reads.
You can turn on the READ_COMMITTED_SNAPSHOT database option. With that option on, row versioning instead of locking is used to provide the default READ_COMMITTED isolation behavior.
There is some cost when this option is enabled. There is an additional 14-bytes for each row plus the overhead of maintaining the row version store in tempdb. However, the overhead can be more than offset by concurrency improvements depending on your workload. You also need to make sure applications are not coded to rely on the default locking behavior.
Program to insert into my 2 tables is written through Entity Framework and to SELECT the data is through a STORED PROC at SQL SERVER level. There is a point when SELECT and INSERT is getting done at the same time simultaneously. And when hitting that point, I got the below error:
Transaction (Process ID) was deadlocked on resources with another process and has been chosen as the deadlock victim. Rerun the transaction.
How can I get rid of this DEADLOCK problem here? Need the best way to solve it.
Option 1: Implementing NOLOCK? What would be the PROS and CONS here for it?
Option 2: IF there is any way to exceed the DEADLOCK wait time so that it can wait for the resource for a longer time than usually it does? If yes, then HOW?
Option 3: Suggest Me?
Thanks,
Rahuul Dutta
A deadlock cannot be cured by increasing lock timeout. The resources are locked in such a way that it cannot be resolved by itself, regardless of how much time you can give it. A special background process in SQL Server, a deadlock monitor, periodically (rather often, actually) runs and if it identifies a deadlock it kills the 'lighter' transaction immediatelly.
The deadlocks are usually dealt with in one of several ways: by providing an alternative data access path for the SELECT query (ie adding a mnnclustered index), minimizing the transaction duration (by better indexing, again), or using one of snapshot isolation levels.
The least effort solution here will be setting the read committed snapshot isolation level. This way the SELECT query will not issue any shared locks on data, but still read only the committed data, which is a huge plus over using the NOLOCK hint (or read uncommitted isolation level).
You can change your transaction isolation level. Best option for deadlocks would be snapshot isolation i think. If you cannot turn this option on in your server or if you run into I/O issues, read committed should still prevent deadlocks from read/write dependencies. Make sure that you don't run into anomalies, read committed will allow non-repeatable reads and phantom reads.
First of all, thanks a lot for your precious answers!
With the help of your answers, some research and a call with Microsoft DBA team, I have got the following solution.
Solution: To implement this solution we have to change the database property to Read Committed Snapshot. This will help the Select statements in avoiding the blocks in case of locks by other sessions on the same table.
- To cater this solution the database will create a snapshot of the data in tempdb. Therefore we must have sufficient space in tempdb. Also if possible we must shift the tempdb to a new disk to split the I/O. This will improve the performance.
The following kb article helps in enabling Read Committed Snapshot property of the database:
http://technet.microsoft.com/en-us/library/ms175095(v=SQL.105).aspx
Alternately we can change this property through SSMS by right clicking the database---options---Miscellaneous----Is Read Committed Snapshot On. We have to change the value of this property to TRUE.
We do not have to restart the server to enable this property however we must note that 'When setting the READ_COMMITTED_SNAPSHOT option, only the connection executing the ALTER DATABASE command is allowed in the database. There must be no other open connection in the database until ALTER DATABASE is complete. The database does not have to be in single-user mode.'
This means we need a small amount of downtime from the application side.
Hope the MOM above would help you all too. :)
Thanks,
Rahuul Dutta
My app needs to batch process 10M rows, the result of a complex SQL query that join tables.
I'll plan to be iterating a resultset, reading a hundred per iteration.
To run this on a busy OLTP production DB and avoid locks, I figured I'll query with a READ UNCOMMITTED isolation level.
Would that get the query out of the way of any DB writes? avoiding any rows/table locks?
My main concern is my query blocking any other DB activity, I'm far less concerned with the other way around.
Side Notes:
1. I'll be reading historical data, so I'm unlikely to meet uncommitted data. It's OK if I do.
2. The iteration process could take hours. The DB connection would remain open through this process.
3. I'll have two such concurrent batch instances at most.
4. I can tolerate dup rows. (by product of read uncommitted).
5. DB2 is the target DB, but I want a solution that fits other DBs vendors as well.
6. Will snapshot isolation level help me clear out server memory?
Have you actually encountered any real locks on read?
As far as I'm concerned, the only reason that READ UNCOMMITED existed in SQL standard was to allow non-locking reads. So I don't know DB2, but I blindly bet that it does not lock data during read in READ UNCOMMITED mode. Most modern RDBMS systems however don't lock data at all during read (*). So READ UNCOMMITED is either not available (in Oracle, for example) or is silently promoted to READ COMMITED (PostgreSQL).
If you can freely choose the engine, either check DB2 transaction isolation level handling or go for Oracle/PostgreSQL/other.
(*) More precisely, they don't exclusively lock the data. Some shared locks can be placed on queried tables so no DDL alters them during read.
My answer applies to SQL Server.
Read committed releases lock after every row read (approximately). Locking is probably not your problem.
I recommend you use the safer READ COMMITTED. Better yet, use snapshot isolation. That removes many locking problems. There are disadvantages as well, sou you better read a little about it.
My main concern is my query blocking any other DB activity
Snapshot isolation makes all locking concerns go away for read-only transactions. No blocking either way, full data consistency. Be aware that long-running transactions can cause TempDB to fill with snapshot versions.
The DB connection would remain open through this process.
That's a problem because a network hiccup, app deployment or mirroring failover would kill your batch process.
Be aware, that read uncommitted can cause queries to sporadically fail outright. You need retry logic or tolerate failed jobs.
In sql server Transaction isolation level Read uncommitted cause no lock on table.
Hi I'm trying to see what's locking the database and found 2 types of locking. Optimistic and Pessimistic Locking. I found some articles on Wiki but I would like to know more ! Can someone explain me about those locking ? We should only use locking when we need exclusive access to something? Locking only happens when we use transaction?
Thanks in advance.
Kevin
Optimistic locking is no locking at all.
It works by noting the state the system was in before you started making your changes, and then going ahead and just making those changes, assuming (optimistically) that no one else will want to make conflicting updates. Just as you are about to atomically commit those changes, you would check if in the mean-time someone else has also updated the same data. In which case, your commit fails.
Subversion for example using optimistic locking. When you try to commit, you have to handle any conflicts, but before that, you can do on your working copy whatever you want.
Pessimistic locks work with real locks. Assuming that there will be contention, you lock everything you want to update before touching it. Everyone else will have to wait for you to commit or rollback.
When using a relational database with transaction support, the database usually takes care of locking internally (such as when you issue an UPDATE statement), so for normal online processing you do not need to handle this yourself. Only if you want to do maintenance work or large batches do you sometimes want to lock down tables.
We should only use locking when we need exclusive access to something?
You need it to prevent conflicting operations from other sessions. In general, this means updates. Reading data can normally go on concurrently.
Locking only happens when we use transaction?
Yes. You will accumulate locks while proceeding with your transaction, releasing all of them at the end of it. Note that a single SQL command in auto-commit mode is still a transaction by itself.
Transactions isolation levels also specify the locking behaviour. BOL refers:Transaction isolation levels control:
Whether locks are taken when data is read, and what type of locks are requested.
How long the read locks are held.
Whether a read operation referencing rows modified by another transaction:
Blocks until the exclusive lock on the row is freed.
Retrieves the committed version of the row that existed at the time the statement or transaction started.
Reads the uncommitted data modification.
The default levels are:
Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read)
Read committed (Database Engine default level)
Repeatable read
Serializable (the highest level, where transactions are completely isolated from one another)