I think I have a misunderstanding about how to use Savepoints. Perhaps someone can clear it up for me. I present my example as what I am trying to do, and what I have experienced.
My app is doing a certain procedure.
Before that procedure (and associated DB operations) I create a savepoint.
During that procedure, I initiate a select for update,
which creates a number of locks:
lock1 - duration=transaction, class=row, type=intent row=big number
lock2 - duration=transaction, class=row, type=WriteNoPK row=big number
Should that java procedure succeed, the associated DB transaction is completed via a commit.
However, if the java procedure fails, I want also to rollback any associated DB operations.
I have been attempting this by:
conn.rollback(mySavepoint);
However, this has not been releasing the table locks created (above) by the DB operations (that I thought I just rolled back by conn.rollback(mySavepoint);)
I have tested this behaviour with two databases: Sybase, and Derby.
Why is this the case?
Do I really need to commit after the conn.rollback(mySavepoint) ???
It just seems a bit counter-intuitive.
I am working with ms sql with struts framework.
While calling procedure I put autocommit false in program.
when the procedure run I have to commit one seperate transaction and it must be affect the table externally
But it never be save until conn.commit() statement execute in program.
Is it any other way to commit the transaction in procedure itself, to affect the table on the end of the single transaction in procedure?
Pl. tell me if you know.
T.Saravanan
You should start and commit/rollback a transaction at the same level, otherwise you are introducing a lot of unpredictable paths - and frankly some bad design. So: if you need to commit at the server, use BEGIN TRAN / COMMIT TRAN in the TSQL to handle the transaction locally.
Note, though, that TSQL exception/error handling is not as rich as handling errors at a caller such as java/C#. If the problem is that you want to disassociate this work from another unrelated transaction, then it depends on how your calling code works:
if it is using connection-level transactions, then you will need to use a separate connection; just run the transaction on a different connection using the java/C#/whatever transaction API (i.e. the same as your existing code, by the sound of it, but on a different connection)
if it is using things like scope-based transactions (TransactionScope in C#; not sure about java etc - but this is an LTM or DTC transaction) then you can explicitly create a new scope that is bound to either a new (isolated) transaction, or the nil-transaction (i.e. the inner scope is not enlisted)
As for affecting the tables... SQL Server generally does optimistic changes, i.e. yes the changes are applied immediately (so that commit is cheap, and rollback is more expensive) - however, the isolation level will generally prevent other SPIDs from seeing the data. A competing SPID with a low isolation level (or using the NOLOCK hint) will see the uncommitted data, but this may be a phantom/non-repeatable read if the data eventually gets rolled back.
Which procedure is more performant for an update which affects zero rows?
UPDATE table SET column = value WHERE id = number;
IF SQL%Rowcount > 0 THEN
COMMIT;
END IF;
or
UPDATE table SET column = value WHERE id = number;
COMMIT;
In other words if an Update affect ZERO rows and a commit is issued am I incurring any added expense at all?
I have a system which is being hampered by log file sync waits... and I'm wondering if issuing a commit; against a transaction which affects zero rows will write that statement to the log or not and thus cause more contention on LGWR.
COMMIT does force the log file sync so the system will have to wait indeed.
However, ROLLBACK does too and at some time either of them will have to happen.
So if you issue neither COMMIT nor ROLLBACK, you are just staying with an open transaction which sooner or later will cause a log sync wait.
Probably, you want to batch you UPDATE operations rather than waiting for a first successful update and committing it.
There are risks in this. Technically while the UPDATE may affect zero rows, it can fire before or after update triggers on the table (not at row level). Those triggers could potentially "do something" that requires a commit/rollback.
Safer to check to see if LOCAL_TRANSACTION_ID is set.
There are any number of reasons which can underlie waits for log file sync. It seems unlikely that the main culprit is committing SQL statements which have updated zero rows. It is true that issuing too many commits can be the cause of this problem. For instance, if the application is set up to commit after every statement (e.g. by using AUTOCOMMIT=TRUE) instead of designing proper transactions. If this is the cause then there is not much you can do, short of a major rewrite of the application.
If you want to delve deeper into the root causes of your problem I recommend you read this exhaustive (and exhausting) article by Pythian's Riyaj Shamsudeen on Tuning ‘log file sync’ Event Waits.
I have a vendor reporting product executing queries to pull report data, no inserts, no updates just reading data.
We have double our heap size 3 times and are now at 1024 4k pages, The app will run fine for a week then we will begin to see DB2 SQL error: SQLCODE: -954, SQLSTATE: 57011 indicating the transaction log is not able to accomodate the request.
Its not the size of the reports since they run fine after a recycle. I spoke with another DBA on this. He believe the problem was in a difference between ORACLE and DB2 in that the vendor code is crappy and not issuing commits on the selects. This is causing the references to not be cleaned up and is slowly accumulating as garbage in the heap.
I wanted to know if this is accurate as I thought only inserts and updates needed to have commits included. Is there any IBM documentation on this?
We are currently recycling on a weekly basis to alleviate the problem but I would like to have a good handle on the issue before going back to the vendor asking them to alter their code.
Any transaction needs to be properly terminated -- why did you think that only applies to inserts and updates? Consider running transactionally a "select a from b where c > 12" and then "select a from b where c <= 12"; within a transaction the DB has to guarantee that every a gets returned exactly once either from the first or second select, not both (assuming c is never null;-). Without transactionality, some a's might fall between the cracks or be returned twice if their corresponding c was changed by a different transaction, and that's just not ACID!-)
So when you do not need separate SELECT queries to be transactional wrt each other, tell the DB! And the way you tell, is by terminating the transaction after each select (normally commit is what you use for the purpose, though I guess you could, indifferently, choose to use rollback here;-).
Per Alex's response, the first SQL activity after any CONNECT, COMMIT, or ROLLBACK initiates a transaction.
To get a handle on your resource issue (transaction logs full), you should investigate your application that issues the reports - ensure that transactions are being closed out explicitly in code. I've seen cases where application developers rely upon the Garbage Collector to clean up database objects - while those objects are waiting for cleanup, the database resources (transactions) are held open.
It's always good practice to explicitly COMMIT or ROLLBACK your transactions as soon as you are done with the data - regardless of the programming methodology you use.
I get this error when committing transaction on a SELECT query, but despite the error it does return a Result-Set that include queried data.
tran.Commit();
error [hy011] [ibm] cli0126e the operation is invalid sqlstate=hy011
I changed my code to tran.Rollback(); and the error disapered.
Can anyone explain this behavior?
I have a read query that I execute within a transaction so that I can specify the isolation level. Once the query is complete, what should I do?
Commit the transaction
Rollback the transaction
Do nothing (which will cause the transaction to be rolled back at the end of the using block)
What are the implications of doing each?
using (IDbConnection connection = ConnectionFactory.CreateConnection())
{
using (IDbTransaction transaction = connection.BeginTransaction(IsolationLevel.ReadUncommitted))
{
using (IDbCommand command = connection.CreateCommand())
{
command.Transaction = transaction;
command.CommandText = "SELECT * FROM SomeTable";
using (IDataReader reader = command.ExecuteReader())
{
// Read the results
}
}
// To commit, or not to commit?
}
}
EDIT: The question is not if a transaction should be used or if there are other ways to set the transaction level. The question is if it makes any difference that a transaction that does not modify anything is committed or rolled back. Is there a performance difference? Does it affect other connections? Any other differences?
You commit. Period. There's no other sensible alternative. If you started a transaction, you should close it. Committing releases any locks you may have had, and is equally sensible with ReadUncommitted or Serializable isolation levels. Relying on implicit rollback - while perhaps technically equivalent - is just poor form.
If that hasn't convinced you, just imagine the next guy who inserts an update statement in the middle of your code, and has to track down the implicit rollback that occurs and removes his data.
If you haven't changed anything, then you can use either a COMMIT or a ROLLBACK. Either one will release any read locks you have acquired and since you haven't made any other changes, they will be equivalent.
If you begin a transaction, then best practice is always to commit it. If an exception is thrown inside your use(transaction) block the transaction will be automatically rolled-back.
Consider nested transactions.
Most RDBMSes do not support nested transactions, or try to emulate them in a very limited way.
For example, in MS SQL Server, a rollback in an inner transaction (which is not a real transaction, MS SQL Server just counts transaction levels!) will rollback the everything which has happened in the outmost transaction (which is the real transaction).
Some database wrappers might consider a rollback in an inner transaction as an sign that an error has occured and rollback everything in the outmost transaction, regardless whether the outmost transaction commited or rolled back.
So a COMMIT is the safe way, when you cannot rule out that your component is used by some software module.
Please note that this is a general answer to the question. The code example cleverly works around the issue with an outer transaction by opening a new database connection.
Regarding performance: depending on the isolation level, SELECTs may require a varying degree of LOCKs and temporary data (snapshots). This is cleaned up when the transaction is closed. It does not matter whether this is done via COMMIT or ROLLBACK. There might be a insignificant difference in CPU time spent - a COMMIT is probably faster to parse than a ROLLBACK (two characters less) and other minor differences. Obviously, this is only true for read-only operations!
Totally not asked for: another programmer who might get to read the code might assume that a ROLLBACK implies an error condition.
IMHO it can make sense to wrap read only queries in transactions as (especially in Java) you can tell the transaction to be "read-only" which in turn the JDBC driver can consider optimizing the query (but does not have to, so nobody will prevent you from issuing an INSERT nevertheless). E.g. the Oracle driver will completely avoid table locks on queries in a transaction marked read-only, which gains a lot of performance on heavily read-driven applications.
ROLLBACK is mostly used in case of an error or exceptional circumstances, and COMMIT in the case of successful completion.
We should close transactions with COMMIT (for success) and ROLLBACK (for failure), even in the case of read-only transactions where it doesn't seem to matter. In fact it does matter, for consistency and future-proofing.
A read-only transaction can logically "fail" in many ways, for example:
a query does not return exactly one row as expected
a stored procedure raises an exception
data fetched is found to be inconsistent
user aborts the transaction because it's taking too long
deadlock or timeout
If COMMIT and ROLLBACK are used properly for a read-only transaction, it will continue to work as expected if DB write code is added at some point, e.g. for caching, auditing or statistics.
Implicit ROLLBACK should only be used for "fatal error" situations, when the application crashes or exits with an unrecoverable error, network failure, power failure, etc.
Just a side note, but you can also write that code like this:
using (IDbConnection connection = ConnectionFactory.CreateConnection())
using (IDbTransaction transaction = connection.BeginTransaction(IsolationLevel.ReadUncommitted))
using (IDbCommand command = connection.CreateCommand())
{
command.Transaction = transaction;
command.CommandText = "SELECT * FROM SomeTable";
using (IDataReader reader = command.ExecuteReader())
{
// Do something useful
}
// To commit, or not to commit?
}
And if you re-structure things just a little bit you might be able to move the using block for the IDataReader up to the top as well.
If you put the SQL into a stored procedure and add this above the query:
set transaction isolation level read uncommitted
then you don't have to jump through any hoops in the C# code. Setting the transaction isolation level in a stored procedure does not cause the setting to apply to all future uses of that connection (which is something you have to worry about with other settings since the connections are pooled). At the end of the stored procedure it just goes back to whatever the connection was initialized with.
Given that a READ does not change state, I would do nothing. Performing a commit will do nothing, except waste a cycle to send the request to the database. You haven't performed an operation that has changed state. Likewise for the rollback.
You should however, be sure to clean up your objects and close your connections to the database. Not closing your connections can lead to issues if this code gets called repeatedly.
If you set AutoCommit false, then YES.
In an experiment with JDBC(Postgresql driver), I found that if select query breaks(because of timeout), then you can not initiate new select query unless you rollback.
Do you need to block others from reading the same data? Why use a transaction?
#Joel - My question would be better phrased as "Why use a transaction on a read query?"
#Stefan - If you are going to use AdHoc SQL and not a stored proc, then just add the WITH (NOLOCK) after the tables in the query. This way you dont incur the overhead (albeit minimal) in the application and the database for a transaction.
SELECT * FROM SomeTable WITH (NOLOCK)
EDIT # Comment 3: Since you had "sqlserver" in the question tags, I had assumed MSSQLServer was the target product. Now that that point has been clarified, I have edited the tags to remove the specific product reference.
I am still not sure of why you want to make a transaction on a read op in the first place.
In your code sample, where you have
// Do something useful
Are you executing a SQL Statement that changes data ?
If not, there's no such thing as a "Read" Transaction... Only changes from an Insert, Update and Delete Statements (statements that can change data) are in a Transaction... What you are talking about is the locks that SQL Server puts on the data you are reading, because of OTHER transactions that affect that data. The level of these locks is dependant on the SQL Server Isolation Level.
But you cannot Commit, or ROll Back anything, if your SQL statement has not changed anything.
If you are changing data, then you can change the isolation level without explicitly starting a transation... Every individual SQL Statement is implicitly in a transaction. explicitly starting a Transaction is only necessary to ensure that 2 or more statements are within the same transaction.
If all you want to do is set the transaction isolation level, then just set a command's CommandText to "Set Transaction Isolation level Repeatable Read" (or whatever level you want), set the CommandType to CommandType.Text, and execute the command. (you can use Command.ExecuteNonQuery() )
NOTE: If you are doing MULTIPLE read statements, and want them all to "see" the same state of the database as the first one, then you need to set the isolation Level top Repeatable Read or Serializable...