I was fiddling around in a table testing some inserts and updates.
I always started my commands with a BEGIN; and when I saw the results, I would run a ROLLBACK; afterwards.
I accidentally hit F5 twice with ROLLBACK; highlighted, and it rolled back to being empty.
Is there a way to undo the ROLLBACK;?
Edit: as it turns out, this didn't play out as I'd thought. I didn't hit F5 twice causing some double-rollback, it just happened that there was a delete being run at the same time as I was interacting with the DB and nobody had informed me. Still, I'll keep this around for now to show that there is seemingly no way to undo a rollback.
Thanks to #visch for the answer:
I don't believe so tutorialspoint.com/postgresql/postgresql_transactions.htm
Edit: to provide more context and make everyone happy, you can't undo a rollback because the transaction is effectively blown away in the following circumstances:
An error within the DB occurs
The connection is lost
Commit is run
Rollback is run
So once any of those conditions are met, the transaction is wiped - no records are stored of transactions because that's the whole point
Related
By mistake, I performed this query in informix using dbaccess session.
Delete from table #without where condition
Realizing my mistake, that I should have used TRUNCATE, I did another foolishness.
I killed the dbaccess session. But the table is exclusively locked and I am not able to do any action on that table.
What are the steps I can do to remove the lock and truncate the table.
1) Restart Informix server
2) onmode -z <sessionid> # Does not work.
I see hell lot of sessions created for the delete query
Is there any other easy way to fix this issue?
Assuming that you are not using Informix SE...
Is the database logged? If so, did you run the statement inside an explicit (BEGIN WORK) transaction?
Analysis
If you've got an unlogged database, then each row that the server's deleted is gone. If you stop the DELETE, it will not undo the partially complete changes. Using an unlogged database means that you do not want guaranteed statement level recovery.
If you've got a regular logged database and no explicit transaction, then the statement is probably still running after the DB-Access session is terminated. Because it is running as a singleton statement, it will complete and commit. Until it does that, if you forcibly take the server down, then fast recovery will rollback the statement (transaction). Given that I see '5 hours ago', I fear your chances of taking the server down in time now are limited.
If you've got a logged database with an explicit transaction, or a MODE ANSI database (where you're always in a transaction), then when the DELETE statement completes, the server will wait for the COMMIT, realize that the session connection is terminated, and will rollback the uncommitted work.
Recovery
If you've got an unlogged database, you can only recover to your last archive. Because it is unlogged, you can't recover it from the logical logs (but other databases in the same instance that are logged can be recovered up to the last logical log).
If you've got a logged database and you can take the server down — preferably under control, but crashing it if necessary — before the DELETE statement completes, then fast recovery will deal with the issue.
If the DELETE has completed and committed and you have good backups, you can consider a point-in-time restore of the database. It will take it offline while you do that (but if the data from the table is all missing, your DB is not going to be functional for a while).
If none of these scenarios applies, then you should contact IBM Technical Support, who may be able to perform minor (and not so minor) miracles.
But, as you may have noticed, a lot depends on the type of database (unlogged, logged, MODE ANSI) and whether there was an explicit transaction in effect when you ran the statement.
The trouble with DBMS is that they're trusting creatures. If you're authorized to do an operation, they assume that you intend to do what you say you want to do, and they go ahead and do it to the best of their ability. When you don't ask it to do what you intended to request, life gets tricky; the DBMS still trusts you and does what you actually asked it to do.
I'm writing some SQL code that needs to be executed when rows are inserted in a database table, so I'm using an AFTER INSERT trigger; the code is quite complex, thus there could still be some bugs around.
I've discovered that, if an error happens when executing a trigger, SQL Server aborts the batch and/or the whole transaction. This is not acceptable for me, because it causes problems to the main application that uses the database; I also don't have the source code for that application, so I can't perform proper debugging on it. I absolutely need all database actions to succeed, even if my trigger fails.
How can I code my trigger so that, should an error happen, SQL Server will not abort the INSERT action?
Additionally, how can I perform proper error handling so that I can actually know the trigger has failed? Sending an email with the error data would be ok for me (the trigger's main purpose is actually sending emails), but how do I detect an error condition in a trigger and react to it?
Edit:
Thanks for the tips about optimizing performance by using something else than a trigger, but this code is not "complex" in the sense that it's long-running or performance intensive; it simply builds and sends a mail message, but in order to do so, it must retrieve data from various linked tables, and since I am reverse-engineering this application, I don't have the database schema available and am still trying to find my way around it; this is why conversion errors or unexpected/null values can still creep up, crashing the trigger execution.
Also, as stated above, I absolutely can't perform debugging on the application itself, nor modify it to do what I need in the application layer; the only way to react to an application event is by firing a database trigger when the application writes to the DB that something has just heppened.
If the operations in the trigger are complex and/or potentially long running, and you don't want the activity to affect the original transaction, then you need to find a way to decouple the activity.
One way might be to use Service Broker. In the trigger, just create message(s) (one per row) and send them on their way, then do the rest of the processing in the service.
If that seems too complex, the older way to do it is to insert the rows needing processing into a work/queue table, and then have a job continuously pulling rows from there are doing the work.
Either way, you're now not preventing the original transaction from committing.
Triggers are part of the transaction. You could do try catch swallow around the trigger code, or somewhat more professional try catch log swallow, but really you should let it go bang and then fix the real problem which can only be in your trigger.
If none of the above are acceptable, then you can't use a trigger.
I am connecting to a SQL Server using no autocommit. If everything is successful, I call commit. Otherwise, I just exit. Do I need to explicitly call rollback, or will it be rolled back automatically when we close the connection without committing?
In case it matters, I'm executing the SQL commands from within proc sql in SAS.
UPDATE: It looks like SAS may call commit automatically at the end of the proc sql block if rollback is not called. So in this case, rollback would be more than good practice; it would be necessary.
Final Update: We ended up switching to a new system, which seems to me to behave the opposite of our previous one. On ending the transaction without specifying committing or rolling back, it will roll back. So, the advice given below is definitely correct: always explicitly commit or rollback.
It should roll back on close of connection. Emphasis on should for a reason :-)
Proper transaction and error handling should have you always commit when the conditions for commit are met and rollback when they aren't. I think it is a great habit to always commit or rollback when done and not rely on disconnect/etc. All it takes is one mistake or incorrectly/not closed session to create a blocking chain nightmare for all :-)
When working with database transactions, what are the possible conditions (if any) that would cause the final COMMIT statement in a transaction to fail, presuming that all statements within the transaction already executed without issue?
For example... let's say you have some two-phase or three-phase commit protocol where you do a bunch of statements, then wait for some master process to tell you when it is ok to finally commit the transaction:
-- <initial handshaking stuff>
START TRANSACTION;
-- <Execute a bunch of SQL statements>
-- <Inform master of readiness to commit>
-- <Time passes... background transactions happening while we wait>
-- <Receive approval to commit from master (finally!)>
COMMIT;
If your code gets to that final COMMIT statement and sends it to your DBMS, can you ever get an error (uniqueness issue, database full, etc) at that statement? What errors? Why? How do they appear? Does it vary depending on what DBMS you run?
COMMIT may fail. You might have had sufficent resources to log all the changes you wished to make, but lack resources to actually implement the changes.
And that's not considering other reasons it might fail:
The change itself might not fit the constraints of the database.
Power loss stops things from completing.
The level of requested selection concurrency might disallow an update (cursors updating a modified table, for example).
The commit might time out or be on a connection which times out due to starvation issues.
The network connection between the client and the database may be lost.
And all the other "simple" reasons that aren't on the top of my head.
It is possible for some database engines to defer UNIQUE index constraint checking until COMMIT. Obviously if the constraint does not hold true at the time of commit then it will fail.
Sure.
In a multi-user environment, the COMMIT may fail because of changes by other users (e.g. your COMMIT would violate a referential constraint when applied to the now current database...).
Thomas
If you're using two-phase commit, then no. Everything that could go wrong is done in the prepare phase.
There could still be network outage, power less, cosmic rays, etc, during the commit, but even so, the transactions will have been written to permanent storage, and if a commit has been triggered, recovery processes should carry them through.
Hopefully.
Certainly, there could be a number of issues. The act of committing, in and of itself, must make some final, permanent entry to indicate that the transaction committed. If making that entry fails, then the transaction can't commit.
As Ignacio states, there can be deferred constraint checking (this could be any form of constraint, not just unique constraint, depending on the DBMS engine).
SQL Server Specific: flushing FILESTREAM data can be deferred until commit time. That could fail.
One very simple and often overlooked item: hardware failure. The commit can fail if the underlying server dies. This might be disk, cpu, memory, or even network related.
The transaction could fail if it never receives approval from the master (for any number of reasons).
No matter how wonderfully a system may be designed, there is going to be some possibility that a commit will get into a situation where it's impossible to know whether it succeeded or not. In some cases, it may not matter (e.g. if a hard drive holding the database turns into a pile of slag, it may be impossible to tell whether the commit succeeded or not before that occurred but it wouldn't really matter); in others cases, however, this could be a problem. Especially with distributed database systems, if a connection failure occurs at just the right time during a commit, it will be impossible for both sides to be certain of whether the other side is expecting a commit or a rollback.
With MySQL or MariaDB, when used with Galera clustering, COMMIT is when the other nodes in the cluster are checked. So, yes important errors can be discovered by COMMIT, and you must check for these errors.
The in-house application framework we use at my company makes it necessary to put every SQL query into transactions, even though if I know that none of the commands will make changes in the database. At the end of the session, before closing the connection, I commit the transaction to close it properly. I wonder if there were any particular difference if I rolled it back, especially in terms of speed.
Please note that I am using Oracle, but I guess other databases have similar behaviour. Also, I can't do anything about the requirement to begin the transaction, that part of the codebase is out of my hands.
Databases often preserve either a before-image journal (what it was before the transaction) or an after-image journal (what it will be when the transaction completes.) If it keeps a before-image, that has to be restored on a rollback. If it keeps an after-image, that has to replace data in the event of a commit.
Oracle has both a journal and rollback space. The transaction journal accumulates blocks which are later written by DB writers. Since these are asychronous, almost nothing DB writer related has any impact on your transaction (if the queue fills up, then you might have to wait.)
Even for a query-only transaction, I'd be willing to bet that there's some little bit of transactional record-keeping in Oracle's rollback areas. I suspect that a rollback requires some work on Oracle's part before it determines there's nothing to actually roll back. And I think this is synchronous with your transaction. You can't really release any locks until the rollback is completed. [Yes, I know you aren't using any in your transaction, but the locking issue is why I think a rollback has to be fully released then all the locks can be released, then your rollback is finished.]
On the other hand, the commit is more-or-less the expected outcome, and I suspect that discarding the rollback area might be slightly faster. You created no transaction entries, so the db writer will never even wake up to check and discover that there was nothing to do.
I also expect that while commit may be faster, the differences will be minor. So minor, that you might not be able to even measure them in a side-by-side comparison.
I agree with the previous answers that there's no difference between COMMIT and ROLLBACK in this case. There might be a negligible difference in the CPU time needed to determine that there's nothing to COMMIT versus the CPU time needed to determine that there's nothing to ROLLBACK. But, if it's a negligible difference, we can safely forget about about it.
However, it's worth pointing out that there's a difference between a session that does a bunch of queries in the context of a single transaction and a session that does the same queries in the context of a series of transactions.
If a client starts a transaction, performs a query, performs a COMMITor ROLLBACK, then starts a second transaction and performs a second query, there's no guarantee that the second query will observe the same database state as the first query. Sometimes, maintaining a single consistent view of the data is of the essence. Sometimes, getting a more current view of the data is of the essence. It depends on what you are doing.
I know, I know, the OP didn't ask this question. But some readers may be asking it in the back of their minds.
In general a COMMIT is much faster than a ROLLBACK, but in the case where you have done nothing they are effectively the same.
The documentation states that:
Oracle recommends that you explicitly end every transaction in your application programs with a COMMIT or ROLLBACK statement, including the last transaction, before disconnecting from Oracle Database. If you do not explicitly commit the transaction and the program terminates abnormally, then the last uncommitted transaction is automatically rolled back. A normal exit from most Oracle utilities and tools causes the current transaction to be committed. A normal exit from an Oracle precompiler program does not commit the transaction and relies on Oracle Database to roll back the current transaction.
http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/statements_4010.htm#SQLRF01110
If you want o choose to do one or the other then you might as well do the one that is the same as doing nothing, and just commit it.
Well, we must take into account what an SELECT returns in Oracle. There are two modes. By default an SELECT returns data as that data looked in the very moment the SELECT statement started executing (this is default behavior in READ COMMITTED isolation mode, the default transactional mode). So if an UPDATE/INSERT was executed after SELECT was issued that won't be visible in result set.
This can be a problem if you need to compare two result sets (for example debta and credit sides of an general ledger app). For that we have a second mode. In that mode SELECT returns data as it looked at the moment the current transaction began (default behavior in READ ONLY and SERIALIZABLE isolation levels).
So, at least sometimes it is necessary to execute SELECTs in transaction.
Since you've not done any DML, I suspect there'd be no difference between a COMMIT and ROLLBACK in Oracle. Either way there's nothing to do.
I'd think a Commit would be more efficient; since generally you'd expect most DB transactions to be committed; so you would think the DB optimizes for this case (as opposed to trying to be more efficient for a rollback).