I'm trying to set up a custom error message to pass to MS Access (from SQL Server) when a user enters a duplicate key (instead of system message 2627). I've read up on sp_addmessage and RAISERROR and TRY/CATCH blocks which all make perfect sense. But nowhere I've looked does it seem to say where you put the RAISERROR code (and TRY/CATCH block) so it will actually pass back to the application. So, where does the code go?
Don't think about it in terms of users entering duplicate keys. Instead, think in terms of users just entering keys, some of which turn out to be duplicates when you try to insert them. It's a subtle difference, but it helps you here, because it means you think in terms of having your code available for all new table inserts instead of just a specific type of insert.
When a user enters a key, an INSERT sql statement runs. If the key is a duplicate and you have the constraints defined on the table to the prevent that, the INSERT statement fails. If your Access application has you writing custom SQL, you can wrap this in a TRY/CATCH, and put the RAISERROR in the CATCH block. If your Access application is such that you never see any SQL, you may be stuck, and have to put up with the built-in behavior.
Related
Using MS SQL Server, a Trigger calls a Stored Procedure which internally makes a select, will the return values be the new or old ones?
I know that inside the trigger I can access them by FROM INSERTED i inner join DELETED, but in this case I want to reuse (cannot change it) an existing Stored Procedure that internally makes a select on the triggered table and processes some logic with them. I just want to know if I can be sure that the existing logic will work or not (by accessing the NEW values).
I can simply try to simulate it with one update... But maybe there are other cases (example: using transactions or something else) that I maybe not be aware and never test it that could result in a different case.
I decided to ask someone else that might know better. Thank you.
AFTER triggers (the default) fire after the DML action. When the proc is called within the trigger, the tables will reflect changes made by the statement that fired the trigger as well changes made within the trigger before calling the proc.
Note changes are uncommitted until the trigger completes or explict transaction later committed.
Since the procedure is running in the same transaction as the (presumably, "after") trigger, it will see the uncommitted data.
I hope you see the implications of that: the trigger is executing as part of the transaction started by the DML statement that caused it to fire, so the stored procedure is part of the same transaction, so a "complicated" stored procedure means that transaction stays open longer, holding locks longer, making responses back to users slower, etc etc.
Also, you said
internally makes a select on the triggered table and processes some logic with them.
if you just mean that the procedure is selecting the data in order to do some complex processing and then write it to somewhere else inside the database, ok, that's not great (for reasons given above), but it will "work".
But just in case you mean you are doing some work on the data in the procedure and then returning that back to the client application, Don't do that
The ability to return results from triggers will be removed in a future version of SQL Server. Triggers that return result sets may cause unexpected behavior in applications that aren't designed to work with them. Avoid returning result sets from triggers in new development work, and plan to modify applications that currently do. To prevent triggers from returning result sets, set the disallow results from triggers option to 1.
Is there some configuration option for MS SQL Server which enables more verbose error messages.
Specific example: I would like to see the actual field values of the inserted record which violates a constraint during an insert, to help track down a bug in stored procedures which I haven't been able to reproduce.
I don't believe there is any such option. There are trace flags that give more information about deadlocks, but I've never heard of one that gives more information on a constraint violation.
If you control the application that is causing the crash then extending it's handling (as Jenn suggested) to include parameter values etc. Once you have the parameter values you can get a copy of live setup on a non-live server and start debugging the issue.
For more options, can any of the users affected reliably reproduce the issue? If they can then you might be able to run a profiler trace to capture the actual statements / parameter values being sent to the database. Of course, if you can figure out the steps to reproduce the issue then you can probably use more traditional debugging methods...
You don't say what the constraint is, I'm assuming it is a fairly complex constraint. If so, could it be broken down into several constraints so you can get more of a hint about the problem with the data?
You could also re-write the constraint as a trigger which could then include more information in the error that it raises. Although this would obviously need testing before being deployed to a production server!
Personally, I would go with changing the error handling of the application. It is probably the less risky change.
PS The application that I helped write, and now spend my time supporting, logs quite a lot of data when an unhandled exception occurs. If it is during a save then our data access layer attaches the complete list of all commands that were being run as part of the save transaction including parameter values. This has proved to be invaluable on many occasions, including some when tracking down constraint violations.
In a stored proc, what I do to get better informatino in a complex SP about the errors is take advantage of the fact that table variables are not affected by a rollback. So I put the information I want to use to troubleshoot into table variables at the time I create it and then if I hit the catch block and rollback, after the rollback I insert the data from the table variable into an exception table along with some meta data like the datetime.
With some thought you can design an exception table that will capture what you need from just about any proc (for instance you could concatenate all the input variables into one field, you could put in the step number that failed (of course then you have to assign stepnumbers to a variable) or you could log every step along the awy and then the last one logged is the one it failed on. Belive me when you are looking at troubleshooting a 100 line SP, this can come in handy. If I have dymanic SQl inteh proc, I can log the SQL variable that contains the dynamic code that was run.
The beauty of this is now you don't have to try to reproduce the error, you know what the input parameters were and any other information you find useful. Yes it can take a bit of time to set up once, but once you do it is relatively easy to get in the habit of putting it into any complex proc that you will want to log errors on.
You might also want to set a an nongeneralized one if you want to return spefic data values of a select used for an insert or the result set of a select that would tell you waht what wopuld have been being updated or deleted. Then you would have that only if the proc failed. This is a little more work than the general exception table but may be needed in some complex cases.
I noticed today a couple of errors from my application where it had attempted to insert a record into a table and received a timeout error due to a temporary network delay. The application is coded to recognise this and retry in such cases, which it obediently did. But on the retry it experienced a primary key violation - essentially because the first insert statement had actually completed, but the timeout had occurred transmitting the response back to the client. A primary key violation is considered by the application to be a serious logic error that shouldn't happen and hence it aborted the entire process.
The question to me is what layer should logically be responsible for handling this sort of thing? Ideally I would have thought the SQL client library (in this case ADO.NET 4.0) should do so, but it has no mechanism for auto-retries that I know of. Given it doesn't, it seems like there's a case for low-level wrapper around the SQL client library that can, but I don't see how it could be written without more access to information about when the timeout occurred: e.g. in this sort of example it's possible that
a) the INSERT statement is simply inserting new records using auto-incrementing keys, and hence retrying after a timeout would cause insertion of a duplicate record, OR
b) the primary key violation WAS in fact a logic error, and the initial attempt to insert a a record would have generated the same violation if the timeout hadn't occurred
OTOH I'm sure I could think of examples where whether to retry could only be determined at the application level (especially if it required user confirmation).
I'm actually slightly surprised I'd never seen this particularly sequence of errors before, as it seems quite likely in practice.
Apparently one solution to this particular problem is to use 'SET XACT_ABORT ON'.
This reportedly causes SQL server to roll-back a statement if there's a run-time error on the client. I'm curious why it's not the default behaviour actually.
The IDENTITY property does not guarantee uniqueness, ever. This means that when you have concurrent INSERT statements on a given table, your auto-incrementing value may be duplicated across multiple active transactions.
One way to deal with this is to issue the SET TRANSACTION ISOLATION LEVEL SERIALIZABLE statement before beginning your transaction. This will prevent conflicting inserts, meaning tat no two inserts may be executed in parallel.
The other, more common way, is to simply re-try your transaction or statement, until you do not receive a primary key violation. This should only be done if you are not performing an identity insert and are letting SQL Server take care of the primary key value and it is auto-incrementing.
One other way to handle this in a sort of acceptable way is to use UNIQUEIDENTIFIER type primary keys. These are almost guaranteed to be unique. You can after a long while, start to receive duplicates; this will require the same logic as the re-try and you lose physical layout optimizations for primary key indexes. This is more of an alternative or note than a recommendation.
All in all, if you want to handle errors and you are using SQL Server 2012, you can simply use the following template to make sure your transaction is appropriately rolled back and that the error moves to the surface:
BEGIN
SET NOCOUNT ON;
SET XACT_ABORT OFF;
BEGIN TRANSACTION;
BEGIN TRY
-- Do stuff here
END TRY
BEGIN CATCH
ROLLBACK TRANSACTION;
THROW;
END CATCH
COMMIT TRANSACTION;
END
Also note that this template will not work for nested calls to stored procedures and will roll back the entire chain of procedures if used in that fashion.
EDIT
Since you're using ADO .NET, you can disable connection pooling and take care of connection management yourself. If a connection times out while pooling is off, it'll fault on the server side and destroy the transaction, causing a rollback.
This is a pretty specific question, albeit possibly subjective, but I've been using this pattern very frequently while not seeing others use it very often. Am I missing out on something or being too paranoid?
I wrap all my UPDATE,DELETE,INSERT operations in stored procedures, and only give EXECUTE on my package and SELECT on my tables, to my application. For the UPDATE and DELETE procedures I have an IF statement at the end in which I do the following:
IF SQL%ROWCOUNT <> 1 THEN
RAISE_APPLICATION_ERROR(-20001, 'Invalid number of rows affected: ' || SQL%ROWCOUNT);
END IF;
One could also do this check in the application code, as the number of rows affected is usually available after a SQL statement is executed.
So am I missing something or is this not the safest way to ensure you're updating or deleting exactly what you want to, nothing more, nothing less?
I think this is a fine way to go. If the pl/sql proc is expected to always update/delete/insert a row and it's considered an error otherwise, then what better place to put this check than in the pl/sql proc itself? That way, no matter what client side code (C#, JAVA, PHP, Rails, etc.) uses this proc, you have this error check centralized in one place.
However, I'm not sure you need the check for an insert. If the insert fails, you should get some sort of DB exception, so no need to check for it explicitly unless you are wrapping the error in some other error message and re-raising it.
In most cases I'd use an ORM like Hibernate, which does a similar thing in order to handle Optimistic locking. Also it will use the PK in the where clause.
So I would consider this kind of stored procedure a waste of time:
- A lot of effort for minimal benefit
- Makes usage of tools like ORMs harder, which solve more and more important problems.
I've a feeling this might not be possible, but here goes...
I've got a table that has an insert trigger on it. When data is inserted into this table the trigger fires and parses a long varbinary column. This trigger performs some operations on the binary data and writes several entries into a second table.
What I have recently discovered is that sometimes the binarydata is not "correct" (i.e. it does not conform to the spec it is supposed to - I have NO control over this whatsoever) and this can cause casting errors etc.
My initial reaction was to wrap things in TRY/CATCH blocks, but it appears this is not a solution either, as the execution of the CATCH means the transaction is doomed and I get a "Transaction doomed in trigger" error.
What is imperitive is that the data still gets written to the initial table. I don't care if the data gets written to the second table or not.
I'm not sure if I can accomplish this or not, and would gratefully receive any advice.
what you could do is commit a transaction inside a trigger and then perform those cast.
i don't know if that's a possible solution to your problem though.
another option would be to create a function IsYourBinaryValueOK which would check the column value. however the check would have to be done with like to not cause an error.
It doesn't sound like this code should run in an insert trigger since it is conceptually two different transactions. You would probably be better off with asynchronous processing such as service broker, a background nanny task that looks for 'not done' work, etc. You could also handle it by using a sproc to do the insert in one transaction and then having it call the do-other-work code afterwards.
If you absolutely have to do it in the trigger then you basically need an autonomous transaction. For some ideas see this link (the techniques apply to sql 2005 as well).