T-SQL Tuesday #24: Prox ‘n’ Funx

Procedures and Functions. Well, this could be interesting. Everyone else’s posts, that is.

T-SQL Tuesday

#24, brought to us by Brad Schulz

OK, T-SQL Tuesday Twenty-Four: Two-year anniversary edition! Pretty sweet of Adam Machanic (blog | @AdamMachanic) to come up with this whole thing two years ago. A good guy, he is. This month’s topic is Prox ‘n’ Funx, brought to all of us by Brad Schulz (blog). I kind of feel bad here—I don’t really know much about Brad, but he’s an MVP, and flipping around his blog brings up some pretty cool posts. I really like this one, for example.

I actually have what I think is a decent little thing to talk about for this topic. It could have been a stretch topic for T-SQL Tuesday #11 about misconceptions, even if [I hope] not exactly a widespread one. This topic revolves around transaction control and error handling within stored procedures, which can be a crazy topic in and of itself. Books could be written on the topic. Specifically, the conversation that I found myself involved in one day was about what happens to any open explicit transactions when a procedure runs into an error.

Let’s Talk About Procs Dying

Once upon a time, someone said to me that if an error occurs within a procedure which contains an explicit transaction, that transaction will remain open, potentially blocking other sessions on the system. In short, that’s not true, and it’s fairly easy to prove. We’ll work through a quick little script to do this, and will include modern TRY…CATCH error handling, because that will come in when we get to the main point (I say “modern” there like it’s a new thing, but coming from a guy who has worked with SQL 2000 more than anything else, that distinction feels necessary). It actually doesn’t matter if there is any error handling during the first part of this exercise, as the results will be the same. This is a pretty contrived setup, but that’s pretty much what I’m going for.

First, create a table & put a couple of rows into it. The two columns we’re most worried about are “FakeNumberColumn” and “ActualNumberColumn” (note their data types), so named because the error which will be triggered in a little bit will be a type conversion error.
-- Create Table
CREATE TABLE dbo.TransactionTest2008
(     TransactionTestID       INT                     NOT NULL    IDENTITY(1,1),
RowDescription          VARCHAR(50)             NOT NULL,
FakeNumberColumn        VARCHAR(10)             NOT NULL,
ActualNumberColumn      INT                     NULL

-- Populate it with a couple of rows
INSERT INTO dbo.TransactionTest2008 (RowDescription, FakeNumberColumn)
SELECT 'Actually a Number 1', 10
SELECT 'Actually a Number 2', 100

Now for the really contrived part: a Stored Procedure that will throw an error if FakeNumberColumn contains something that won’t implicitly convert to a numeric:

CREATE PROCEDURE TransactionTester2008

SET ActualNumberColumn = FakeNumberColumn * 2
--   select *
-- from dbo.TransactionTest2008

     -- Wait for 10 seconds so we have a chance to look at DBCC OPENTRAN (unless of course it dies first)
     WAITFOR delay '00:00:10'


BEGIN catch
-- Some kind of error has occured
PRINT 'Welcome to Catchville'


   -- Raise an error with the details of the exception
DECLARE @ErrMsg NVARCHAR(4000), @ErrSeverity INT
@ErrSeverity = ERROR_SEVERITY()

   RAISERROR(@ErrMsg, @ErrSeverity, 1)

(The commented-out SELECT statement can be un-commented if you like, to see the state of the table at that point of execution.)

As it is now, the proc will run successfully. The 10 second WAITFOR in it gives you time to run DBCC OPENTRAN in another query window to see the proc’s open transaction.

EXEC TransactionTester2008

Open TransactionNow we’ll make things somewhat interesting. Insert another row into our table to put a letter into FakeNumberColumn, then run the proc again.

INSERT INTO dbo.TransactionTest2008 (RowDescription, FakeNumberColumn)
SELECT 'Not really a Number', 'F'

EXEC TransactionTester2008

Things won’t go so well this time…

Bombed Proc RunWe get the PRINT message about being in Catchville, so we know that our exception was caught and execution finished in the CATCH block. At this point, go run DBCC OPENTRAN again, and you will see that there isn’t a transaction open. This would be the expected behavior. No transactions are left open; the in-process activities are rolled back.

I should also note that a less-severe error, such as a constraint violation on an INSERT, will only cause an error in that particular statement. The Engine will skip over that statement & continue processing normally. That behavior has led to some near-brown pants moments while running a huge pile of INSERTs, putting in some provided business data, but that’s what explicit transactions are for!

Now, About Timeouts…

OK, that section wound up pretty long. Here’s where I’m actually getting to what I want to talk about…

We’ve established that errors in Stored Procedures will not lead to transactions being left open under normal circumstances. There is a situation where things don’t go so well: when a client/application connection times out for one reason or another. If this happens, the client side will close its end of the connection, and after the in-progress query SQL Server is running completes, nothing else really happens. This can leave open transactions, which, of course, are bad, bad, bad.

Starting with where we left off above, we can simulate an application timeout by cancelling the running SP in Management Studio.

First, delete the error-producing row from dbo.TransactionTest2008:

DELETE FROM dbo.TransactionTest2008
WHERE RowDescription = 'Not really a Number'

Execute TransactionTester2008 again, and this time, while in the 10-second WAITFOR, cancel the query in Management Studio. Even with the TRY…CATCH block in place, the explicit transaction is left open (check with DBCC OPENTRAN). What this means is that whatever application (or DBA!) running the statement(s) is responsible for closing an open transaction if a session times out or is cancelled. In my experience, if one is in the habit of wrapping everything you do in explicit BEGIN/COMMIT/ROLLBACK TRAN, they’ll be less likely to cancel a script they’re running and then sit there blocking half of the rest of the world. Not that I’ve been there, or anything…

There is a safety net here: XACT_ABORT. I first learned about XACT_ABORT while doing some work with Linked Servers a few years ago. What XACT_ABORT does when set to ON is to force SQL Server to terminate and roll back the entire batch if any error occurs. Here’s the Books On Line page for XACT_ABORT.

In our case, flipping that setting to ON within our test SP will change what happens when a query timeout (or cancel) happens. Add “SET XACT_ABORT ON” at the begging of the above SP and re-create it thusly (either drop and recreate or add the line and change it to ALTER PROCEDURE):

CREATE PROCEDURE TransactionTester2008


Run the SP as before, and again, while in the 10-second WAITFOR, cancel the query. Now if checking for an open transaction, there won’t be one—it was rolled back by the engine when the timeout (cancel) occurred, because of XACT_ABORT. No locks are still being held, no other sessions will be blocked by the timed-out session. Smooth sailing 🙂

As Usual, Be Careful

Before hauling off and adding this to a bunch of SPs, beware the Law of Unintended Consequences. Test extensively, because no matter how self-contained a fix may seem, who knows what else may be affected. This is no where this is as true as it is in old, overly-complicated, poorly-understood legacy systems. I know DBAs like to fix problems and prevent problems from happening in the first place, but please make sure that no new issues are introduced while trying to “fix” things.

Oh; don’t forget to clean up after yourself!

DROP TABLE dbo.TransactionTest2008
DROP PROCEDURE TransactionTester2008


T-SQL Tuesday #21: “This Ugly Hack is Only Temporary”

…Unless “nothing is temporary around here” also applies to your shop. Then…well…good luck with that (I feel your pain).

Wednesday is better than Tuesday anyway; everyone knows that

It’s been a while since I’ve participated in a T-SQL Tuesday; haven’t been writing at all, really, as you can see. Summer apparently does that to me. If you’re reading this, then it means I got it shoehorned in.

T-SQL Tuesday is the brainchild of Adam Machanic (blog | @AdamMachanic) that started, apparently, 21 months ago. Its purpose is for bloggers to get together and cover a particular topic every month. This is good for readers and also is an easy way to get a blog topic, if one has a hard time coming up with them (like me). This month’s topic is hosted by Adam, and has been billed as “revealing your crap to the world.” There will be lots of good stuff to read this month, calling it now.

RI Enforced by aTrigger? Check.

I’m going to preface this one by saying that it never actually saw the light of day. Sometime between Design & Development, the project that it was part of got canned, so the Development instance was the only place this was ever deployed to. But, it was still crap, and it was crap that I came up with, at that, so here we go.

There once was a big ugly “Location” table. This table was way wider than it should have been, mainly due to the fact that “normalization” was a term missing from the historical evolution of the system it was a part of. As you can imagine, this table contained a lot of character columns of varying widths (up to and including TEXT). To make matters worse, in lots of cases, the UI allows free text entry, which makes much of the contents suspect, at best.

A project came along that basically would require normalizing out a couple of columns related to a certain location attribute. For the sake of discussion, this could be just about anything—we’ll use “Preferred Shipping Carrier” as an example.

At the beginning of design, this “PreferredShipCarrier” column is full of shady entries. There are some “UPS”es, “FedEx”es, and “None”s, but there’s also a lot of NULLs (which shouldn’t really happen), “N/A”s, and “UPPS”es, all of which are incorrect for one reason or another. Obviously as part of normalizing this column out, there would need  to be some corresponding data cleanup, but that’s beside the point.

Where things go south is how this was to be implemented. Since the column in the table is VARCHAR, a real Foreign Key relationship can’t be set up to the INT Surrogate Key in the new “PreferredShipper” table. This is the first sign of trouble. Second sign is that we can’t change the UI—it has to stay in its free-text entry state. This leaves the database exposed to bad data, as there can’t be RI to enforce valid entries.

Enter a Trigger. This can be used to, upon an attempted row insert into the Location table, look up the value entered for Location.PreferredShipCarrier, and throw an error if it doesn’t find a matching row. I hated it, but it got the job done.

Auditing with Triggers, too? What is up with you and Triggers?

SQL 2000 was terrible for auditing—it didn’t have all of the new fancy stuff that we have now for this sort of thing. That means, when you’re dealing with 2000, and the business wants an audit trail of who changed what when, your options are/were pretty limited.

At one point, a requirement came along to implement audit trails on some tables. This means I needed to duplicate tables, adding some metadata columns to them (audit timestamp, who dunnit, what the action was, etc), and then put a trigger on each of the source tables. Some of these particular tables were busy, so the triggers & log tables got a lot of action and I hated having to do this to get what was needed.

If the trigger was fired for a DELETE, it would log the row that was deleted. If an UPDATE was happening, it would first check to see if any column contents were actually changing, and if so, log the previous state of the row to the log table. These triggers will grow relative to the width of the table (because they contain three explicit lists of the tables’ columns), so if the tables being audited are wide, the trigger will get pretty long. Additionally, since triggers in SQL Server are set-based operations, running on all rows that are being DELETEd or INSERTed, special care needs to be taken so they can operate when more than one row is operated on. This can make them a bit ticklish to write.

I know this isn’t necessarily crap, as when you’ve got to audit, you’ve got to audit. I don’t like it if for no other reason than the extra clutter it puts in to the DB and just the general idea of triggers all over the place. All manner of things can happen in triggers, and if you are operating in an unfamiliar DB or troubleshooting a goofy problem, things could be going on in there behind the curtain making troubleshooting harder.

In short: Friends don’t let friends mash F5 on CREATE TRIGGER.