Debugging, like error handling, is a design choice that is fairly easy to make at the outset of writing a stored procedure, and is usually a lot easier to get in there if you do it from the get-go.
The number of times I’ve had to go back and add debugging into something only to introduce debugging bugs is actually a bit tragic.
One of the first things I added debugging to is sp_Blitz, because sometimes queries would fail, and we’d have no idea which one was failing.
Sure, there would be an error message, and a line number, but that all goes pear shaped when you’re dealing with dynamic SQL and nested dynamic SQL and whatnot.
My bright idea was to add lines like this to know which check was starting:
IF @Debug IN (1, 2) RAISERROR('Running CheckId [%d].', 0, 1, 185) WITH NOWAIT;
They’ve changed a little over the years, but it took a few tries to get all the substitution numbers right, and to get them in the right place so they didn’t prevent checks from running.
But now if you @Debug, everything is cool. And if a check fails, you can figure out which one and be a top notch open source contributor by reporting (and even fixing) issues.
Where I’m making a really pleasant face in the thumbnail, apparently.
You should check those out if you haven’t already.
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Just like Erland, it has three parts and three appendices. If you want to learn how to do a whole lot of things right, give yourself a couple days to read through and digest all that.
What I’m here to talk about is some of the stuff you should think about before implementing any error handling at all.
Namely, what do you hope to do with certain errors and conditions?
As far as I’m concerned, there are four outcomes:
Print an informational message and keep chugging along
Raise and log the error somewhere and stop everything (maybe rollback, maybe don’t)
Raise and log the error somewhere and keep going
Use the type of error to retry the thing you’re trying to do
If you’re not going to do one of those things, there’s no point in even considering error handling (and to a large extent, transactions) in your procedures.
Let’s talk through some examples of each one.
Is This Right?
Here’s a simple example: You have a stored procedure that accepts start and end date parameters.
What do you want to do if someone passes in a start date that’s after the end date?
Throw an error and call someone a bonehead
Be nice and swap them around
Be psychic and change them to dates that they probably meant
Add or subtract time from one or both of the parameters
But let me ask you: if this is coming from a a front end application, why would you wait until the query gets to SQL Server to figure out that the start date is after the end date?
The application should tell you that. The application is capable of telling you that. The application server is a lot cheaper than the SQL Server.
You don’t need error handling in SQL Server for that.
Is This Permanent?
There may be times when it’s sensible to halt a stored procedure when it hits an error.
Let’s say a critical step is to populate a temporary object. If that step fails, all further effort is wasted because no results will be returned.
You could also have a check to look at the ROWCOUNT_BIG() after the insert to have it bail if zero rows end up in the temporary object, too. But that’s not an error, really, that’s just pure human kindness.
Other times, like if your query is just returning a bunch of selects (or if a minor snag somewhere only has a minor impact on the overall results), an error in one place might not be enough of an issue to stop everything.
If you want everything to stop and rollback, use SET XACT_ABORT ON; at the beginning of your stored procedure.
If you want to accept small errors as long as you get the larger result set, don’t do anything.
If you want to log and swallow transient errors, use an empty TRY/CATCH block around queries that may not matter to the outcome.
There are situations where all of those things make sense. Make sure you choose the right one for your arrangement.
A good example of this is in sp_BlitzLock. Sometimes part of the script will fail to get Agent job names depending on permissions and stuff, but that doesn’t impact the overall analysis much.
Try And Retry
There are plenty of good uses for retrying things. Like the start/end date example above, this is usually more wisely done in application code.
But if your stored procedure isn’t being called by an application, doing it in T-SQL isn’t a horrible alternative.
In my life, I’ve used retry logic in stored procedures to:
Catch deadlock errors and attempt the modification again
Wait for a lock timeout and try again
Check in sp_getapplock locks and try again
Look for available records in a queue and try again
I’m sure there are more, but these are some simple examples where writing retry logic into T-SQL was the best available option.
You Don’t Always Need Error Handling
Some processes require a certain amount of robustness in terms of a few things:
Error handling and logging
Retrying certain types of transactions
Self-healing in the event of certain failures
I didn’t talk much about the self-healing part, because it’s pretty complicated depending on what you’re doing.
But as a small preview, let’s say you’re building a process to move data from one big table to a more normalized schema.
You have to anticipate failures, and be able to pick back up from any point when the process is re-executed.
Exciting times we live in, no?
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Not putting new things on new lines (from join on where and or order by group by)
Not indenting things
Leaving AS out of table aliases
Leaving out column aliases
Top expressions without parentheses
Not aligning parentheses
There are others, but I’m tired of thinking about things that are demonstrably wrong to do, like putting ice in scotch or sugar in wine.
Thank you for complying.
Not All Code
When you think about formatting code, you probably think primarily of organizing a single query so that it’s inoffensive to civilized society.
But you should also apply this to your code as a whole, too. People who use words wrong will call this “holistic”.
Create all your temporary objects in the same section of code
Declare all your variables in the same section of code
Set as many of your variables as you can in the same block of code
Do as much of your validation in the same section of code as you can
Few things (aside from the list at the beginning of the post) are more grating than seeing random things being created and declared at random points in the code.
It makes things far more difficult to scope, follow, and understand.
I’ll lend some leniency to using SELECT INTO here, because sometimes that’s the best way to get a fully parallel insert.
I’ll also lend some leniency to cursors, because sometimes it’s only sensible to declare them when they’ll be used.
Consistency Is Key
Formatting should follow a consistent set of (my) rules. It is sometimes possible to get used to inferior style choices as long as long as they’re predictable.
What’s impossible to get used to is code formatting that’s constantly changing gears.
There are many ways to format code out there in the world. Some are more acceptable than others.
For example, some companies may shy away from online code formatting tools because who knows who might see the code.
So much valuable intellectual property exists in your “joins” and “column names” and such. Someone else may steal the bright idea to also key on “identifiers”.
I personally use SQL Prompt, because it gets me about 80% of the way to how I want code to look.
While I do wish there were more ways to implement single formatting rules, like changing how columns are aliased from the shabby “column AS new_name” to the correct “new_name = column” format, it’s often trivial to tweak certain things after applying everything.
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Most code I look at has been a disaster in the making for many years, worked on by many people who seemed to find one thing that sort of worked, and kept doing it.
I can’t lie: there are areas of my life where I do the same thing. SQL Server is the one area where I confidently know the rules, when I can break them, and explain reasons for both.
It’s not that I don’t make mistakes, it’s just that I learn well from them.
Usually.
There are others things that I’ll do the same way for ages, thinking they’re needlessly complicated, before looking into less needlessly complicated ways of doing them.
What’s shocking is that sometimes there’s no better way of doing things.
Like xquery. What were they thinking?
Find Yourself
Possibly the least helpful, but most humorous, way of leaving comments, is a large block of green text up at the top of a module.
There are all sorts of helpful insights buried in those comments to help me as a consultant understand my audience.
But… even though a lot of changes are described, there are often dozens to hundreds of queries in those stored procedures and function, or the view definition has 50 joins that join to 50 views that have 50 joins in them.
Fixed performance??? Avoided parameter sniffing??? Needed DISTINCT to fix a bug???
WHERE? WHICH QUERY?
SHOW ME TO YOUR MAGICK!
If you use a ticketing system, and there’s an issue number in your comment way up at the top of your stored procedure, add the ticket number in where you changed the code.
If there’s not, make one up. Give me some way to figure out where a change was made.
The only thing more frustrating is seeing some chunk of code quoted out, with no explanation of why it was quoted out.
Explain Yourself
In my stored procedures, I like to quote any code blocks as I go so they’re easier for me to find when I scroll through things.
Not because I can’t read my own code (okay, sometimes I get a lil’ lost), but because there’s often a lot of repetitive code, and it’s usually easier to read plain-English green text when I’m looking for something.
This code isn’t hard to understand, but it’s a tiny chunk of code, and pretty easy to miss if I were looking for it.
Leaving little notes about even small sections of code can make life a lot easier for others trying to work for it.
Block It Off
While we’re on the subject, I must beg this of you: USE BLOCK QUOTES
Like I talk about in the post, using the -- method can screw a lot of things up.
Depending on where you get the query from, and where the query originates, the whole query might end up on one line.
Having a bunch of double-dash comments makes it really hard to distinguish what’s code, what’s comment, and what’s quoted out code.
Worse, it breaks most every automatic code formatting tool out there.
They can’t figure it out either.
Tidbits
Comments in code are good for all sorts of things. Comments on the internet generally are mistakes and should be ignored.
Leaving comments not only helps others, but can really help you.
Think about how many times you’ve written code with a bunch of BEGIN/END blocks, and you get totally lost in which begin goes with which end and how many ends you need to stop getting an error at the very end of the stored procedure.
Heck, even while writing this post I went through a couple of my bits of code to add better comments in.
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
SET ANSI_NULLS ON;
SET ANSI_PADDING ON;
SET ANSI_WARNINGS ON;
SET ARITHABORT ON;
SET CONCAT_NULL_YIELDS_NULL ON;
SET QUOTED_IDENTIFIER ON;
SET NUMERIC_ROUNDABORT OFF;
SET IMPLICIT_TRANSACTIONS OFF;
SET STATISTICS TIME, IO OFF;
GO
This aligns my stored procedures with the necessary settings to accomplish a couple things:
Allow the optimizer to use indexed views, filtered indexes, and computed columns
Avoid errors when modifying tables involved with indexed views, filtered indexes, and computed columns
Many ORMs mess up with ARITHABORT, and let’s be honest, most of them time ORMs aren’t calling stored procedures.
If you’ve spent some time right-clicking around in SSMS at all, you might have seen how missing index requests and various module templates are generated.
They all start off with enabling ANSI_NULLS and QUOTED_IDENTIFIER, but there are some others that it makes total sense to include.
That’s why I have that list up there.
AS BEGIN
Inside of your stored procedures, you may want to do some other stuff, depending on how you’ll use it.
Inside of a real, production-ready stored procedure, you’ll probably want to do this:
SET NOCOUNT, XACT_ABORT ON;
If your stored procedure are being called by a Java or Python app, you’ll probably want this too:
SET IMPLICIT_TRANSACTIONS OFF;
Because by default, they both use implicit transactions, and you probably don’t want those happening across all the queries in a stored procefure.
That can cause a whole hell of a lot of blocking.
NOCOUNT is of course a good idea to not send additional results back.
XACT_ABORT is a great idea for stored procedures with multiple queries that modify data, because it allows you to roll all of the changes they made back without a TRY/CATCH block surrounding the whole ordeal.
If your query is just reading a bunch of data from different places and there’s nothing to roll back, XACT_ABORT matters not a bit.
Otherwise, use it, because it is wise to do so.
Locally Owned
If you’re writing a stored procedure (like most of my troubleshooting procedures) where you don’t want to:
Fail completely if you hit one error
Get blocked by all the transient stuff that locks system views and functions
Accidentally get query plans for your troubleshooting procedures
You’ll probably want to do something like this:
SET STATISTICS XML OFF;
SET NOCOUNT ON;
SET XACT_ABORT OFF;
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
That’s what I do inside all of my admin procedures to avoid those things.
Of course, you should not use some of those if you care about:
Easily troubleshooting query performance (getting actual execution plans in SSMS)
Leaving some changes in place when another part of the procedure fails (XACT_ABORT)
Not returning data from potentially dirty reads (READ UNCOMMITTED)
SET NOCOUNT on is still a generally wise idea, regardless of the intent or application of the code.
Deal with it.
Exercise
I’ve been bitten by a lot of different weird things by not doing this stuff in the past.
At one point I had suggested a really helpful filtered index to a client that used Cold Fusion as a front end, which used some weird driver (Adobe, maybe?) that used a bunch of bad ANSI settings.
Implementing the index immediately caused a bunch of queries coming from there to start failing. It was an easy fix — rebuild the index without the filter — tada!
But the better fix was to move the queries to a stored procedure that requested the correct SET options, and re-filter the index.
Consulting life teaches you a lot of weird lessons, probably the first is to be paranoid of everything, and never trust that even the largest companies are doing anything correctly.
In tomorrow’s post, we’ll talk about one of my favorite subjects: helpful and professional (ha ha ha) code comments.
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
I’ve been keeping a mental list of things of things I often have to adjust or fix in stored procedures I work on.
Some of them are a touch pedantic, but the majority of them are lessons and practices I’ve learned (often the hard way) over the years while working on things:
Performance tuning
Building processes
Fixing issues and errors
The thing is, I see the same mistakes over and over again. You probably do too, but you may not know they’re mistakes.
Judging by the code I see, most developers out there don’t.
No offense, it’s just… if all you do is begin a transaction and then try to commit it at the end, with no thought given to error handling or rolling back, what is the point of you?
If you do this, go ahead and tell me why in the comments. I’d love to know.
Anyway.
Table Of Contents
Here’s what I’ll be talking about over the next month or so.
The list might change a little, but it’s the basic outline of what I care to talk about.
Some posts might be shorter than others, because there’s not a lot to say about certain things beyond “this is right, and this is why”.
Hopefully these lessons start to sink in.
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Optimizing SQL Server Query Plans With Eager Index Spools In Them
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Optimizing SQL Server Query Plans With Row Count Spools In Them
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Optimizing SQL Server Query Plans With Table Spools In Them
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
Optimizing Complex SQL Server Queries By Using Batch Mode
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that, and need to solve database performance problems quickly. You can also get a quick, low cost health check with no phone time required.
