FOR XML and STUFF you might not know

If you are like me it’s not enough for you dynamic SQL to just be executed at design time, you have to make time to design the code you are dynamically building. What am I talking about? The first part is about not executing dynamic SQL as part of a data tier application wherever possible. That is to say dynamic SQL is not going to be able to reuse cached plans the way static and parameterized SQL can. The second part is about including formatting in your dynamic SQL, at least enough to make it readable.

Using dynamic SQL to write your code can be a huge time saver when you have a lot of code to write. It can also be a huge time suck when you have debug, optimize, or just get it all into one string ((N)VARCHAR field).

Dynamic SQL can make debugging challenging for a number of reasons. For starters Management Studio will be unable to detect a syntax error until its executed. Even after it executes you won’t be able to double click on the error message and jump to the line in the query editor. You can however use GO to separate your code into batches which double clicking the error will jump to beginning of. It is also difficult to divine from the error message (ERROR_LINE()) which line is actually causing the error.

ThisIsNotStuffOrXML

Let’s not get sidetracked. If you want to noodle on those debugging consider this screenshot from SQL Server Management Studio 2014. If I were going to go off on a tangent it might be about the fact that the GO in the dynamic string does not behave the way it does outside of the string. That is to say the entire string fails in spite of the GO where only the batch containing the error does in the static SQL. And don’t get me started on how the execution plan calls a script a batch when determining cost relative to batch. Also worth noting is that the breakpoint encompasses both statements and the assignment where with static SQL a single statement is broken on.

When it comes to optimization of dynamic SQL I am going to simply link some reading material and comment. Jeremiah Peschka at Brent Ozar Unlimited has a great post about filtered indexes that kind of contradicts what I am saying about optimizing dynamic SQL. I say kind of because Jeremiah is using dynamic SQL to take advantage of a filtered index he created when a parameter might cause it to be skipped over. Andrew Kelly at SQL Server magazine mentions in this article that compiling is cpu intensive, he also goes on to show how sp_executesql allows us to cache plan. Derik Hammer explains and demonstrates the need to match the SQL text in order to get plan reuse. Turgay Sahtiyan echoes Derek, plus he is a Senior Premier Field Engineer (I was almost a PFE once). And last but certainly not least, Kimberly Tripp gave us some code to play with and a vigorous discussion with here post on multipurpose procedures. Of course, for all that good stuff we can use to avoid using dynamic SQL that is bad at execute time, our monitoring tools are probably only going to show us the generated SQL. As I tend to fall into the DBA roll, I make it a point to always attempt to optimize without changing the code that executes… at least when I don’t own it. But that is a topic for a whole other discussion perhaps on whether you can separate workloads into separate instances for ad hoc and prepared workloads.

The last obstacle is a matter of practical consideration. When we have multiple statements we might build and execute them separately. When we have nesting strings building dynamic SQL for us we might find it easier to have parts of our code in separate columns that copy as a row and paste as a single script. But what about when we are building dynamic strings that roll up some one to many values like columns and tables. Are we going to have to paste one long line and then format it? That would be another time suck to avoid. Enough teasing the title of the post said we were going to show an example of FOR XML and STUFF.
singleline Well, that example makes a single line, and I don’t feel like formatting myself. Sure, I could use a tool to auto-format but I don’t have it installed and I really want to automate the generation of these scripts and output them directly to script files. What if we try to stick in carriage return and a line feed?

multilineAscii

What the heck is this weird emoticon? Is it a winking fish or something? No, it’s proof enough that the output of the FOR XML PATH statement is typed as XML. That is to say its an XML escape character. Specifically it’s the XML break character for carriage return. What happened to the line feed? They don’t use it paired with carriage return in the UTF world so SQL Server was goodly enough to drop it. You can try using NCHAR instead of CHAR but the results are the same. What if we use the break character?

nopenotit

Darn, now the ampersand is being escaped. Ok, we’ll just have to replace it.

Ok

There you have it. FOR XML and STUFF you might not know.

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