A Fine Slice Of SQL Server
Erik Darling isn’t content with simple searches:
Like having a built-in type to make dynamic SQL more easily managed, it would also be nice to have some mechanism to manage dynamic searches.
Of course, what I mean by dynamic searches is when you have a variety of parameters that users can potentially search on, with none or few of them being required.
Erik provides two techniques and contrasts the two, so check it out.
Comments closedErik Darling goes back to the well on a popular and important topic:
One of the most fun parts of my job is telling people that parameter sniffing is a good thing. They’re often shocked, because the phrase has such ominous connotations to developers.
The parameters! They get sniffed! Performance! Something! Stuff!
Parameter sniffing does not mean the sky is falling. Like I’ve said before, if it were such a bad idea, it would have been abandoned.
Read on for more thoughts on the topic.
Comments closedI probably overuse temp tables. I’m the first one to admit it. But often when I choose to use one, it’s a choice between:
- Spending a long time trying to business understand logic for a company I don’t work for
- Tinkering with query syntax and using really weird tricks and hints to get different plan shapes
- Waiting a long time to create or tweak indexes on huge tables (usually on “dev” hardware)
Erik provides a good example of where a temp table fits well, contrasting it to a common table expression that fares poorly. My bias tends to come in the opposite direction from Erik’s: I’m susceptible to playing query golf and getting everything I need back in one optimized call. But part of craftsmanship is knowing how and when to use each tool.
Comments closedErik Darling offers some advice:
Wrapper stored procedures are useful for things like:
- Transforming declared local variables into parameters
- Preventing code from compiling when it isn’t used
- Generating different query plans to deal with parameter sniffing
The upside of using this over dynamic SQL is that you have a convenient object name attached to the code.
Read on for the downside to this, as well as a pair of videos on the topic.
Comments closedErik Darling does not approve:
Like many other things we’ve discussed thus far, local variables are a convenience to you that have behavior many people are still shocked by.
You, my dear and constant reader, may not be shocked, but the nice people who pay me money to fix things seem quite astounded by what happens when you invoke local variables.
So I find myself in a difficult position: do I dredge up more red meat for the millions of die-hard SQL Server performance nuts who come here for the strange and outlandish, or produce evergreen content for people who pay my substantial bar tabs.
You have at least a 50% chance to guess what Erik does next.
Comments closedThere are two forms of conditional logic that I often have to fix in stored procedures:
- Branching to run different queries at different times
- Complicated join and where clause logic
The problems with both are similar in terms of performance. You see, when smart people tell you that SQL is a declarative language, and not a procedural language, they’re usually trying to get you to stop using cursors.
And that’s not always wrong or bad advice, trust me. But it also applies here.
Read on for exceptions to the rule and how you can make your life a bit easier if you do have this in place.
Comments closedErik Darling plays the roles of both Goofus and Gallant here:
All sorts of bad things happen when you do this. You can’t index for this in any meaningful way, and comparing non-string data types (numbers, dates, etc.) with a double wildcard string means implicit conversion hell.
You don’t want to do this. Ever.
Unless you want to hire me.
Click through for good advice on the proper use of data types and input parameters.
Comments closedErik Darling goes into isolation:
I’ve talked about isolation levels a bit lately because I need you all to understand that no isolation level is perfect, and that most everyone is completely wrong about how they really work.
For a very high percentage of workloads, Read Committed Snapshot isolation is the best choice. Why?
Read on for that answer. I think Erik is right about people misunderstanding how the different isolation levels work, as well as the root cause of not having a great place to try it out. You can build out demos of how different transaction isolation levels will work but some of the nuanced operations can be hard for one person with a couple new query tabs open to emulate.
Comments closedErik Darling continues a series on stored procedures:
Transactions and error handling often go hand-in-glove, to make better use of
XACT_ABORT, manually manage rollbacks, and try to make the best use of all those ACID properties that database people seem to care deeply about.
Click through for Erik’s advice around transaction handling.
Comments closedErik Darling shares some tips:
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.
If you’ve never brought down a system with your monitoring process or introduced bugs via debugging code, you’re not living life to its fullest.
Comments closed