Can you apply gaps and islands logic on a string? Sure you can.
Continue readingIntermediate
How to add “created” and “updated” timestamps without triggers
You have a table that you want to add “created” and “updated” timestamp columns to, but you can’t update the application code to update those columns. In the bad old times, you had to write a trigger to do the hard work for you. Triggers introduce additional complexity and potentially even a performance impact.
So here’s a nicer way to do it, trigger-free.
Continue readingHow MERGE on two different rows can still deadlock you
I recently ran into a curious deadlock issue. I have a process that performs a lot of updates in a “state” table using multiple, concurrent connections. The business logic in the application guarantees that two connections won’t try to update the same item, so we shouldn’t ever run into any locking issues. And yet, we keep getting deadlocks.
What’s going on here? Hint: it has to do with isolation levels and range locks.
Continue readingConnecting a SQL Server client on Linux using Active Directory authentication
or: How I learned to stop worrying, and love all-caps domain names.
I’m a complete beginner at Linux, so I should preface this post with the fact that these are my humble notes after hours of pulling my hair. It’s not really a fully-fledged how-to article, and there are lot of things I’m not covering. But I figured it may help someone out there at some point.
Continue readingIs a sort faster when the data is already sorted?
Whenever SQL Server needs to sort a data stream, it will use the Sort operator to reorder the rows of the stream. Sorting data is an expensive operation because it entails loading part or all of the data into memory and shifting that data back and forth a couple of times. The only time SQL Server doesn’t sort the data is when it already knows the data to be ordered correctly, like when it has already passed a Sort operator or it’s reading from an appropriately sorted index.
But what happens if the data is ordered correctly, but SQL Server doesn’t know about it? Let’s find out.
Continue readingThe curious case of the Top N Sort
Watching Brent Ozar’s 2017 PASS Summit session on Youtube the other day, I learned that the Top N Sort operation in SQL Server behaves dramatically differently, depending on how many rows you want from the TOP.
Continue readingHow to run your CTE just once, and re-use the output
You may already know that common table expressions, like views, don’t behave like regular tables. They’re a way to make your query more readable by allowing you to write a complex SQL expression just once, rather than repeating it all over your statement or view. This makes reading, understanding and future refactoring of your code a little less painful.
But they’re no magic bullet, and you may end up with some unexpected execution plans.
Continue readingOptimizing for something else
Ah, the feeling when you think of a new blog post topic, only to realize that you already wrote a post about that six years ago.
Well, I’ve done the work, so I might as well have a little fun with it.
Continue readingHow to use switching to make metadata changes online
Metadata changes, like modifying a clustered index, or many types of column changes, will create locks in SQL Server that will block users from working with that table until the change is completed. In many cases, those locks will extend to the system objects, so you won’t even be able to expand the “Tables” or “Views” nodes in Management Studio.
I want to show you how you can perform those changes using a copy of the table, then instantly switching the table with the copy. The secret is partition switching, and contrary to popular belief, you won’t need Enterprise Edition, or even partitions, to do it.
Continue readingScalar function inlining in SQL Server 2019
This insanely cool new performance-related update is one of nicest features in SQL Server 2019, and certainly one I’ve been looking forward to for a long time.
If you’ve done any work around performance tuning and user-defined scalar functions, I’m pretty sure you’ll love this.
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