How SQL Server compiles, optimizes, and executes individual queries.
573 associated posts292 primary posts
For this month's T-SQL Tuesday, I invited y'all to tell me how you know a query's gonna be bad when you open it. Riccardo Perico and Shane O'Neill says WHILE loops and/or CURSORs are pretty good indicators. I agree, because especially when you're opening code in the year 2026, and someone still hasn't figured this out…
After the PASS Data Community Summit in Frankfurt, Germany, I headed back to Iceland and spent a few more days gallivanting around the country in a Tesla Model Y camper. (Well, "camper" is a glamorous word for it: CampEasy.com puts a mattress in the back. But still.) Today, I'm at Djúpalónssandur, a marvelous black pebble…
I work with databases for a living.
I teach people, and I help them via consulting.
On a 12-hour layover in Keflavik, I pulled over my Suzuki Jimny (love these little things, and we can't buy them in the US) in some of the freshest real estate in the world and took your top-voted questions from https://pollgab.com/room/brento. This is a 360-degree video, so you can move the camera angle around with your mouse.
In this live stream recording, I'll show you how to use the latest features of sp_BlitzCache to rapidly improve performance on an existing server. I'll show you how to find your server's top bottleneck, find the queries causing that bottleneck, and then demonstrate how to get advice from AI on how to tune them in seconds.
Our cruise ship pulled into port in Victoria, Canada, so before we head out to the Butchart Gardens, let's go through your top-voted questions from https://pollgab.com/room/brento.
https://www.youtube.com/watch?v=MDTuQFiXo2s
I love me some documentation.
For years, I've pointed folks to the SQL Server 2008 documentation on Forced Parameterization, a really useful tool for reducing plan cache bloat, getting more accurate reusable query plans, and enabling SQL Server 2019 and 2022's Intelligent Query Plan features, many of which rely on the same query text coming in repeatedly over time in order to tune it. That documentation was pretty sparse, though.
When there are IF branches or conditional logic, "Edit Query Text" only shows you the branches that actually executed. It's easy to miss whole swaths of code that didn't happen to execute one particular time.
To demo it, let's create a simple stored procedure with an IF branch that sometimes executes, and sometimes doesn't:
In theory, SQL Server performance monitoring is pretty simple:
Review the server's top wait types
Find the queries causing those wait types
Fix those queries, or improve the way the server reacts to them (indexes, settings, etc.)
When you turn on last actual plans in SQL Server 2019 and newer:
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Let's go through a LOT of your top-voted questions from https://pollgab.com/room/brento on a VERY early Saturday morning.
https://www.youtube.com/watch?v=V7lGcm8CyEM
The short answer: in the real world, only the first column works. When SQL Server needs data about the second column, it builds its own stats on that column instead (assuming they don't already exist), and uses those two statistics together - but they're not really correlated.
For the longer answer, let's take a large version of the Stack Overflow database, create a two-column index on the Users table, and then view the resulting statistics:
My tripod is probably never gonna recover from the salt water, but the water was so nice that I couldn't resist. This is a 360 video, so you can grab the screen and move it around to see Magens Bay in St Thomas, US Virgin Islands, as I go through your top-voted questions from https://pollgab.com/room/brento.
The official Azure SQL Dev's Corner blog recently wrote about how to enable soft deletes in Azure SQL using row-level security, and it's a nice, clean, short tutorial. I like posts like that because the feature is pretty cool and accomplishes a real business goal. It's always tough deciding where to draw the line on how much to include in a blog post, so I forgive them for not including one vital caveat with this feature.
Yes, I'm back on a cruise ship with another 360 degree video. Lest you think I'm being wildly irresponsible (or responsible perhaps?) with your consulting and training money, be aware that this particular cruise was free thanks to the fine folks in the casino department at Norwegian Cruise Lines. In between beaches and blackjack, let's go through your top-voted questions from https://pollgab.com/room/brento.
Want to ask your own questions or pick the ones I discuss? Head over to https://pollgab.com/room/brento to get involved. In the meantime, let's hang out in the backyard, where Beni makes a special guest appearance.
https://youtu.be/aMMBj4owm3A
On the world's largest floating electronic dance music festival, I took your top-voted database questions from https://pollgab.com/room/brento.
https://www.youtube.com/watch?v=Dk1kb6fM21E
Normally, when SQL Server updates statistics on an object, it invalidates the cached plans that rely on that statistic as well. That's why you'll see recompiles happen after stats updates: SQL Server knows the stats have changed, so it's a good time to build new execution plans based on the changes in the data.
However, updates to system-created stats don't necessarily cause plan recompiles.
I really wanted to film Office Hours on the Great Wall of China, but the instant I got out there, I realized it was a lost cause. That place was SO overwhelming - tons of people, frigid winds, heights, totally overwhelming in every way. So, instead you get a nice peaceful park in downtown Zhengzhou,…
Back in March 2025 when Microsoft first announced that REGEX support was coming to SQL Server 2025 and Azure SQL DB, I gave it a quick test, and the performance was horrific. It was bad in 3 different ways:
The CPU usage was terrible, burning 60 seconds of CPU time to check a few million rows
It refused to use an index
The cardinality estimation was terrible, hard-coded to 30% of the table
