Execution Plans

What’s the Buggiest Feature in SQL Server?

Last Updated May 5, 2021

Not the one you like the least, or the one that requires the most work. What’s the one that you can’t use because it’s just flat out busted? For me, it’s SSMS’s Live Query Statistics, aka live query plans. The idea is so awesome – you click Query, Include Live Query Statistics, and then when you…

Execution Plans Don’t Have the Yellow Bang They Really Need.

Last Updated April 29, 2021

Brent Ozar

Execution Plans

When SQL Server is purely making an estimate up out of thin air, with no freakin’ idea what the real answer is, it doesn’t give you any kind of warning. It just produces an estimate that looks scientifically accurate – until you know how it’s actually getting calculated. Let’s ask SQL Server how many users have…

Changing Statistics Cause Longer Compilation Times

Last Updated February 13, 2021

Brent Ozar

Execution Plans, Statistics

I need to be up front with you, dear reader, and tell you that you’re probably never going to need to know this. I try to blog about stuff people need to know to get their job done – things that will be genuinely useful in your day-to-day performance tuning and management of SQL Server.…

Can You Get Parameter Sniffing on Updates and Deletes?

Last Updated January 27, 2021

Brent Ozar

Execution Plans

Sure you can – parameter sniffing is everywhere. Anytime that you have one piece of code that needs to handle a varying number of rows, you’re probably gonna have to worry about parameter sniffing. I’ll start with any Stack Overflow database and run a couple of non-parameterized update statements. I’m using literals here, not parameters:…

Finding the One Query to Tune in a Multi-Query Batch

Last Updated May 14, 2026

Brent Ozar

Execution Plans

When you have a stored procedure or application query that has multiple queries inside of it, how can you tell which query to focus on? For starters, do NOT believe the percentages you see in execution plans. Let’s take a classic performance tuning issue: scalar functions. I’ll start with the Stack Overflow database – if…

MAXDOP Isn’t Really MAXDOP. It’s More Like DOP.

Last Updated November 30, 2020

Brent Ozar

Execution Plans

Here’s how Books Online describes the Max Degree of Parallelism setting: You can use the max degree of parallelism option to limit the number of processors to use in parallel plan execution. And here’s what the SQL Server 2019 setup screen says: When an instance of SQL Server runs on a computer that has more…

“UPDATE, INSERT, and DELETE are not normally processed in parallel”

Last Updated September 8, 2020

Brent Ozar

Execution Plans

Years ago, when troubleshooting performance, I stumbled across this Microsoft documentation on parallel query processing that says: Certain types of statements cannot be processed in parallel unless they contain clauses, however. For example, UPDATE, INSERT, and DELETE are not normally processed in parallel even if the related query meets the criteria. But if the UPDATE…

Watch Brent Tune Queries at SQLSaturday Oslo

Last Updated August 29, 2020

Brent Ozar

Execution Plans, Videos

This morning, I had the honor of presenting virtually at SQLSaturday Oslo. The session title, Watch Brent Tune Queries, explains it all. Enjoy! Watch Brent Tune Queries - SQLSaturday OsloWatch this video on YouTube In the session, I used the 2018-06 Stack Overflow database, but any recent full-sized copy (not the 2010-2013 copies) will work.…

Date Tables are Great for Users, but Not So Great for Performance

Last Updated August 5, 2020

Brent Ozar

Execution Plans, Videos

Date tables help users filter data based on day of week, holiday, this year vs last year, and other kinds of filters that business users usually wanna add to their reports. If you haven’t used a date table before, here are a couple of primers on how to create one and use it: Creating a…

Parallelism Can Make Queries Perform Worse.

Last Updated May 26, 2020

Brent Ozar

Execution Plans, Parameter Sniffing

While I was building lab queries for my all-new Fundamentals of Parameter Sniffing course – first live one is next week, still time to get in – I ran across a query with delightfully terrible behavior. I’m always torn when I build the hands-on labs. I want to make the challenges easy enough that you…

How to Get Better Estimates for Modification Queries

Last Updated April 27, 2020

Brent Ozar

Execution Plans, T-SQL

When you’re doing DUI operations against tables with millions of rows, you have to be really careful about SQL Server’s estimates. Ideally, you want your delete/update/insert (what – what’d you think I meant?) queries to be as simple as possible – even a simple join can cause SQL Server to do wildly, wildly incorrect estimates,…

Making TRY_CAST and TRY_CONVERT Queries Faster with Indexed Computed Columns

Last Updated March 1, 2020

Brent Ozar

Computed columns, Execution Plans, T-SQL

I know this is gonna sound crazy, but let’s say you had a table where people stored all kinds of things in one column: dates, integers, file names, sale prices, file names, you name it. And let’s say your application frequently ran a query looking for dates in that column, like this: Transact-SQL SELECT *…

[Video] Watch Brent Tune Queries

Last Updated January 31, 2020

Brent Ozar

Execution Plans, Videos

Ever wonder somebody else does it? Watch over my shoulder as I spend 9 minutes in PowerPoint explaining the big picture, and then about 40 minutes working on this stored procedure in the StackOverflow2013 database: Watch Brent Tune Queries 2020Watch this video on YouTube If you enjoy that, the Watch Brent Tune Queries page has…

Where Should You Tune Queries: Production, Staging, or Development?

Last Updated January 22, 2020

Brent Ozar

Execution Plans

The #1 fastest way to tune queries is in the production database, on the production server. I know. Put the knife down. You’re not happy about that, but hear me out: I’m specifically talking about the fastest way to tune queries. In production, you can guarantee that you’re looking at the same data, hosted on the…

Can SELECT * Make a Query Go…Faster?!?

Last Updated January 7, 2020

Brent Ozar

Execution Plans

Most of the time, you’ll hear advice that you should select as few columns as possible so that SQL Server can do less work. However, one of my clients hit a neat edge case scenario where SELECT * queries actually ran faster than picking a subset of columns. I can’t share the exact query, but I…

How to Make SELECT COUNT(*) Queries Crazy Fast

Last Updated December 22, 2019

Brent Ozar

Columnstore Indexes, Execution Plans

When you run a SELECT COUNT(*), the speed of the results depends a lot on the structure & settings of the database. Let’s do an exploration of the Votes table in the Stack Overflow database, specifically the 2018-06 ~300GB version where the Votes table has 150,784,380 rows taking up ~5.3GB of space. I’m going to measure…

The Two Ways to Fix Non-Sargable Queries

Last Updated December 16, 2019

Brent Ozar

Execution Plans

In the database world, when we say something isn’t sargable, what we mean is that SQL Server is having a tough time with your Search ARGuments. What a sargable query looks like In a perfect world, when you search for something, SQL Server can do an index seek to jump directly to the data you…

How to Think Like the Engine: When a Seek Isn’t

Last Updated November 19, 2019

Brent Ozar

Execution Plans, How to Think Like the SQL Server Engine, Indexing

In our last episode, I introduced the concept of scan predicates: execution plan operations that weren’t able to seek directly to the rows they needed. Let’s take another query: Transact-SQL DropIndexes; GO CREATE INDEX IX_LastAccessDate_Id_DisplayName_Age ON dbo.Users(LastAccessDate, Id, DisplayName, Age); GO SELECT Id FROM dbo.Users WHERE LastAccessDate > '1800/01/01' AND DisplayName = N'Brent Ozar'; GO…

How to Think Like the Engine: Index Column Order Matters a LOT.

Last Updated November 19, 2019

Brent Ozar

Execution Plans, How to Think Like the SQL Server Engine, Indexing

We’ve been working with the clustered index of the Users table, which is on the Identity column – starts at 1 and goes up to a bajillion: And in a recent episode, we added a wider nonclustered index on LastAccessDate, Id, DisplayName, and Age: Transact-SQL CREATE INDEX IX_LastAccessDate_Id_DisplayName_Age ON dbo.Users(LastAccessDate, Id, DisplayName, Age); GO 123…

How to Think Like the SQL Server Engine: What’s the Tipping Point?

Last Updated November 17, 2019

Brent Ozar

Execution Plans, How to Think Like the SQL Server Engine

In our last episode, I’d expanded our query to include DisplayName and Age – two columns that weren’t in our nonclustered index: Transact-SQL SELECT LastAccessDate, Id, DisplayName, Age FROM dbo.Users WHERE LastAccessDate > '2018-09-02 04:00' ORDER BY LastAccessDate; 1234 SELECT LastAccessDate, Id, DisplayName, Age FROM dbo.Users WHERE LastAccessDate > '2018-09-02 04:00' ORDER BY LastAccessDate; So as a result,…