Plansplaining

Plansplaining, part 10. Just passing through

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Welcome to part ten of the plansplaining series. Each of these posts takes an execution plan with an interesting pattern, and details exactly how that plan (or pattern) works. In this post we will look at a query and execution plan that may appear perfectly normal and unexpected at first sight, but that has some perhaps confusing execution counts. Sample query The sample query below will (as almost always) run in all versions of the AdventureWorks sample database. It returns a list of all staff, and for people that have a sales-related job title it adds the total of sales…

Plansplaining, part 9. Recursive CTEs

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I had to skip three months, but finally it is here: part eight of the plansplaining series. Each of these posts takes an execution plan with an interesting pattern, and details exactly how that plan works. I am pretty sure that (almost) everyone reading this blog knows that a CTE (Common Table Expression) is an independent subquery that can be named and then referenced (multiple times if needed) in the main query. This makes CTEs an invaluable tool to increase the readability of complex queries. Almost everything we can do with a CTE can equally well be done by using…

Plansplaining, part 8. To join the impossible join

This is part eight of the plansplaining series. Each of these posts takes an execution plan with an interesting pattern, and details exactly how that plan works. In this post we look at a query that, given the known limitations of the available join operators, appears to be impossible to execute. But it’s a legal query so it should run. And it does. But how? Sample query The query below can be executed in any version of the AdventureWorks sample database. Don’t bother understanding the logic, there is none. It is merely constructed to show how SQL Server handles what…

Plansplaining, part 7. The Constant Scan that returns no data

This is part seven of the plansplaining series. Each of these posts takes an execution plan with an interesting pattern, and details exactly how that plan works. In this post we look at a deceptively simple query: a simple SELECT with an ISNULL to show either a row returned or a placeholder value. And yet there is more going on under the covers than one might expect. Sample query The query below can be executed in any version of the AdventureWorks sample database. It returns a code string representing the version number on the 2016 and 2017 versions; on older…

Plansplaining, part 6. Aggregates with OVER.

This is the sixth post in the plansplaining series. Each of these blog posts focuses on a sample execution plan that exposes an uncommon and interesting pattern, and details exactly how that plan works. In the first post, I covered each individual step of each operator in great detail, to make sure that everyone understands exactly how operators work in the pull-based execution plans. In this post (and all future installments), I will leave out the details that I now assume to be known to my readers. If you did not read part 1 already, I suggest you start there.…

Plansplaining, part 5. Bitmaps

This is the fifth post in the plansplaining series. Each of these blog posts focuses on a sample execution plan that exposes an uncommon and interesting pattern, and details exactly how that plan works. In the first post, I covered each individual step of each operator in great detail, to make sure that everyone understands exactly how operators work in the pull-based execution plans. In this post (and all future installments), I will leave out the details that I now assume to be known to my readers. If you did not read part 1 already, I suggest you start there.…

Plansplaining, part 4. Let’s repartition the streams.

This is the fourth post in the plansplaining series. Each of these blog posts focuses on a sample execution plan that exposes an uncommon and interesting pattern, and details exactly how that plan works. In the first post, I covered each individual step of each operator in great detail, to make sure that everyone understands exactly how operators work in the pull-based execution plans. In this post (and all future installments), I will leave out the details that I now assume to be known to my readers. If you did not read part 1 already, I suggest you start there.…

Plansplaining, part 3. How repeating work saves time

This is the third post in the plansplaining series. Each of these blog posts focuses on a sample execution plan that exposes an uncommon and interesting pattern, and details exactly how that plan works. In the first post, I covered each individual step of each operator in great detail, to make sure that everyone understands exactly how operators work in the pull-based execution plans. In this post (and all future installments), I will leave out the details that I now assume to be known to my readers. If you did not read part 1 already, I suggest you start there.…

Plansplaining, part 2. Why scan and spool instead of seek?

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This is the second post in the plansplaining series. Each of these blog posts focuses on a sample execution plan and details exactly how that plan works. In the first post, I covered each individual step of each operator in great detail, to make sure that everyone understands exactly how operators work in the pull-based execution plans. In this post (and all future installments), I will leave out the details that I now assume to be known to my readers. If you did not read part 1 already, I suggest you start there. Sample query The query in this post…

Plansplaining, part 1. The unexpected aggregation and assert

When I look at an execution plan I sometimes right away see how the operators work together. Other times I need to dive into the details and put in effort before I really understand it. And occasionally, I think I understand but then am proven wrong after I start looking at the details. However, understanding all the details of an execution plan is really important when you want to optimize performance. If I see an execution plan where I do not understand the role of each and every operator, I know I do not truly understand how the query is…
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