How a row goal affects estimates

Most of our queries are intended to process a complete data set. But not all. There are cases where we restrict the number of rows. Or cases where SQL Server does this. In all such cases, and a few others, the query optimizer uses a mechanism called a “row goal” to ensure that the execution plan is optimized for a subset of the total result set, not all of it. Some examples Without trying to be complete, let’s list a few cases where a row goal would be set. Literal row limitation in the query The most simple is when…
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Execution plans – learning opportunities

Most people know they have to invest in themselves, so that they can get better at their job. And if that job involves query performance on SQL Server, then part of that investment should definitely go towards execution plans. I happen to teach about execution plans. A lot. And there are a few opportunities in the near future that I want to make sure you are aware of. (If you are allergic to self-promotion, you might want to skip this post!) The basics Perhaps you are here because you have heard that execution plans are an important tool for query…
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Execution Plan Video Training – Announcing the next block

It’s time! Time to formally announce the release date for the next block of videos for the SQLServerFast Execution Plan Video Training. The basic level of block 2, Reading data, will go live on May 15. What to expect? Block 2 of the SQLServerFast Execution Plan Video Training covers everything involved with reading data. The basic level, consisting of five videos (“chapter”) of 25 to 40 minutes each, focuses on what many people still see as the standard for SQL Server: disk-based rowstore indexes and heaps. The first chapter of the block gives a detailed overview of how this type…
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T-SQL Tuesday #137 – Use notebooks every day? No way!

As you probably know, T-SQL Tuesday is a monthly event where bloggers are invited to write about a set topic. Our host for April 2021 is Steve Jones (b|t) this time, and he has asked us to write about Jupyter notebooks: do we use them, would we want to; and why or why not? The basics Let’s start with the basics, because I’m not convinced that everyone reading this already knows what Jupyter notebooks are. Simply put, a notebook is a single file where you can store a combination of blocks with formatted text and blocks with code, where the…
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Plansplaining, part 17. Temporal tables (part 2)

Welcome to part seventeen of the plansplaining series, where I will continue my look at how temporal tables affect execution plans. The previous post focused on data modifications in temporal tables. Now let’s see what happens if we use temporal logic to our basic retrieval queries. This post focuses on the basics only; do keep in mind that as soon as you want to join data from multiple temporal queries you will run into some specific issues that you need to be aware of; this will be the subject for a later post. Sample tables For the code samples in…
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T-SQL Tuesday #136 – Float does NOT suck!

Another month has passed; time for another T-SQL Tuesday post. The monthly blog party is hosted by Brent Ozar (b|t) this time. And he wants us to write about our favourite (or least favourite) data type. I don’t really have a data type I like. I also don’t hate data types. The thing I do hate is when the wrong data type is used for the purpose. So instead of doing what Brent asks, I will write a short post about a very common misconception about a data type that triggers me whenever I read it. Which happens a lot.…
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Plansplaining, part 16. Temporal tables (part 1)

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Welcome to part sixteen of the plansplaining series. The first of a few posts about how temporal tables affect execution plans. In this post, I’ll build on the last four posts on data modifications, building on the more generic discussion of data modification in the previous four posts. Later posts will look at data retrieval and some specific scenarios. If you have never heard of temporal tables, I suggest reading Louis Davidson’s five part series about this feature. It seems some of the formatting has been lost in time, but the content is still good and valid. Part 1 Part…
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T-SQL Tuesday #135 – My tools to stay alive

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It’s February, it’s Tuesday, and we all know what that means. Time for another T-SQL Tuesday post. Our host today is Mikey Bronowski (b|t), who asks us to write about tools we use to make our jobs easier. This looked familiar. And when I checked, I found out that I did, indeed, talk about tools before. That post was specifically about tools for work. I could of course now write about new tools I discovered, or tools for other areas in my work such as presenting or recording videos. But since Mikey doesn’t restrict it to work-related tools only, I…
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T-SQL Tuesday #134 – Give me a break!

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The January 2021 edition of T-SQL Tuesday is hosted by James McGillivray (b|t). His question is: if you’ve been working too hard, if you’ve been spending too much time in front of a screen, what do you do to wind down? The answer to that is, for me, three-pronged. Tier 1: Do other work I work on a very varied set of activities, and I have a lot of freedom on how I choose to distribute my time: paid customer work, sometimes for multiple customers. Preparing, rehearsing, and delivering live presentations. Research and writing for the Execution Plan Reference. Authoring…
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Plansplaining, part 15. Foreign keys, part 4

Welcome to part fifteen of the plansplaining series. In the three previous parts I looked at the operators and properties in an execution plan that check a modification doesn’t violate foreign key constraints. That part is done. But I’m not done with foreign keys yet. We normally expect foreign keys to throw an error on violations. But that’s actually only the default option: they can also be set to be self-correcting. This is done using the ON UPDATE and ON DELETE clauses, which provide the user with several choices on how to handle child data that would become orphaned, and…
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