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Plansplaining

Plansplaining, part 19. Temporal tables (part 4)

Hugo Kornelis
2 Comments
Welcome to part nineteen of the plansplaining series, where I will finally wrap up the discussion on temporal tables. In the first three parts, we looked at execution plans for modifying data in a temporal table, and for basic and advanced temporal queries. In this last part, we’ll shift away for execution plans (so the plansplaining tag is perhaps a bit misleading) and look at what happens if you join data from multiple temporal tables. Sample data I once more use the same tables as in all previous post, but this time I want a bit more control over the…

Plansplaining, part 18. Temporal tables (part 3)

Hugo Kornelis
1 Comment
Welcome to part eighteen of the plansplaining series. Like the previous posts, this one too focuses on temporal tables and their effect on the execution plan. After looking at data modifications in temporal tables and at querying with a most basic temporal form of temporal query, let’s look at the more advanced variations for temporal querying. We’re still looking at getting data from a single query only in this post. We’ll look at joins in the next post. Sample tables For the code samples in this post, I keep building on the demo tables and sample data from the previous…

Plansplaining, part 17. Temporal tables (part 2)

Hugo Kornelis
4 Comments
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…

Plansplaining, part 16. Temporal tables (part 1)

Hugo Kornelis
5 Comments
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…

Plansplaining, part 15. Foreign keys, part 4

Hugo Kornelis
4 Comments
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…

Plansplaining, part 14. Foreign keys, part 3

Hugo Kornelis
3 Comments
Welcome to part fourteen of the plansplaining series, where I wrap up the mini-series on how simple foreign keys have huge effects on execution plans for data modifications. We already looked at inserting data in the referencing (child) table, and at deleting data from the referenced (parent) table as well as updates in the child table. We did not and will not look at deleting from the child table or inserting in the parent table: those operations can by default never violate the foreign key constraint, so no additional logic is needed. So that means there is only one thing…

Plansplaining, part 13. Foreign keys, part 2

Hugo Kornelis
2 Comments
Welcome to part thirteen of the plansplaining series. In this part I continue the exploration of how execution plans check foreign keys on data modification. In the previous part, we looked at foreign key checking when data is inserted. Inserts in the referencing (child) table can, by definition, not cause foreign key violations, so no checking is done in those cases. But for inserts in the referenced (parent) table, SQL Server needs to check that the inserted value does indeed exist. We saw how both the Nested Loops and the Merge Join operators have some specific optimizations to do that…

Plansplaining, part 12. Foreign keys, part 1

Hugo Kornelis
5 Comments
Welcome to part twelve of the plansplaining series, where I’ll discuss foreign keys in the context of execution plans. And just to be clear, this is not about how foreign keys and other constraints can be used to simplify execution plans and speed up query executions. That is documented in so many places that I trust you already know how important those constraints are. No, today we’ll look at the extra work that SQL Server needs to do to ensure that your changes don’t violate any of your foreign key constraints. Sample tables I will use a very simple scenario…

Plansplaining, part 11. Merge plans

Hugo Kornelis
2 Comments
Welcome to part eleven of the plansplaining series. You will have noticed that this is no longer a monthly series. But when I was asked recently to provide some insight in how a MERGE statement is reflected in an execution plan, I realized that the plansplaining format works fine for this. But first a word of warning. The MERGE statement, introduced in SQL Server 2008 as an easier alternative for “delete / update / insert” logic, turned out to have issues when it was released. And now, in 2020, many of those issues still exist. So I’ll just point you…

Plansplaining, part 10. Just passing through

Hugo Kornelis
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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…

Hugo Kornelis

Married, father of 2 children.

Awarded 13x MVP (2006-2016; 2019-now), in the areas SQL Server and Data Platform.

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