Development

“Index Seek” Doesn’t Mean Much.

Execution Plans
13 Comments
When you see “index seek” on an execution plan, that doesn’t mean SQL Server is jumping to exactly the row you’re looking for. It only means that SQL Server is seeking on the first column of the index. This is especially misleading on indexes where the first column isn’t very selective. To explain, I’ll take…
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What Do the LCK_M_SCH_M and LCK_M_IS Wait Types Indicate?

You’re investigating your SQL Server’s top wait stats, and you’re noticing a lot of LCK% waits. Normally, that indicates blocking and deadlocks, but you’re just not getting complaints from your end users, and you’re wondering what’s causing it. It might be overzealous index rebuilds. Let’s demo why by starting a new query in the Stack…
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Error Handling Quiz Week: Tryin’ TRY/CATCH

T-SQL
32 Comments
Let’s say we have two tables, Parent and Child, and we need to guarantee that they both get populated at once. We’ll write a single stored procedure to do both inserts: DROP TABLE IF EXISTS dbo.Parent; DROP TABLE IF EXISTS dbo.Child; CREATE TABLE dbo.Parent (ID INT IDENTITY(1,1), DateAdded DATETIME2); CREATE TABLE dbo.Child (ID INT IDENTITY(1,1),…
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Mastering Query Tuning

5 Ways to Measure a Query

Execution Plans
2 Comments
In my free How to Think Like the Engine class, we start out by measuring query performance using logical reads. That’s the number of 8KB data pages that SQL Server has to read in order to find your query’s results. That’s the measure I use the most because generally speaking, the less data your server…
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What Is a Cost-Based Optimizer?

Execution Plans
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When you execute a query, the database server has to figure out things like: Which table to process first Which index to use on that table Whether to seek on that index or scan it Which table to process next How to join the data between those two tables When to sort the data For…
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Most T-SQL Queries Don’t Even Try to Handle Errors.

T-SQL
45 Comments
David Tovee asked a great question in yesterday’s Mastering Query Tuning class. He asked his fellow students, “How many of you actually use TRY/CATCH?” I turned it into a Twitter poll because I wanted to check a wider audience: When the developers at my company write new queries, they use TRY/CATCH and exception handling: —…
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Why Are Linked Server Queries So Bad?

Development
90 Comments
No, I don’t want to be your valentine. Remember when you were in school, and you had a crush on someone? You would write a note asking them to be your valentine, and you’d ask a mutual friend to pass the note over to them. The adult equivalent is linked server queries. When your query…
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What does SET NOCOUNT ON do?

T-SQL
13 Comments
When you’re working with T-SQL, you’ll often see SET NOCOUNT ON at the beginning of stored procedures and triggers. What SET NCOUNT ON does is prevent the “1 row affected” messages from being returned for every operation. I’ll demo it by writing a stored procedure in the Stack Overflow database to cast a vote: CREATE…
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Brent Reading Book

9 Signs Your T-SQL Might Fail a Code Review

T-SQL
28 Comments
It’s hard to set absolute rules about, “Feature X should absolutely never be used.” However, there are some features that set off alarm bells when I see them. Usually, when I start asking more questions about when we’re using those particular features, I get answers of, “Oh, I didn’t know that was a problem.” As…
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