What’s New in SQL Server 2019: Adaptive Memory Grants

When you run a query, SQL Server guesses how much memory you’re going to need for things like sorts and joins. As your query starts, it gets an allocation of workspace memory, then starts work. Sometimes SQL Server underestimates the work you’re about to do, and doesn’t grant you enough memory. Say you’re working with…
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A Strange Place For A Memory Grant

If You Hang Around Fetish Bars Long Enough… You learn things about memory grants in execution plans. Stuff like the things that usually need memory Hash Join Hash Match Sort Parallelism Exchanges But there’s something that rarely gets discussed, even in the deepest, darkest, dankest dungeons. Let’s loosen the clamps and look at it. Keys…
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Concurrency Week: How Entity Framework and NHibernate Can Cause Poison RESOURCE_SEMAPHORE Waits

Development, Memory Grants
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I’ve already blogged about my dislike for ORMs from a production DBA performance tuning standpoint only. I get that they’re useful to developers. I get it. But I’m focused on performance. A quick recap of what I don’t like about ORMs from that other blog post: Implicit conversions due to nvarchar variables vs varchar columns…
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Hash Join Memory Grant Factors

Buskets Much like Sorts, Hash Joins require some amount of memory to operate efficiently  — without spilling, or spilling too much. And to a similar degree, the number of rows and columns passed to the Hashing operator matter where the memory grant is concerned. This doesn’t mean Hashing is bad, but you may need to…
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Memory Grants: SQL Server’s Other Public Toilet

Sharing Is Caring When everything is going well, and queries are behaving responsibly, one need hardly think about memory grants. The problem becomes itself when queries start to over and under estimate their practical needs. Second Hand Emotion Queries ask for memory to do stuff. Memory is a shared resource. What kind of stuff? Well, usually…
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“But It Worked in Development!” – 3 Hard Performance Problems

Development, Memory Grants
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You’ve been performance tuning queries and indexes for a few years, but lately, you’ve been running into problems you can’t explain. Could it be RESOURCE_SEMAPHORE, THREADPOOL, or lock escalation? These problems only pop up under heavy load or concurrency, so they’re very hard to detect in a development environment. In a very fast-paced session, I’ll…
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SQL Server 2017 CU3 adds tempdb spill diagnostics in DMVs and Extended Events

Where have you been all my life? Whenever I’m teaching people about query tuning, two things come up almost without fail: parameter sniffing and tempdb spills. They’re not mutually exclusive — some queries will seemingly always spill to tempdb, whether parameter sniffing is present of not, and some queries experiencing issues with parameter sniffing will never spill…
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What’s the Difference Between Estimated and Actual Execution Plans?

I’m going to use the dbo.Users table in the StackOverflow demo database and run a pretty simple query: Transact-SQL SELECT * FROM dbo.Users WHERE DisplayName LIKE 'Brent%'; 123 SELECT *FROM dbo.UsersWHERE DisplayName LIKE 'Brent%'; First, hit Control-L in SSMS and get the estimated execution plan. Here it is: Estimated execution plan: https://www.brentozar.com/pastetheplan/?id=H1TeRlZke Click on the…
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An Introduction to Query Memory

Microsoft has been quietly making some amazing improvements for performance tuners in SQL Server 2012, 2014, and 2016. This week, we’re going to introduce you to just how awesome they are. (They being the improvements, not Microsoft. You already knew they were awesome.) Using the freely available StackOverflow database, let’s start with a simple query…
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