Development

Development

Say you’ve got PAGELATCH_UP or PAGELATCH_EX waits, and you’re wondering what might be causing ’em. I’m going to keep the brutally simple approach of building a stored procedure to simply dump hard-coded values into a table at high speed. I don’t want to select from other tables since they might introduce some other types of…

How Table Variables Mess With Parallelism

June 7, 2018

Execution Plans, SQL Server

This Is A Parallel Query Hi There This Is A Temp Table Transact-SQL CREATE TABLE #Posts (OwnerUserId INT); 1 CREATE TABLE #Posts (OwnerUserId INT); This Is A Parallel Insert Into A Temp Table Sort of asterisk (The Insert isn’t parallel, but the part of the plan leading up to it is) This Is A Table…

Stupid T-SQL Tricks

June 6, 2018

Bad Idea Jeans, T-SQL

Presented without comment: Transact-SQL CREATE TABLE dbo.[FROM] ([SELECT] INT, [WHERE] INT, [LIKE] INT); GO SELECT [SELECT] FROM [FROM] WHERE [WHERE] LIKE [LIKE]; GO 1234 CREATE TABLE dbo.[FROM] ([SELECT] INT, [WHERE] INT, [LIKE] INT);GOSELECT [SELECT] FROM [FROM] WHERE [WHERE] LIKE [LIKE];GO Next up, can you break up a query with spaces? Yep: Transact-SQL SELECT * FROM…

How Many Kinds Of Joins Are There?

May 30, 2018

Adaptive Joins, Execution Plans

Quite So There are three in SQL Server, not counting the Adaptive Join which is just a placeholder for either Nested Loops or Hash Joins. If you look closely at each of those joins, you’ll see all sorts of interesting things pop up. Hopefully none of them will be Sorts. NOT EXISTS Nested Loops has:…

Behind Every Trivial Plan Is A Good Demo

May 29, 2018

A While Back I started this Q&A on Stack Exchange about ISNULL. It’s one of the most common client issues I see, and putting stuff like that out in a different community to get more opinions on it is valuable. Also, I’m saving up points for a Stack Overflow Swiss Army Knife. That’s what you…

A Strange Place For A Memory Grant

May 24, 2018

Execution Plans, SQL Server

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…

How to Reduce the CPU Overhead of Dynamic SQL

May 23, 2018

Tara Kizer

Development, T-SQL

Dynamic SQL is a good choice for catch-all type queries, but you have to be careful how you execute the dynamic string. For frequently executed queries, “EXEC sp_executesql @sql”is a good choice but at the risk of encountering parameter sniffing issues. For queries that aren’t executed very often, “EXEC (@sql)” can be used, but you…

Trivial Plans, Simple Parameterization and Check Constraints

May 22, 2018

Trivial Plans Are Weird They’re the optimizer’s “Yes, Dear”. The thing is, sometimes “Yes, Dear” can miss some important details. Cost based decisions about operators Potential index improvements Contradiction detection for check constraints Simple parameterization, according to… uh… Official Microsoft documentation? Can only occur in a Trivial Plan. Which makes sense. Simple parameterization is considered…

How to Create Deadlocks and Troubleshoot Them

May 17, 2018

Deadlocks

Watch in awe and wonder as I create a deadlock, then use sp_BlitzLock after the fact to show you which queries and tables were involved: Here are the scripts to run in the left hand window: Transact-SQL CREATE TABLE dbo.Lefty (Numbers INT PRIMARY KEY CLUSTERED); INSERT INTO dbo.Lefty VALUES (1), (2), (3); CREATE TABLE dbo.Righty…

Quick Tips For Debugging Large Stored Procedures

May 16, 2018

T-SQL

Hell Here! Get Your Fresh Hell, Here! Let’s face it — as far as developer tools go, SSMS is pretty bad. Intellisense? The jokes write themselves. Don’t get me wrong, SSMS is a good management tool, but it’s not a good development tool (and what kind of maniac sticks debug — F6 — next to execute —…

How to Screw Up Your Database Updates in 3 Simple Steps

May 15, 2018

Alex Yates

Development

Alex Yates Hi. I’m not Brent. My name’s Alex, and I care about databases, DevOps and data protection. Last year I gave a session on GroupBy all about how to do Database DevOps right. Now, dear reader, I’m going to give you a cheerful, three-step crash course about how to screw it all up. All…

Building SQL ConstantCare®: I Love Unit Tests

May 14, 2018

Development, SQL ConstantCare

You know what my biggest regret is about sp_Blitz? No unit tests. Seriously. Back then, I wrote a bunch of T-SQL checks to see if a server had memory configuration errors, corruption, obscure trace flags, etc. Over time, sp_Blitz got bigger and bigger, and we started getting code contributions from strangers. Things got even more challenging…

When Query Plans Lie Part 2

May 9, 2018

Execution Plans, Indexing, SQL Server

Getting Weirder In Part 1, we looked at how query plans can tell us little lies. Now we’re going to look at how those little lies can turn into bigger lies. Adding An Index Right now, this is our query: Transact-SQL SELECT u.UpVotes, u.DownVotes FROM dbo.Users AS u WHERE u.Reputation >= 100000; 123 SELECT u.UpVotes,…

When Query Plans Lie Part 1

May 7, 2018

Freaking Out Brent Is a full time job. Some days I can get him to hit the desk drawer tequila, and other days I crash and burn. This day was so monumental I almost hired someone to do an action movie narration of this blog post. Let’s Start Easy Over in Stack Overflow, we’re going…

Hey, That’s Not My Sort!

May 3, 2018

Understand Your Plan Mr. Optimizer does the rock n roll hoochie koo with Mrs. Optimizer, c. 1953. When reading query plans, you may sometimes see a sort when you didn’t explicitly ask for data to be sorted. Sometimes they show up to support order-preserving operators like stream aggregates, merge joins, or segments. Other times, they…

Concurrency Week: How to Delete Just Some Rows from a Really Big Table

April 27, 2018

Locking, Blocking, and Isolation Levels

Say you’ve got a table with millions or billions of rows, and you need to delete some rows. Deleting ALL of them is fast and easy – just do TRUNCATE TABLE – but things get much harder when you need to delete a small percentage of them, say 5%. It’s especially painful if you need…

Concurrency Week: An Odd Case of Blocking

April 26, 2018

Locking, Blocking, and Isolation Levels

It’s Not A Trigger Because I know, I know. It’s always a trigger. Ha ha ha this query on [Table Unrelated] is causing blocking on [Other Table Unrelated]. I’m not going to insult you and waste your time with that. It’s also not a view. You’re better than that. I’m better than that. Let’s move…

Concurrency Week: How Entity Framework and NHibernate Can Cause Poison RESOURCE_SEMAPHORE Waits

April 25, 2018

Tara Kizer

Development

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…

Concurrency Week: Can SELECTs Win Deadlocks?

April 24, 2018

Deadlocks