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[Video] The Top 10 Developer Mistakes That Won’t Scale on Microsoft SQL Server

Last Updated July 30, 2020

Development, Videos

You’re a C# or Java developer building an application that stores data in Microsoft SQL Server, and you want to avoid common pitfalls that will cause you scalability problems down the road. In this session from my live stream, you’ll learn why you probably want to avoid triggers, SELECT *, user-defined functions, dynamic SQL, big…

A One-Slide Summary of the Differences Between TDE and Always Encrypted

Last Updated July 31, 2020

The folks on Twitter liked this, so sharing it here: It’s a one-slide summary from a SQL Critical Care client’s deck, so obviously it’s abridged, but I think it does a pretty good job of summing things up. Some highlights of the conversation: A solid 20 minutes of my talk from last year’s Summit, summed…

Remember “Nothing Stops a Hekaton Transaction?” Yeah, About That.

Last Updated July 29, 2020

In-Memory OLTP (Hekaton)

When In-Memory OLTP first came out, Microsoft liked to go around bragging at conferences that “Nothing stops a Hekaton transaction.” I remember hearing that and kinda chuckling at the time, and I was reminded of it by a recent client who was running into blocking issues with…yep, Hekaton. To reproduce it: Do a large insert…

The 201 Buckets Problem, Part 2: How Bad Estimates Backfire As Your Data Grows

Last Updated July 7, 2020

In the last post, I talked about how we don’t get accurate estimates because SQL Server’s statistics only have up to 201 buckets in the histogram. It didn’t matter much in that post, though, because we were using the small StackOverflow2010 database. But what happens as our data grows? Let’s move to a newer Stack…

The 201 Buckets Problem, Part 1: Why You Still Don’t Get Accurate Estimates

Last Updated July 7, 2020

I’ll start with the smallest Stack Overflow 2010 database and set up an index on Location: Transact-SQL USE StackOverflow2010 GO CREATE INDEX Location ON dbo.Users(Location); GO SELECT COUNT(*) FROM dbo.Users; GO 123456 USE StackOverflow2010GOCREATE INDEX Location ON dbo.Users(Location);GOSELECT COUNT(*) FROM dbo.Users;GO There are about 300,000 Users – not a lot, but enough that it will…

You Can Disable Parameter Sniffing. You Probably Shouldn’t.

Last Updated June 27, 2020

Parameter Sniffing

During my parameter sniffing classes, people get a little exasperated with the complexity of the problem. Parameter sniffing is totally hard. I get it. At some level, it’d be great to just hit a magic button and make the whole thing go away. So inevitably, somebody will ask, “What about the database-level setting called Parameter…

Sit Down. We Need to Talk About Your Scalar Functions.

Last Updated June 25, 2020

Development, SQL Server 2019

Thanks for coming today. I apologize for baiting you in here with a picture of toilet paper. That’s not even in the US – that’s from my Iceland trip, before I flew back here to the US. Look, we all make mistakes. That’s not the point of this post. You see, I’ve been reading this…

[Video] How to Find Queries Using OPTION RECOMPILE (And Their Parameters)

Last Updated June 23, 2020

Development, Videos

For years, I hated troubleshooting servers with high CPU usage caused by queries constantly asking for new execution plans. Hated it. SQL Server just doesn’t make it easy to find queries with recompile hints. Then Erik Darling’s sp_HumanEvents came along. And now troubleshooting frequent compilations is as easy as this: Transact-SQL EXEC dbo.sp_HumanEvents @event_type =…

Updating Statistics Causes Parameter Sniffing.

Last Updated June 3, 2020

Parameter Sniffing, Statistics

In my free How to Think Like the Engine class, I explain that SQL Server builds execution plans based on statistics. The contents of your tables inform the decisions it makes about which indexes to use, whether to do seeks or scans, how many CPU cores to allocate, how much memory to grant, and much…

Parallelism Can Make Queries Perform Worse.

Last Updated May 26, 2020

Execution Plans, Parameter Sniffing

While I was building lab queries for my all-new Fundamentals of Parameter Sniffing course – first live one is next week, still time to get in – I ran across a query with delightfully terrible behavior. I’m always torn when I build the hands-on labs. I want to make the challenges easy enough that you…

Your Views Aren’t The Problem. Your Code Is.

Last Updated May 6, 2020

“I hate views,” the DBA said. “They kill performance. And nested views are even worse.” Wrong. I’ll prove it. I’m going to use the 50GB StackOverflow2013 database, and I’ll start by creating a couple of indexes on the Users table to help our queries, then create a view on the Users table: Transact-SQL USE StackOverflow2013;…

Free Webcast on Dynamic SQL Pro Tips

Last Updated May 1, 2020

GroupBy Conference, T-SQL

Dynamic SQL is one of the most powerful tools in the database developer’s arsenal. When you need a complex search stored procedure that takes all kinds of parameters (price, category, location, color), dynamic SQL can run extremely quickly by leveraging better indexes. However, when done wrong, it’s extremely painful to troubleshoot. I’ve been working with…

Can You Use NVARCHAR As a Universal Parameter? Almost.

Last Updated May 27, 2020

A perfect storm of unusual problems caused a client to ask, “What if we just used NVARCHAR(4000) as the default datatype parameter for any query, regardless of what datatype the table has in it – like numbers or dates?” It actually works pretty well in most situations, believe it or not. The Users table of the…

Foreign keys

If You Have Foreign Keys, Don’t Update Fields That Aren’t Changing.

Last Updated December 29, 2021

Foreign Keys, T-SQL

If you update a row without actually changing its contents, does it still hurt? Paul White wrote in detail about the impact of non-updating updates, proving that SQL Server works hard to avoid doing extra work where it can. That’s a great post, and you should read it. But foreign keys add another level of…

How to Get Better Estimates for Modification Queries

Last Updated April 27, 2020

Execution Plans, T-SQL

When you’re doing DUI operations against tables with millions of rows, you have to be really careful about SQL Server’s estimates. Ideally, you want your delete/update/insert (what – what’d you think I meant?) queries to be as simple as possible – even a simple join can cause SQL Server to do wildly, wildly incorrect estimates,…

Why Ordering Isn’t Guaranteed Without an ORDER BY

Last Updated April 11, 2020

Indexing, T-SQL

If your query doesn’t have an ORDER BY clause, you can’t reliably predict the order of your results over time. Sure, it’s going to look predictable at first, but down the road, as things change – the indexes, the table, the server’s configuration, the size of your data – you can end up with some ugly…

[Video] Altering Datatypes With (Almost) No Downtime

Last Updated October 30, 2020

Sometimes, your INT columns run out of space, and you need to alter them to be a larger datatype, like BIGINT. Usually, when you do this, you’re met with screams of agony from your end users because this will totally lock the table and rewrite the whole thing: Transact-SQL ALTER TABLE dbo.Posts ALTER COLUMN OwnerUserId…

How to Track Performance of Queries That Use RECOMPILE Hints

Last Updated May 26, 2020

Parameter Sniffing, Query Store (QDS), sp_BlitzCache

Say we have a stored procedure that has two queries in it – the second query uses a recompile hint, and you might recognize it from my parameter sniffing session: Transact-SQL CREATE OR ALTER PROC dbo.usp_SearchUsers @Reputation INT AS BEGIN /* Query 1, always the same: */ SELECT COUNT(*) FROM dbo.Users; /* Query 2, recompiles…

Making TRY_CAST and TRY_CONVERT Queries Faster with Indexed Computed Columns

Last Updated March 1, 2020

Computed columns, Execution Plans, T-SQL

I know this is gonna sound crazy, but let’s say you had a table where people stored all kinds of things in one column: dates, integers, file names, sale prices, file names, you name it. And let’s say your application frequently ran a query looking for dates in that column, like this: Transact-SQL SELECT *…

WHERE GETDATE() BETWEEN StartDate AND COALESCE(CancelDate, EndDate) Is Even Harder to Tune.

Last Updated February 12, 2020

Development, Indexing

In my last post, we started with a memberships table, and each membership had start & end dates. I’m going to create the table and populate it with everyone having an active membership – their StartDate is the same as their Stack Overflow account creation date, and their EndDate is around a year or two…