development

CTEs, Views, and NOLOCK

Last Updated March 25, 2017

This is a post because it surprised me It might also save you some time, if you’re the kind of person who uses NOLOCK everywhere. If you are, you’re welcome. If you’re not, thank you. Funny how that works! I was looking at some code recently, and saw a CTE. No big deal. The syntax…

Connect Item: Query Plan Hash in sys.dm_exec_query_stats vs Query Plan Hash in the query plan

Last Updated March 18, 2017

Erik Darling

SQL Server

Shifty Henry said to Bugs You know those moments when you’re looking at something for so long, and it’s still not working, and you’re pretty sure you’re losing that final grip on your sanity? Well, that feeling was multiplied this week by an odd bug that I found, where Query Plan Hashes in system views…

sp_BlitzCache Tip: Cutting Through Heavy Dynamic SQL Use

Last Updated November 16, 2017

Erik Darling

First Responder Kit, sp_BlitzCache, T-SQL

It keeps me up at night Out of the box, sp_BlitzCache will give you enough stuff to work on in your server’s plan cache to last a month of leap days. This is great for most people, but if you use a ton of dynamic SQL in stored procedures, it can make things difficult. What’s…

Indexed Views And Data Modifications

Last Updated March 9, 2017

Erik Darling

Indexing

Love/Hate When indexed views help, they make aggregation queries super fast. There are some kind of horrible limitations and requirements. When they hurt, your modifications slow to a crawl because SQL has to not only keep your horrible indexes synchronized, it also has to keep your indexed view indexes in sync now, too. But SQL…

No seriously, don’t use SQL_VARIANT for that

Last Updated March 9, 2017

Erik Darling

Development, T-SQL

I’d been meaning to write this for a while I half-stumbled on the weirdness around SQL_VARIANT a while back while writing another post about implicit conversion. What I didn’t get into at the time is that it can give you incorrect results. When I see people using SQL_VARIANT, it’s often in dynamic SQL, when they don’t…

Why Cost Threshold For Parallelism Shouldn’t Be Set To 5

Last Updated March 3, 2017

Erik Darling

Configuration Settings

CURRENT_YEAR() Here it is, 2017. A full 20 years since SQL Server 7. Back then, there was a developer named Nick. Nick was on the development team, and apparently the query-costing-magic development was done on his machine. All of it. The magical metric was that for every second a query ran, it would cost another…

Max Worker Threads: Don’t Touch That

Last Updated February 15, 2017

Erik Darling

SQL Server

More isn’t faster I’ve had people give me all sorts of janky reasons for changing Max Worker Threads. I’ve even had people tell me that someone from Microsoft told them to do it. The thing is, all changing that setting does is help you not run out of worker threads. It doesn’t make queries run…

Crappy Missing Index Requests

Last Updated November 16, 2017

Erik Darling

Execution Plans, Indexing, sp_BlitzIndex

When you’re tuning queries It’s sort of a relief when the first time you get your hands on it, you get the plan and there’s a missing index request. Even if it’s not a super high-value one, something in there is crying for help. Where there’s smoke, there’s a bingo parlor. But does adding missing…

Simulating Workload With ostress And Agent Jobs

Last Updated February 14, 2017

Erik Darling

Load Testing, SQL Server

This question comes up a lot Especially during Office Hours, and the answer is usually… not great. You can spend a lot of money on people and complicated software to design, run, and monitor workloads against test environments, or you can throw together tests with some free tools like SQL Query Stress or Microsoft’s RML…

Inline Table Valued Functions: Parameter Snorting

Last Updated May 26, 2020

Erik Darling

Parameter Sniffing, SQL Server, T-SQL

You’ve probably heard about parameter sniffing But there’s an even more insidious menace out there: Parameter Snorting. It goes beyond ordinary parameter sniffing, where SQL at least tried to come up with a good plan for something once upon a compile. In these cases, it just plain gives up and throws a garbage number at…

Memory Grants and Data Size

Last Updated January 8, 2019

Erik Darling

Execution Plans, Memory Grants, SQL Server

What does memory do? In SQL Server, just about everything. We cache our data and execution plans in it (along with some other system stuff), it gets used as scratch space for temporary objects, and of course queries use it for certain operations, most notably sorting and hashing. And of course, now Hekaton comes along…

First Responder Kit Updates: So Many Updates, Brent Quit

Last Updated February 13, 2017

Erik Darling

First Responder Kit, SQL Server

I got 128 GB of RAM just to develop sp_BlitzCache with! Also, thanks to @RichBenner @BrentOzar and @douglane for code, and @LitKnd and @HenrikStaunPoulsen for great ideas! Grab the newly updated First Responder Kit, and here’s what’s in it: sp_Blitz Improvements #677 Finally! A bug that isn’t ours! Microsoft forget to let SE clean up…

High Compiles and Multi-Statement Table Valued Functions

Last Updated February 14, 2017

Erik Darling

SQL Server, T-SQL, TempDB

Way back in 2016 I wrote about what to do if sp_BlitzFirst warns about high compiles. During GroupBy, Adam Machanic gave a great talk on new features in SQL Server 2016. It reminded me of a blog post I wanted to write about one common culprit of high compiles: Multi-Statement Table Valued Functions. Leaving aside…

Analyzing Temporary Stored Procedures with sp_BlitzCache

Last Updated November 16, 2017

Erik Darling

First Responder Kit, sp_BlitzCache, TempDB

All the cool kids Know that when you’re trying to figure out why a stored procedure is slow sometimes, you should use a temporary stored procedure instead of pulling the code out and running it ad hoc. We frequently point people to Erland Sommarskog’s “Slow in the Application, Fast in SSMS“, because it’s an excellent…

Indexes For Kids

Last Updated February 13, 2017

Erik Darling

Indexing, SQL Server

Kid Tested When you work with something a lot, you tend to extrapolate concepts from it out to everything you see. That one person collecting tickets is a serial zone. That food truck line is a HEAP. The empty subway car that smells like a toilet is Replication. Stuff like that. If you have one…

Filtered Indexes and Variables: Less Doom and Gloom

Last Updated March 12, 2018

Erik Darling

Indexing, SQL Server

It Is Known That when you use filtered indexes, they get ignored when your queries are parameterized. This is a Plan Caching Thing©, of course. The simplest example is a bit column with a filtered index. If your index is on WHERE Bit = 1, it doesn’t have data for WHERE Bit = 0. That…

Filtered Statistics Follow-up

Last Updated May 26, 2020

Erik Darling

Parameter Sniffing, SQL Server, Statistics

During our pre-con in Seattle A really sharp lady brought up using filtered statistics, and for a good reason. She has some big tables, and with just 200 histogram steps, you can miss out on a lot of information about data distribution when you have millions or billions of rows in a table. There’s simply…

Is NOLOCK Ever The Right Choice?

Last Updated February 13, 2017

Erik Darling

Locking, Blocking, and Isolation Levels, SQL Server

I’ve poked a lot of fun at NOLOCK I mean, it sucks. You can get incorrect data, and it can throw errors if your data is particularly volatile. Generally, outside of testing a query or grabbing some sample data, I’m going to avoid it. Crappy in every language But what if… What if it’s about…

Hidden in SQL Server 2017 CTP v1.1: sys.dm_db_stats_histogram

Last Updated May 12, 2017

Erik Darling

SQL Server 2017, Statistics

It’s Friday night, so I’m waiting for new CTP Releases As soon as I got the email, I started reading the release notes. Some interesting stuff, of course. Batch mode queries now support “memory grant feedback loops,” which learn from memory used during query execution and adjusts on subsequent query executions; this can allow more…

Indexing Temp Tables

Last Updated February 13, 2017

Erik Darling

Execution Plans, Indexing, SQL Server, TempDB

People often don’t give this thought Which is a shame, because I see people sticking fairly large amount of data into temp tables. On the rare occurrence that I do see them indexed, it’s a nonclustered index on a column or two. The optimzer promptly ignores this index while you select 10 columns and join…