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performance tuning

Crappy Missing Index Requests

Last Updated November 16, 2017

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

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

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

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…

High Compiles and Multi-Statement Table Valued Functions

Last Updated February 14, 2017

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

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

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

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

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

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…

Indexing Temp Tables

Last Updated February 13, 2017

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…

Things I Would Love to Put In sp_BlitzCache

Last Updated April 21, 2017

Execution Plans, First Responder Kit, SQL Server

This is my favorite script to work on Getting stuff to work sp_BlitzCache is so rewarding because XML querying is so ridiculous. I wish Mikael Eriksson would write a book already; it’d make my life a whole lot easier. Along the way, I see a lot of things that I would love to warn people…

Selective XML Indexes: Not Bad At All

Last Updated February 14, 2017

Indexing, SQL Server

Promises, Promises I said I’d follow up on this, and here it is. I’m by no means the world’s foremost authority on these, I just played with a simple example until I figured out what worked the best for my query. So you can all play along at home, I’m using XML we all have…

Date Math In The WHERE Clause

Last Updated February 13, 2017

Execution Plans, SQL Server

Oh, THAT SARGability I realize that not everyone has a Full Metal Pocket Protector, and that we can’t all spend our days plunging the depths of query and index tuning to eek out every CPU cycle and I/O operation from our servers. I mean, I don’t even do that. Most of the time I’m just…

Spills SQL Server Doesn’t Warn You About

Last Updated February 13, 2017

Execution Plans, SQL Server

Don’t make me spill Table variables get a lot of bad press, and they deserve it. They are to query performance what the TSA is to air travel. No one’s sure what they’re doing, but they’ve been taking forever to do it. One particular thing that irks me about them (table variables, now) is that…

Top 5 Overlooked Index Features

Last Updated February 13, 2017

Indexing, SQL Server

I see a lot of indexes Most of them are pretty vanilla. Some are pretty memorable, like one that I saw with 93 INCLUDES. A lot of things that people could do with indexes get overlooked, either because they don’t know about them, or they’re not sure when they’ll help. So, here goes my attempt…

Can Indexes My Query Doesn’t Use Help My Query?

Last Updated February 13, 2017

Indexing, SQL Server

This came up during Office Hours And I love questions like this, because they reveal some interesting underpinnings of how the cardinality estimator works. It was something along the lines of “I have a slow query, and when I add an index the query goes faster even though it doesn’t use the index I created.”…

Why You Should Test Your Queries Against Bigger Data

Last Updated February 14, 2017

Load Testing, SQL Server

I Like Testing Things Especially Brent’s patience. And I like watching people test things. SQL Server is sort of perfect for that. You make some changes, you hit f5, and you wait for something to finish. If it’s faster, you ship it. Right? I mean, we all know you just added NOLOCK hints anyway. A…

MAX Data Types Do WHAT?

Last Updated March 12, 2018

Users are horrible, I know You can just never rely on them to pass in reasonable search strings. When you write stored procedures to retrieve information for them, it’s really tempting to define all your string variables as MAX length. As long as the actual data type, N/VARCHAR matches, you’re cool, right? Avoid that pesky…

Set Statistics… Profile?

Last Updated February 13, 2017

Forgotten, But Not Gone Two of the best built-in tools to figure out if your query tuning efforts are headed in the right direction are SET STATISTICS TIME ON and SET STATISTICS IO ON. If you’re really fancy, you can use SET STATISTICS TIME, IO ON. By fancy I mean lazy. By lazy I mean…