Indexing

When you index a nullable field, are the rows with nulls stored in the index? It’s easy enough to find out by creating a table with a nullable field, and then creating an index on it: Transact-SQL CREATE TABLE dbo.Employees ( ID INT IDENTITY(1, 1) PRIMARY KEY CLUSTERED, EmployeeName VARCHAR(50) NULL ); GO INSERT INTO…

Can deleting rows make a table…bigger?

March 5, 2019

Michael J. Swart posted an interesting question: he had a large table with 7.5 billion rows and 5 indexes. When he deleted 10 million rows, he noticed that the indexes were getting larger, not smaller. Here’s one way that deletes can cause a table to grow: The rows were originally written when the database didn’t…

What does Azure SQL DB Automatic Index Tuning actually do, and when?

February 19, 2019

Azure SQL DB, Indexing

Azure SQL DB’s Automatic Tuning will create and drop indexes based on your workloads. It’s easy to enable – just go into your database in the Azure portal, Automatic Tuning, and then turn “on” for create and drop index: ENGAGE Let’s track what it does, and when. I set up Kendra Little‘s DDL trigger to log index changes, which…

How to Think Like the Engine – 2019 Edition

January 25, 2019

Indexing, SQL Server, Videos

You’re a developer or a DBA, and you’re comfortable writing queries to get the data you need. You’re much less comfortable trying to design the right indexes for your database server. In this series of videos, you’ll learn how the SQL Server engine looks at your indexes and builds your query results. In 90 minutes…

New Fundamentals of Index Tuning Course

January 23, 2019

You’re a developer who needs to make your application go faster, and you don’t want to (or can’t!) change the queries. You’ve never been to an official training class, and your manager isn’t about to spring for travel. In my new Fundamentals of Index Tuning course, you’ll learn: How to design indexes for a query…

sp_BlitzIndex's new statistics histograms at the bottom

How Should We Show Statistics Histograms in sp_BlitzIndex?

January 10, 2019

Indexing, sp_BlitzIndex

If you’re a graduate of my free How to Think Like the SQL Server Engine course – and you’d better be, dear reader – then you’re vaguely familiar with DBCC SHOW_STATISTICS. It’s a command that shows you the contents of a statistics histogram. When I’m doing the first two parts of the D.E.A.T.H. Method – (D)eduplicating identical…

The Pleasure Of Presenting With Joe Obbish

January 3, 2019

Execution Plans, GroupBy Conference, Indexing, Videos

Grouple On At the December 2018 GroupBy I had the honor of co-presenting with one of the smartest guys around, Joe Obbish. Here’s the video of our session, “The Problems With Optional Operators”, and a link to the resources. Enjoy! Resources: Slides and Demos

Why Do Some Indexes Create Faster Than Others?

January 3, 2019

To warn you ahead of time, this post is an exploration without an answer (yet). There’s some interesting stuff in here, but no conclusions. If that’s not your kind of post, feel free to skip it. If it is, well, here goes nothin’… Wild, Wild Life Creating indexes is kind of a funny thing. By…

When Does SARGability Matter Most?

December 27, 2018

!erehT iH I know what you’re thinking. Another post about how you should just never do this one thing and all your queries will magically end up faster. Just watch the video. For more reading, check out these posts: Sargability: Why %string% Is Slow Optional Parameters and Missing Index Requests Computed Columns: Reversing Data…

Do Functions Stop You From Using Indexes?

December 20, 2018

Indexing, T-SQL

Say I’ve got a function in my WHERE clause: Transact-SQL SELECT DisplayName FROM dbo.Users WHERE LTRIM(RTRIM(DisplayName)) = 'Brent Ozar'; 123 SELECT DisplayName FROM dbo.Users WHERE LTRIM(RTRIM(DisplayName)) = 'Brent Ozar'; If I have an index on DisplayName, will SQL Server use it? Sure: Function in the WHERE clause Even though SQL Server can’t seek to “Brent Ozar,” it will…

Indexed View Matching With GROUP BY And DISTINCT

December 5, 2018

Bit Of A Kick I’ve been playing with indexed views a little bit lately for some demos in my Bits Precon. There are a whole bunch of limitations in creating indexed views. One of them is that you can’t base the query on DISTINCT. Fair enough, but you can do GROUP BY. And what’s pretty…

Indexed View Creation And Underlying Indexes

December 4, 2018

Accidental Haha While working on some demos, I came across sort of funny behavior during indexed view creation and how the indexes you have on the base tables can impact how long it takes to create the index on the view. Starting off with no indexes, this query runs in about six seconds. Transact-SQL DECLARE…

Tales From Overindexing: Too Many One Column Indexes

November 29, 2018

Execution Plans, Indexing, SQL Server

Master Plan Sometimes you see a query, and it’s hitting one table, and then the query plan looks like a vase full of spaghetti. Usually, there’s a view involved. Usually. Sometimes, there’s just really weird indexing. Here’s A Thing, Look At It Promises This is a query against one table. No views, no functions, no…

Adventures In Foreign Keys 5: How Join Elimination Makes Queries Faster

November 16, 2018

FINALLY… This is the last post I’ll write about foreign keys for a while. Maybe ever. Let’s face it, most developers probably find them more annoying than useful, and if you didn’t implement them when you first started designing your database, you’re not likely to go back and start trying to add them in. Depending…

Adventures In Foreign Keys 4: How to Index Foreign Keys

November 15, 2018

This week, we’re all about foreign keys. So far, we set up the Stack Overflow database to get ready, then tried to set up relationships, and encountered cascading locking issues. Dawn Of The Data I don’t know how long the recommendation to index your foreign keys has been a thing, but I generally find it useful…

How Check Constraints MIGHT Improve Your Queries and Missing Index Requests

October 31, 2018

The more SQL Server knows about your data, the better your query plans can get. Say you’ve got an app that’s designed to store multiple companies in a single database – but you don’t actually use it that way. All of the data in a given database is actually for the same company. We’ll use…

Filtered Indexes vs Parameterization (Again)

October 26, 2018

At First I Was Like… This won’t work at all, because parameters are a known enemy of filtered indexes. But it turns out that some filtered indexes can be used for parameterized queries, whether they’re from stored procedures, dynamic SQL, or in databases with forced parameterization enabled. Unfortunately, it seems limited to filtering out NULLs…

Batch Mode For Row Store: Does It Fix Parameter Sniffing?

October 25, 2018

Batch Mode, Execution Plans, Indexing

Snorting The Future SQL Server 2019 introduced batch mode over row store, which allows for batch mode processing to kick in on queries when the optimizer deems it cost effective to do so, and also to open up row store queries to the possibility of Adaptive Joins, and Memory Grant Feedback. These optimizer tricks have…

Index Tuning Week: Missed Missing Index Opportunities

October 12, 2018