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Tales From Overindexing: Too Many One Column Indexes

Last Updated November 23, 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…

Brent getting caffeinated

How to Troubleshoot Blocking and Deadlocking with Scripts and Tools

Last Updated November 28, 2018

Deadlocks, Monitoring, sp_BlitzLock

When you need to find which queries are blocking other queries, your decision comes down to when the blocking happened. Is it happening now, recently, or coming up soon? During a live emergency, start with sp_WhoIsActive. Adam Machanic’s excellent free sp_WhoIsActive replaces Activity Monitor, sp_who, and sp_who2, and it’s way more powerful. The documentation is extensive –…

Getting Sneaky With Forced Parameterization

Last Updated November 23, 2018

Execution Plans

Silly Rules I’ve blogged about some of the silly rules about where Forced Parameterization doesn’t work. One rule that really irked me is this one: The TOP, TABLESAMPLE, HAVING, GROUP BY, ORDER BY, OUTPUT…INTO, or FOR XML clauses of a query. TOP and FOR XML, get used, like, everywhere. TOP is pretty obvious in its usage.…

Not So Forced Parameterization

Last Updated November 26, 2018

Execution Plans

Asking The Wrong Question Sometimes, when you wanna turn on a feature, you spend so much time wondering if you should, you don’t bother asking if it’ll even work when you do. There are a long list of things that are incompatible with Forced Parameterization, on a page that’s pretty hard to find. Now, there’s…

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

Last Updated December 29, 2021

Execution Plans, Foreign Keys

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

Last Updated December 29, 2021

Execution Plans, Foreign Keys

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…

Adventures In Foreign Keys 3: Why Cascading Deletes Perform Slowly

Last Updated December 29, 2021

Execution Plans, Foreign Keys, Locking, Blocking, and Isolation Levels

Legendreary In the last post, I looked at some issues with implementing foreign keys with cascading actions. Namely that, well, it fell apart pretty quickly just trying to set up. I didn’t even get to the point where I wanted to test all those relationships. But there’s an even worse surprise waiting for you if…

Adventures In Foreign Keys 2: Common Foreign Key Errors

Last Updated December 29, 2021

Execution Plans, Foreign Keys

This week, we’re all about foreign keys. Yesterday’s post covered scripts to set up the Stack Overflow database to get ready, eliminating data that would violate FK relationships. You Had Two Jobs! Let’s say I wanted to implement Foreign Keys to do two things If a user deletes their account, all of their badges, comments, and…

Adventures In Foreign Keys 1: Setting Up Foreign Keys in Stack Overflow

Last Updated December 29, 2021

Execution Plans, Foreign Keys

In A Foreign Key, In A Foreign Table Much of what people want from foreign keys, like referential integrity and join elimination, are only as guaranteed as much as SQL Server can trust your constraints (and even then…). The same goes for check constraints, too. Thankfully, things like Primary Keys and Unique Constraints are sort…

What’s New in SQL Server 2019: Faster Functions

Last Updated June 25, 2020

SQL Server 2019, T-SQL

A while back, we talked you through a public whitepaper about how Microsoft was working on making user-defined functions go faster. Now that the preview of SQL Server 2019 is out, you can start getting your hands on Froid, the performance-boosting feature. Here’s the documentation on it – let’s see how it works. Using the…

The Curse of Cursor Options

Last Updated November 2, 2018

Execution Plans

Red Skies At Night I know it’s hard to believe, but I still see a lot of people using cursors when they shouldn’t. Other times, there’s some scary dungeon part of the code that someone wrote eons ago that no one wants to go anywhere near to fix. Sometimes there’s a decent reason, something like:…

dbo.Posts_MultiTenant

How Check Constraints MIGHT Improve Your Queries and Missing Index Requests

Last Updated October 31, 2018

Execution Plans, Indexing

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…

SUM, AVG, and arithmetic overflow

Last Updated October 26, 2018

You Shoulda Brought A Bigger Int Sometimes you run a query, and everything goes fine. For a while. For example, if I run this query in the 2010 copy of Stack Overflow, it finishes pretty quickly, and without error. Transact-SQL SELECT u.Id, u.DisplayName, SUM(p.Score) AS SumPostScore, AVG(c.Score) AS SumCommentScore FROM dbo.Users AS u JOIN dbo.Posts…

Filtered Indexes vs Parameterization (Again)

Last Updated October 21, 2018

Execution Plans, Indexing

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?

Last Updated May 26, 2020

Batch Mode, Execution Plans, Indexing, Parameter Sniffing

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…

Batch Mode For Row Store: What Does It Help With?

Last Updated January 8, 2019

Batch Mode, Memory Grants

SQL Server 2019 Introduces something pretty cool: the ability for row store indexes to be processed in batch mode. Batch mode is, of course, the preferred processing realm of column store indexes. If you’re still in the dark about column store, you might wanna start by reading up on Niko Neugebauer and Joe Obbish’s blogs about it.…

Execution plan in Azure SQL DB

How to See Execution Plans in Azure Data Studio

Last Updated October 23, 2018

Azure Data Studio, Execution Plans

Azure Data Studio is Microsoft’s cross-platform tool for T-SQL developers. It’s still relatively young, so it has some missing areas of functionality. When you run DMV queries to look at query plans, they show up as XML, which isn’t very friendly. For example, here I’m running sp_BlitzCache in my Azure SQL DB: It works, but…

Is Cost Threshold for Parallelism Measured in Seconds?

Last Updated October 5, 2018

Execution Plans

SQL Server automatically chooses when to divide your query’s work across multiple CPU cores. It makes that decision based on your query’s cost. To see it, let’s throw 1,000,000 tiny rows in a table: Transact-SQL CREATE TABLE dbo.Timeless(ID INT IDENTITY(1,1) PRIMARY KEY CLUSTERED, Stuffing VARCHAR(20)); INSERT INTO dbo.Timeless (Stuffing) SELECT TOP 1000000 'Stuff' FROM sys.all_columns…

Skewing Parallelism For Fun And Profit

Last Updated October 5, 2018

Execution Plans

What Is Skewed Parallelism? When queries go parallel, some assumptions get made: There’s enough work to keep multiple threads busy Each thread will get an equal amount of work to do The ‘equal amount of work’ part is particularly important, because in a parallel plan, each thread gets an equal share of memory up front.…

Using NOLOCK? Here’s How You’ll Get the Wrong Query Results.

Last Updated July 29, 2019

Locking, Blocking, and Isolation Levels

Slapping WITH (NOLOCK) on your query seems to make it go faster – but what’s the drawback? Let’s take a look. We’ll start with the free StackOverflow.com public database – any one of them will do, even the 10GB mini one – and run this query: Transact-SQL UPDATE dbo.Users SET WebsiteUrl = 'https://www.brentozar.com/'; 12 UPDATE…