Parameter Sniffing

In part 1 of this series, I showed you how to get the compiled parameter values for an execution plan that you are investigating. You’ve identified it as a bad execution plan. You’ve already fixed the production emergency by removing the bad plan from the plan cache. You’re almost ready to start testing for a…

Troubleshooting Parameter Sniffing Issues the Right Way: Part 1

March 6, 2018

Tara Kizer

Many of you are troubleshooting Parameter Sniffing issues the wrong way in production. In this three-part series, I’ll show you how to troubleshoot it the right way: Getting the Compiled Parameter Values Getting the App’s SET Options Avoiding the Density Vector This is part 1 in the series. Parameter Sniffing 101 When a query suddenly…

Adaptive Joins, Memory Grant Feedback, and Stored Procedures

February 28, 2018

Adaptive Joins, Execution Plans, Memory Grants, Parameter Sniffing

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Not Exactly The Catchiest Name There’s a TL;DR here, in case you don’t feel like reading the whole darn thing. Batch mode memory grant feedback works with stored procedures It takes several runs to adjust upwards to a final number It seems to only adjust downwards once (in this case by about 40%) This isn’t…

Memory Grants: SQL Server’s Other Public Toilet

February 15, 2018

Execution Plans, Indexing, Memory Grants, Parameter Sniffing

Sharing Is Caring When everything is going well, and queries are behaving responsibly, one need hardly think about memory grants. The problem becomes itself when queries start to over and under estimate their practical needs. Second Hand Emotion Queries ask for memory to do stuff. Memory is a shared resource. What kind of stuff? Well, usually…

SQL Server 2017 CU3 adds tempdb spill diagnostics in DMVs and Extended Events

January 11, 2018

Execution Plans, Memory Grants, Parameter Sniffing, SQL Server 2017

Where have you been all my life? Whenever I’m teaching people about query tuning, two things come up almost without fail: parameter sniffing and tempdb spills. They’re not mutually exclusive — some queries will seemingly always spill to tempdb, whether parameter sniffing is present of not, and some queries experiencing issues with parameter sniffing will never spill…

sp_BlitzQueryStore: A Gentle Introduction

December 7, 2017

Execution Plans, First Responder Kit, Parameter Sniffing, Query Store (QDS), sp_BlitzQueryStore, SQL Server 2016, SQL Server 2017

dba, development, performance tuning, sql server

Odds and ends During the precon, we asked how many people were on 2016, how many people were aware of query store, how many people were using it, and how many people were using sp_BlitzQueryStore. About 1/3 of the hands went up at first, then for each successive question, fewer and fewer hands stayed up.…

Which sp_configure Options Clear the Plan Cache?

September 27, 2017

When you change MAXDOP and run RECONFIGURE, SQL Server clears the plan cache. (It’s documented, by the way, as is the rest of the behavior I’m about to show you, but sometimes I like to learn the hard way by running experiments.) Here’s the easiest way to see it – but make sure to only…

ColumnStore Indexes: Rowgroup Elimination and Parameter Sniffing In Stored Procedures

August 23, 2017

Columnstore Indexes, Development, Execution Plans, Indexing, Parameter Sniffing, SQL Server, SQL Server 2017

Yazoo Over on his blog, fellow Query Plan aficionado Joe Obbish has a Great Post, Brent® about query patterns that qualify for Rowgroup Elimination. This is really important to performance! It allows scans to skip over stuff it doesn’t need, like skipping over the dialog in, uh… movies with really good fight scenes. Car chases?…

Inline Table Valued Functions: Parameter Snorting

February 8, 2017

Parameter Sniffing, SQL Server, T-SQL

dba, development, performance tuning, sql server

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…

Filtered Statistics Follow-up

December 22, 2016

Parameter Sniffing, SQL Server, Statistics

dba, development, performance tuning, sql server

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…

Using Plan Guides to Remove OPTIMIZE FOR UNKNOWN Hints

November 17, 2016

Say you’ve got an application that has tons of OPTIMIZE FOR UNKNOWN hints in the T-SQL, and you’re getting bad query plans. We’re going to use the same StackOverflow query (and the same index on Reputation) that I demoed in the post Why Is This Query Sometimes Fast and Sometimes Slow? This technique produces a…

Why Is This Query Sometimes Fast and Sometimes Slow?

November 16, 2016

You swear you didn’t change anything, but all of a sudden the SQL Server is going doggone slow. What happened? Parameter sniffing might be the problem, and to explain it, let’s see how it works. I’m going to use the StackOverflow database – particularly, the Users table that I demo in How to Think Like the…

Query Tuning Week: How to Run sp_BlitzCache on a Single Query

August 19, 2016

Execution Plans, Parameter Sniffing, sp_BlitzCache

The most popular way of using sp_BlitzCache® is to just run it – by default, it shows you the top 10 most CPU-intensive queries that have run on your server recently. Plus, it shows you warnings about each of the queries – if they’re missing indexes, experiencing parameter sniffing issues, running long, running frequently, doing…

How to Start Troubleshooting Parameter Sniffing Issues

August 16, 2016

When a query is sometimes fast and sometimes slow, for the same input parameters, and you swear nothing else in the environment is changing, that’s often a case of parameter sniffing. After SQL Server starts up and you run a stored procedure, SQL Server builds an execution plan for that proc based on the first set…

An Introduction to Query Memory

August 5, 2016

Execution Plans, Memory Grants, Parameter Sniffing, SQL Server

Microsoft has been quietly making some amazing improvements for performance tuners in SQL Server 2012, 2014, and 2016. This week, we’re going to introduce you to just how awesome they are. (They being the improvements, not Microsoft. You already knew they were awesome.) Using the freely available StackOverflow database, let’s start with a simple query…

Creating Tables and Stored Procedures in TempDB – Permanently

May 4, 2016

Bad Idea Jeans, Humor, Parameter Sniffing, SQL Server, TempDB

No, not #tables – actual tables. Here’s how: USE tempdb; GO /* This one is only available during my session: */ CREATE TABLE #myTempTable (ID INT IDENTITY(1,1), Stuffing VARCHAR(100)); GO /* This one is global, meaning it's available to other sessions: */ CREATE TABLE ##myTempTable (ID INT IDENTITY(1,1), Stuffing VARCHAR(100)); GO /* You can create…

Stats Week: Do Query Predicates Affect Histogram Step Creation?

April 20, 2016

development, performance tuning

Auto Create Statistics is your friend It’s not perfect, but 99% of the time I’d rather have imperfect statistics than no statistics. This question struck me as interesting, because the optimizer will totally sniff parameters to compile an initial plan. If you don’t have index statistics, or system statistics already on a column in a…

When does a Query Get Trivial Optimization?

June 4, 2015

Kendra Little