Bad Parameter Sniffing Decision Flow Chart

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016
Lots of people are confused by how to deal with bad parameter sniffing when it occurs. In an effort to help with this, I'm going to try to make a decision flow chart to walk you through the process. This is a rough, quite rough, first draft. I would love to hear any input. For this draft, I won't address the things I think I've left out. I want to see what you think of the decision flow and what you think might need to be included. Click on it to embiggen. Thanks to the attendees at my SQLSaturday Louisville pre-con for the great questions and the inspiration to get this done. Thank you in advance for any and all feedback.
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There Is a Magic Button, a Rant

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, TSQL
OK guys. I think it's way past time. A bunch of us have been keeping a secret from the rest of you. We know something that you don't. I don't think we should hide this secret from the world any more. Illuminati? Incompetents. Free Masons? I am one, so I already know all those secrets. Bilderbergers, Cthulhu Cultists, MKUltra, New World Order, Rotarians? All of these are nothing compared to the vast conspiracy that I'm about to reveal. We need to just unveil the magic "Run Really Fast" button. We've been keeping that sucker a secret forever. It's been tough. Every so often some unauthorized person almost finds it or a "query tuning expert" (as if that was a real thing) tries to reveal it. But we've kept it secret…
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Why You Should Change the Cost Threshold for Parallelism

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016
I've written several times about the Cost Threshold for Parallelism and it's relationship to your execution plans, how to determine your execution plan cost, and even how to decide what value to set your Cost Threshold to. What I haven't explicitly addressed in extremely clear terms is why you should adjust your Cost Threshold for Parallelism. There are two reasons to modify this value. Cost Threshold for Parallelism Default Value The primary reason to change the Cost Threshold for Parallelism is because the default value is not a good choice for the vast majority of systems. The default value is 5. This means that when a query has an estimated cost greater than 5, it may get a parallel execution plan. Microsoft set the default value for the Cost Threshold…
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Determining the Cost Threshold for Parallelism

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016
In the previous post, I showed how you can get full listings of your execution plan costs. Knowing what the values you're dealing with for the estimated costs on your execution plans can help you determine what the Cost Threshold on your system should be. However, we don't want to just take the average and use that. You need to understand the data you're looking at. Let's explore this just a little using R. Mean, Median, Range and Standard Deviation I've used the queries in the previous blog post to generate a full listing of costs for my plans. With that, I can start to query the information. Here's how I could use R to begin to explore the data: library("RODBC", lib.loc="~/R/win-library/3.2") query <- "SELECT * FROM dbo.QueryCost;" dbhandle <-…
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Estimated Costs of All Queries

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016
One question constantly comes up; What should the Cost Threshold for Parallelism be? The default value of 5 is pretty universally denigrated (well, not by Microsoft, but by most everyone else). However, what value should you set yours to? What Do Your Plans Cost? I have a question right back at you. What do your plans currently cost? Let's say, for argument's sake, that all your plans have an estimated cost (and all plan costs are estimates, let's please keep that in mind, even on Actual plans) value of 3 or less. Do you need to adjust the cost threshold in this case? Probably not. But the key is, how do you look at the costs for your plans? Unfortunately, there isn't a property in a DMV that shows this value. Instead,…
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Data About Execution Plans

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016
If you look at the Properties for the first operator of a graphical execution plan, you get all sorts of great information. I've talked about the data available there and how important it is in this older post. Checking out the properties of a plan you're working on is a fundamental part of tuning that plan. What happens when you don't know which plan you should be working on? What do you do, for example, if you want to see all the plans that are currently using ARITHABORT=FALSE or some other plan affecting setting? The "easy" answer to this question is to run an XQuery against the XML of the query plan itself. You can identify these properties and retrieve the appropriate values from within the plan. However, XQuery consumes quite a…
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Query Optimizer and Data Definition Language Queries

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, TSQL
Data Definition Language queries don't go through the optimizer, right? While normally, my short answer to this question in the past would have been, yes. However, with testing comes knowledge and I want to be sure about the statement. I'm working with a team of people to completely rewrite the SQL Server Execution Plans book. We'll probably be published in April or May. It's going to be amazing. The structure will be completely different and the book will represent five years of additional knowledge in how execution plans work and how to read and interpret them since the last book was written. However, enough on that. Let's answer the question about Data Definition Language. First of all, we need to quickly define our terms. Data Definition Language (DDL) represents the syntax for queries that build…
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OPTIMIZE FOR Hints When Parameter Sniffing is Turned Off

Azure, SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, TSQL
While presenting recently and talking about dealing with bad Parameter Sniffing, I got the question; what happens to OPTIMIZE FOR hints when parameter sniffing is disabled? This is my favorite kind of question because the answer is simple: I don't know. Parameter Sniffing For those who don't know, parameter sniffing is when SQL Server uses the precise values passed into a query as a parameter (this means stored procedures or prepared statements) to generate an execution plan from the statistics using the value from the parameter. Most of the time, parameter sniffing is either helping you, or is not hurting you. Sometimes, parameter sniffing turns bad and hurts you quite severely. Usually, but not always, this is because you either have severely skewed data (some data is very different than the rest, lots…
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Presentations for SQL Server Beginners

Azure, PASS, PowerShell, Professional Development, SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016
[caption id="attachment_2548" align="alignleft" width="300"] Tired from all those blog posts[/caption] For my final blog post in the #gettingstarted, #iwanttohelp series, I decided to curate a set of presentations from the PASS Virtual Chapters. This content is available online. It's free. Most importantly for the person just getting started as a SQL Server data pro, it's good. I'm going to marry each of the presentations with my eleven blog posts in this series. The Importance of a Full Backup in SQL Server For this one I'm going to recommend Tim Radney's session Understanding SQL Server Backup and Restore. I know Tim personally and guarantee this is a good session. Why Is The Server Slow Jes Borland is a very close personal friend and an absolutely outstanding presenter (and person). She has…
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PowerShell to Test a Query

DevOps, PowerShell, SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, TSQL
So you want to do some tuning, but you're not sure how to test a query on it's performance. Not a problem. Here's a very rough script that I use to do some recent testing. This script to test a query is post #11 of the #enterylevel #iwanttohelp effort started by Tim Ford (b|t). Read about it here. The Script The goal here is to load a bunch of parameter values from one table and then use those values to run a query to test it. To do this I connect up to my SQL Server instance, naturally. Then I retrieve the values I'm interested in. I set up the query I want to test. Finally a loop through the data set, calling the query once for each value. [reflection.assembly]::LoadWithPartialName("Microsoft.SqlServer.Smo") | out-null #…
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