Fundamentals of Parameter Sniffing

Sometimes a query just goes slow for no apparent reason.

Created by Brent Ozar


Course Description

Sometimes the exact same query goes slow out of nowhere.

Your current fix is to update statistics, rebuild indexes, or restart SQL Server. It works, but you don’t know why.

You’re facing a problem called parameter sniffing: SQL Server “sniffs” the first set of parameters for a query, builds an execution plan for it, and then reuses that same plan no matter what parameters get called. It’s been the bane of our performance tuning for decades, and it doesn’t appear to be going away anytime soon – in fact, in SQL Server 2019, it even gets worse!

In this one-day class, you’ll follow along with me in the Stack Overflow database on your own laptop or desktop as you learn:

  • * What kinds of queries are vulnerable to parameter sniffing
  • * How to test your code to see if it’s vulnerable
  • * Why tactics like rebuilding indexes and updating statistics fix one query, but make others worse
  • * How to proactively patrol your server to uncover the problems before they bring you down

This course is 100% demos: the only slides are the introductions at the start of the day, and the recap at the end of the day. The rest of the time, we’ll be working in SQL Server Management Studio. Roll up your sleeves and join me!

Want to save a bundle? This class is included in my Recorded Class Season Pass: Fundamentals bundle.