1. Plan cache performance tuning
Arthur Daniels

2. DBA-Art.com Twitter: @ArthurDanSQL
Arthur Daniels
DBA at SAP Fieldglass
Enjoys tuning SQL

3. Contents
First half:
What is the plan cache
Second half:
Demos

4. How execution plans are made
Someone sends a query to your SQL Server
Execution plan is created
SQL stores the plan in cache for reuse

5. Architecture of the plan cache
In the plan cache, every plan is a template
If a query matches, the template will be used!*
Match is determined based the query used to create the template, SET options such as SET ANSI NULLS ON/OFF, along with other requirements based on the type of compiled plan. Plan cache helps with plan re-use and stable performance

6. Compiled plans Adhoc Proc Prepared
Prepared and Proc are ideal for plan re-use. Adhoc will only re-use if the text is an exact match

7. DMVs/Ways to query the Plan Cache
Sys.dm_exec_cached_plans
Sys.dm_exec_sql_text
Sys.dm_exec_query_plan
Sys.dm_exec_query_stats

8. Query hash
SQL Handle
Query Plan Hash
Plan Handle

9. So why do we care? Performance! “Bad” templates in the cache
Finding implicit conversions and queries that need to be tuned

10. Intro to performance tools
SQL Query Stress
Created by Adam Machanic
Allows you to run queries/workloads

14. First, take a look at your cache
Pick any scripts to analyze your plan cache
but seriously, use sp_BlitzCache
Created by Jeremiah Peschka, free, open source

15. All plan caches have quirks
It’s likely all the top findings are bad.
Form an action plan ordered by what you perceive is the highest priority
Log sp_BlitzCache to a table during peak server usage to establish a baseline
That doesn’t mean it’s time to freak out.

16. Prep: Running sp_BlitzCache to a table
Tools used:
Sp_BlitzCache
SQL Query Stress
Run QueryStress-PostType-3 to create a basic load. Then, clear the cache, and run QueryStress-PostType-1 before moving onto Demo 2. This puts the bad plan in cache

17. Implicit Conversions – A Problem

18. Demo 2: What do bad queries look like?
Run bad queries

19. Intro to Bad Parameter Sniffing
SQL Server records parameters
Picks the best execution plan for just that set!

20. Setting up a parameter sniffing scenario
In Stack Overflow, Post Type = 1 are questions
Post Type = 3 are wiki
There’s 1,100,336 questions and 28 wiki

21. Demo 3: Parameter Sniffing
Use SQL Query Stress

22. Solutions to this Let’s fix it!
Manually DBCC FREEPROCCACHE(specific plan)
Scheduling a job to watch for the plan
By Google, licensed under Apache License 2.0 via GitHub

23. Demo 4: Timed clear of plan cache

24. New tool: the Query Store
Requires SQL Server 2016
Tools and Records
What Query Store can’t do

25. Demo 5: Solving the same problem but with Query Store

26. DBA-Art.com Search Plan Cache for warning signs
Bad templates can ruin performance
Force fix inside Query Store
Tools used today:
Query Store
Sp_BlitzCache
SQL Query Stress