You know what your query does, but do you know how it does it? Do you know what type of resources your query uses? This session covered these questions and more as we walked through reading execution plans. We saw how SQL breaks down the execution of your query and what each step tells us about the overall query. These slides provide the additional resources that go into the depth we couldn't get into in the session.

1. Before You Optimize:
Understanding Execution Plans
@IAmTimCorey

2. About Me
• Worked with Microsoft SQL since 6.5
• Started my career in VB6
• Moved to .NET when it came out
• Consultant, Adjunct Professor, Trainer, and
Speaker

3. Agenda
• Introduction to Execution Plans
• Reading Execution Plans
• Limitations
• Additional Tools
• Next Steps

4. What Are Execution Plans?

5. Basic Execution Plan

6. Types of Execution Plans
Estimated Actual
Graphical Ctrl + L Ctrl + M
Text SHOWPLAN_ALL STATISTICS PROFILE
SHOWPLAN_TEXT
XML SHOWPLAN_XML STATISTICS_XML

7. Estimated vs. Actual Details

8. List of Existing Plans
SELECT [cp].[refcounts]
,[cp].[usecounts]
,[cp].[objtype]
,[st].[dbid]
,[st].[objectid]
,[st].[text]
,[qp].[query_plan]
FROM sys.dm_exec_cached_plans cp
CROSS APPLY sys.dm_exec_sql_text(cp.plan_handle) st
CROSS APPLY sys.dm_exec_query_plan(cp.plan_handle)
qp;
* From SQL Server Execution Plans by Grant Fritchey – p. 23

9. Execution Plan Aging
Formula: Cost to compile * # of calls
Lazywriter: scans cache and decrements by one
each scan
Clear Cache: DBCC FREEPROCCACHE
Items are removed when:
• Memory is needed
• The age of the item is zero
• The plan is not currently in use

10. How Do You Read an Execution
Plan?

11. Rebind and Rewind
Occur on certain loop joins. They represent
each time the system has to look through the
inner table again for more data.
Rebind – when the outer data changes
Rewind – when the outer data remains the same
More info: http://bit.ly/P7EUv3

12. Limitations

13. Ad Hoc Query Plans
Ad hoc plans will use the cache as long as:
• The schema is specified throughout
• The text doesn’t change (at all*)
* For queries with one table only, simple
parameterization will allow the parameters to
change while still reusing the cached plan. To
expand this to more complex queries, see this
reference: http://bit.ly/Oi3FDH

14. Parameter Sniffing
SQL Server designs execution plans based upon the
first set of parameters passed in. This can have
adverse effects. To get around it, you can:
• Use WITH RECOMPILE
• Use tailored stored procs
• Set a trace flag to disable parameter sniffing
• Use the OPTIMIZE FOR query hint
• Use sp_recompile to clear cache for stored proc
or those that use a particular table
Read more here: http://bit.ly/MqNVOG

15. Additional Tools

16. SQL Sentry Plan Explorer
http://bit.ly/NDwBqh

17. AutoEPLoader
http://bit.ly/StcFVu

18. SQL Server Profiler

19. How Do I Learn More?

20. Further Resources
• Book by Grant Fritchey - http://bit.ly/MtL0a2
• Index Statistics - http://bit.ly/NTArHo
• Query Parallelism - http://bit.ly/NlcS0s
• Query Hints - http://bit.ly/N6YMKo

21. For questions, comments, and
further information, catch me on
Twitter at @IAmTimCorey or email
me@timothycorey.com