Before You Optimize: Understanding Execution Plans

@IAmTimCorey

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

Agenda

• Introduction to Execution Plans• Reading Execution Plans• Limitations• Additional Tools• Next Steps

What Are Execution Plans?

Basic Execution Plan

Types of Execution Plans

Estimated ActualGraphical Ctrl + L Ctrl + MText SHOWPLAN_ALL

SHOWPLAN_TEXTSTATISTICS PROFILE

XML SHOWPLAN_XML STATISTICS_XML

Estimated vs. Actual Details

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 cpCROSS APPLY sys.dm_exec_sql_text(cp.plan_handle) stCROSS APPLY sys.dm_exec_query_plan(cp.plan_handle)qp;

* From SQL Server Execution Plans by Grant Fritchey – p. 23

Execution Plan Aging

Formula: Cost to compile * # of callsLazywriter: scans cache and decrements by one each scanClear Cache: DBCC FREEPROCCACHE

Items are removed when:• Memory is needed• The age of the item is zero• The plan is not currently in use

How Do You Read an Execution Plan?

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 changesRewind – when the outer data remains the same

More info: http://bit.ly/P7EUv3

Limitations

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

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

Additional Tools

SQL Sentry Plan Explorer

http://bit.ly/NDwBqh

AutoEPLoader

http://bit.ly/StcFVu

SQL Server Profiler

How Do I Learn More?

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

For questions, comments, and further information, catch me on Twitter at

@IAmTimCorey or email me@timothycorey.com