Guiding Query Plans for optimized query performance Amit Bansal

CTOeDominer Systems P Ltd | Peopleware India

Who is Amit Bansal?

CTO, eDominer Systems & Peopleware IndiaConducted more than 200 workshops on SQL Server & BI for top notch IT companies world wideMicrosoft MVP for SQL ServerMicrosoft Certified Trainer Advisory Council memberSpeaker at TechED India, TechED US & TechED EuropeTechnical Reviewer – MSL courses on SQL ServerSME – SQL Server 2008 certificationsUG Lead (Delhi NCR) – CulminisManager – www.WeTogether.in, www.DelhiDevs.com, www.BlogBoard.in

Agenda & Session takeaways

Query Optimizer characteristicsUsing Query & Table HintsPlan Freezing conceptScenarios from the wildWhat are Plan Guides & different typesImplementing Plan GuidesManaging & Validating Plan GuidesPlan Guide considerationsSummary

Query Optimizer characteristics

Cost based optimizationDoes the Query Optimizer always do a fine job?

Does it always select the best query plan?Use Hints to affect the query plan selection

Use Hints as a last resort, Use with cautionTo be used only by experienced DBDs & DBAs

Is there really a problem?Check if there are other issuesIdentify the real cause of a poor planWhat if SQL Server fails to generate a valid plan?

(error 8622 is raised)

Join, Query & Table Hints

FAST N HintRECOMPILEOPTIMIZE FOR HintOPTIMIZE FOR UNKNOWN HintJOIN HintsFORCE ORDER HintINDEX HintFORCESEEK HintREADPAST HintUSE PLAN Hint

FunctionalityIndex HintsJoin HintsParallelismLockingCompilationTable Hints….

Plan Freezing concepts

Forcing / Locking down a query planProvides greater query performance stability & predictabilityEnables organizations to promote stable query plans

Hardware Server replacementsServer upgradesProduction Deployments

Plan Freezing canOptimize query performanceCopy query plans between serversPrevent the plan cache from growing excessively with large compiled plans that are not reused

Scenarios from the wild…

A query started mis-behaving after SQL Server upgradeQuery text cannot be modified to use hints

Eg: Application/vendor code, ISV, etcQuery performance degraded after hardware upgradeYou need to copy plans from one server to the otherOvercoming ‘Parameter Sniffing’ issuesAfter a service pack upgrade – “my query just starting running 4 times slower today and the plan is different from yesterday” ……sigh

What are Plan Guides?

Plan Freezing concept is implemented with Plan GuidesA DB object that associates a set of query hints with the queryCan be created using:

sp_create_plan_guidesp_create_plan_guide_from_handle

Fixed query plan (XML Showplan output) can be also be appliedAvailable in Standard, Enterprise, Evaluation & Developer edition

Types of Plan Guides

Object Plan Guidesmatches queries that execute in the context of a SQL Server object (SP, UDF, DML triggers etc)

SQL Plan Guidesmatches queries that execute in the context of stand-alone Transact-SQL statements and batches ( not part of a database object)

Template Plan Guidesmatches stand-alone queries that parameterize to a specified formTo override the current PARAMETERIZATION setting

Object Plan Guides

You are interested in a T-SQL statement appearing in the context of a SQL Server objectSQL Server object can be:

Stored ProcedureUser Defined Function (Scalar)Multi-statement table-valued user-defined functionsDML Triggers

Object Plan Guidessp_create_plan_guide @name = N'Guide1',@stmt = N'SELECT *FROM Sales.SalesOrderHeader AS h, Sales.Customer AS c, Sales.SalesTerritory AS t WHERE h.CustomerID = c.CustomerID AND c.TerritoryID = t.TerritoryID AND CountryRegionCode = @Country_region',@type = N'OBJECT',@module_or_batch = N'Sales.GetSalesOrderByCountry',@params = NULL,@hints = N'OPTION (OPTIMIZE FOR (@Country_region = N''US''))'

Implementing Object Plan GuidesNameTitleCompany

demo

SQL Plan Guides

You are interested in a particular T-SQL statementStand-aloneOr part of a batch

T-SQL statements sent by CLR objects, extended SPs & Dynamic SQL (EXEC (sql_string)) are processed as batchesThe text has to be exactly the same:

Case, whitespace, etc.

SQL Plan Guidessp_create_plan_guide @name = N'Guide2', @stmt = N'SELECT TOP 1 * FROM Sales.SalesOrderHeader ORDER BY OrderDate DESC',@type = N'SQL',@module_or_batch = NULL, @params = NULL, @hints = N'OPTION (MAXDOP 1)';

Implementing SQL Plan GuidesNameTitleCompany

demo

Template Plan Guides

To override PARAMETERIZATION database settingCan only use the following hints:

PARAMETERIZATION FORCEDPARAMETERIZATION SIMPLE

Little trickySQL Server has to construct a template of the query in the same format that it will be in once it is parameterizedSpecial stored procedure: sp_get_query_template@params is only used for TEMPLATE plan guide

Template Plan GuidesEXEC sp_create_plan_guide @name = N'TemplateGuide1', @stmt = N'SELECT * FROM AdventureWorks.Sales.SalesOrderHeader AS h INNER JOIN AdventureWorks.Sales.SalesOrderDetail AS d ON h.SalesOrderID = d.SalesOrderID WHERE h.SalesOrderID = @0', @type = N'TEMPLATE', @module_or_batch = NULL, @params = N'@0 int', @hints = N'OPTION(PARAMETERIZATION FORCED)';

Implementing Template Plan GuidesNameTitleCompany

demo

Applying a Fixed Query Plan to a Plan Guide

Useful when you know about an existing execution plan that performs better than the one selected by the optimizer for a particular queryCan be applied to OBJECT or SQL plan guidesExtract the XML Showplan from the cache and supply it to the sp_create_plan_guide

Applying a Fixed Query Plan to a Plan Guide

demo

Creating Plan Guides directly from the plan cache

Creates one or more plan guides from a query plan in the plan cacheCan be applied to OBJECT or SQL plan guidesExtract the plan handle from the cache and supply it to the sp_create_plan_guide_from_handle

Creating Plan Guides directly from the plan cache

demo

Managing & Validating Plan Guides

Managing Plan Guidessp_control_plan_guide

ENABLE, DISABLE, DROPValidating Plan guides

sys.fn_validate_plan_guide

Plan Guides considerations

The statement text in the plan guide must exactly match the query textTo verify that your plan guide was used, see for specific elements in XML Showplan output

PlanGuideDB & PlanGuideName (or)TemplatePlanGuideDB & TemplatePlanGuideNameQuery Plan (properties window)

Summary

There are many scenarios where you cannot use hintsPlan Guides come to rescueThree different types (OBJECT, SQL, TEMPLATE)SQL Server 2008 enhancements

Applying a fixed query to a plan guide using XML ShowplanCreating a plan directly from a plan in the plan cacheValidating Plan Guides

Plan Guide Considerations

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question & answer

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