SQL Server Query Optimization, Execution and Debugging Query Performance
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This document summarizes a presentation on SQL Server query optimization, execution, and debugging query performance. The key takeaways are: 1) estimated row counts affect plan selection because the optimizer uses cardinality estimates to determine query plan costs; 2) tools like SSMS, DMVs, and tracing can be used to find estimated and actual row counts and compare query plans; 3) parameterized queries can cause issues if the plan is reused for parameter values that expected different plans. Techniques for influencing plan choice and testing considerations for parameterized queries are also discussed.
![How is the plan cost determined?Demo – watch query plan costs for two similar queries…selectod.OrderID,od.ProductIDFROM [Order Details] odWHERE od.ProductID IN (9,15)SELECT od.OrderID,od.ProductIDFROM [Order Details] odWHERE od.ProductID IN (59,31)ConclusionsSQL Server is using cardinality estimates to derive cost of operatorsCardinality estimates are derived from the statistics](https://image.slidesharecdn.com/sqlserverqueryoptimizationexecutionanddebuggingqueryperformanceupload-100422225604-phpapp01/85/SQL-Server-Query-Optimization-Execution-and-Debugging-Query-Performance-8-320.jpg)
![Demo – Stored ProcedureCREATE PROCEDURE myp @ShipRegion NVARCHAR(15)asSELECT o.Orderid,c.CompanyName,od.QuantityFROM Orders o, [Order Details] od,Customers c WHERE o.Orderid=od.Orderid ANDo.CustomerID=c.CustomerID AND ((o.ShipRegion=@ShipRegion) OR (@ShipRegion is NULL) AND (o.ShipRegion is NULL))](https://image.slidesharecdn.com/sqlserverqueryoptimizationexecutionanddebuggingqueryperformanceupload-100422225604-phpapp01/85/SQL-Server-Query-Optimization-Execution-and-Debugging-Query-Performance-19-320.jpg)
SQL Server Query Optimization, Execution and Debugging Query Performance
- 2. SQL Server Query Optimization, Execution and Debugging Query PerformanceVinod Kumar MTechnology Evangelist – Microsoft@vinodk_sqlwww.ExtremeExperts.comhttp://blogs.sqlxml.org/vinodkumar
- 3. Session Objectives And TakeawaysTakeaways (Learn…): why estimated row counts affect plan selectionhow to find out “estimated” and “actual” row counts in the query planschallenges of parameterized queries how to influence query plan choicetips on design and testing of parameterized queries and stored proceduresbunch of useful demos you can try on your own
- 4. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 5. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 6. SQL Server Relational ServerHigh-Level Architecture Language Processing (Parse/Bind)Query Optimization (Plan Generation, View Matching, Statistics, Costing)Statement/Batch ExecutionQuery Execution(Query Operators, Memory Grants, Parallelism)Metadata, Type system, Expression ServicesUtilities (DBCC, Backup / Restore, BCP, …)Plan Cache ManagementStorage Engine (Access Methods, Database Page Cache, Locking, Transactions, …)SQL-OS (Schedulers, Buffer Pool, Memory Management, Synchronization Primitives, …)SQL Relational Engine
- 7. Query OptimizationQuery Optimization is cost based selection of good enough (potentially not the best) Query PlanInput to Query Optimization is Operator Tree produced byParsing SQL statement (Syntax)Semantic analysis and checking (Algebrizing – this includes type derivation, type checking, binding variables, table and column names, etc.)Output is Query Plan
- 8. How is the plan cost determined?Demo – watch query plan costs for two similar queries…selectod.OrderID,od.ProductIDFROM [Order Details] odWHERE od.ProductID IN (9,15)SELECT od.OrderID,od.ProductIDFROM [Order Details] odWHERE od.ProductID IN (59,31)ConclusionsSQL Server is using cardinality estimates to derive cost of operatorsCardinality estimates are derived from the statistics
- 9. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 10. Query Debugging ToolsSSMS – Management StudioShowplanDMVs – Dynamic Management ViewsTracePerfmon counters
- 11. Optimizer Perfmon CountersUse “SQL Server:SQL Statistics” object counters:
- 16. SQL Re-Compilations/sec TraceExecution events may be result of query plan selectionThe most useful execution event categories Errors and Warnings (e.g. Exchange Spill or Sort Warning)Performance (e.g. Plan Guide Successful or Performance Statistics)Query OptimizationErrors and Warnings – Missing Column Statistics and Missing Join PredicatePerformance - 8 different Showplan Events; the most useful areShowplan XML for Query Compile – with each compileShowplan XML Statistics Profile – with each execution
- 17. DMVssys.dm_exec_query_stats- returns performance statistics and SQL handle for each query plan in cachesys.dm_exec_sql_text- Returns the text of the sql statement based on the SQL handle sys.dm_exec_query_plan- Returns the showplan in XML format for a batch or module based on the SQL handlesys.dm_exec_plan_attributes– TVF returns one row per plan attribute for the plan specified by the plan handle.
- 18. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 19. Demo – Stored ProcedureCREATE PROCEDURE myp @ShipRegion NVARCHAR(15)asSELECT o.Orderid,c.CompanyName,od.QuantityFROM Orders o, [Order Details] od,Customers c WHERE o.Orderid=od.Orderid ANDo.CustomerID=c.CustomerID AND ((o.ShipRegion=@ShipRegion) OR (@ShipRegion is NULL) AND (o.ShipRegion is NULL))
- 20. Estimated and Actual Query Plan Actual Query Plan contains additional informationActual Number of RowsActual Rebinds and RewindsNumber of ExecutionsParameter values (compile time and runtime)Estimated rowcount is per single executionWhen comparing the estimated and actual rowcounts multiply the estimated rowcountby estimated number of executionsSometimes (rarely!) the shape of the Actual Query Plan may differ from the Estimated
- 21. How to Obtain Actual Plan?SSMS (as it was shown)SQLTRACEShowplan XML Statistics Profile eventSET STATISTICS XML ONReturns the Actual Plan in XML format following the resultSET STATISTICS PROFILE ONReturns text format of the query plan with two leading columns: ROWS and EXECUTESUsing DMVBut does not contain “actual” row counts or “actual parameter values”
- 22. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 23. Two Different Days…-- Day One – NULL query comes firstDBCC FREEPROCCACHEEXECUTE myp NULLEXECUTE myp 'BC'-- Day Two – a BC query comes firstDBCC FREEPROCCACHEEXECUTE myp 'BC'EXECUTE myp NULLObserve:Query plan is reused for all parameter values if found in the cacheIf used for “different” value the discrepancies between “estimated” and “actual” row counts are higher
- 24. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 25. One Testing ScenarioA story…Developer created parameterized stored procTester developed a driverTests were successful and sp moved to production After 4 weeks w/out any problems the sp now takes forever to finish and blocks the productionDisaster declared, all hands…A likely cause:The tests were run always with the same query plan created for the first set of parameter values
- 26. Test ResultsIf the plans are equal (watch for plan reuse! Run DBCC FREEPROCCACHE) you are ok (most likely)Else it still may be ok to use one of the generated plans for all parameter values (even those that would not pick the plan)Else you need to use more than one plan to get desirable performance…
- 27. Agenda IntroductionSQL Server OptimizerQuery Debugging ToolsEstimated and Actual Query PlanParameters and Query PlanTesting ConsiderationsHints and Other Workarounds
- 28. One Query Plan is Goodbut some parameter values may generate different planWorkarounds If optimize for “unknown” gives desired plan -> use OPTION (OPTIMIZE FOR UNKNOWN)If optimization for a specific value produces the plan -> use OPTION (OPTIMIZE FOR (@ShipRegion=‘BC’))Use Plan Guides to enforce the same planRewrite the sp and “hide” the passed in parameter (example follows)
- 29. Hiding Parameter ValueALTER PROCEDURE myp @ShipRegion NVARCHAR(15)asDECLARE @LocalShipRegion NVARCHAR (15)SET @LocalShipRegion=@ShipRegionSELECT o.Orderid,c.CompanyName,od.QuantityFROM Orders o, [Order Details] od,Customers c WHERE o.Orderid=od.Orderid ANDo.CustomerID=c.CustomerID AND ((o.ShipRegion=@LocalShipRegion) OR (@LocalShipRegionis NULL) and (o.ShipRegion is NULL))
- 30. Session TakeawaysTakeaways (We have learned…): why estimated row counts affect plan selection (cost)how to find out “estimated” and “actual” row counts in the query plans (tools)challenges of parameterized queries (query plan reuse)how to influence query plan choice(sp rewrite, hints, plan guides)tips on design and testing of parameterized queries and stored procedures (several)
- 31. Referenceshttp://blogs.msdn.com/sqltips/archive/2005/10/05/Top-N-costly-query-plans.aspxUmachandar’s blog “Find Top N costly query plans in adhoc batches or modules...” has some great DMV queries. You can track various times including cumulative time of many executions of the same statement or stored procedure. UC has example how to use the attributes in the XML showplan to correlate the plan costs with actual execution times. http://msdn.microsoft.com/en-us/library/dd672789.aspx Troubleshooting Performance Problems in SQL Server 2008 – white paper produced by MS developers and CSS. The paper is addressing most common performance problems as encountered by the CSS team and it is covering tools new in 2008.
- 32. © 2008 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.