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1、外文翻译原文TestingSQLServer’sQueryOptimizer:Challenges,TechniquesandExperiencesMaterialSource:IEEEDataEngineeringBulletinAuthor:LeoGiakoumakis,CesarGalindo-Legaria1IntroductionToday’squeryoptimizersprovidehighlysophisticatedfunctionalitythatisdesignedtoservealargevarietyofworkl
2、oads,datasizesandusagepatterns.Theyaretheresultofmanyyearsofresearchanddevelopment,whichhascomeatthecostofincreasedengineeringcomplexity,specificallyinvalidatingcorrectnessandmeasuringquality.Thereareseveraluniquecharacteristicsthatmakequeryoptimizersexceptionallycomplexsy
3、stemstovalidate,moresothanmostothersoftwaresystems.Queryoptimizershandleapracticallyinfiniteinputspaceofdeclarativedataqueries(e.g.SQL,XQuery),logical/physicalschemaanddata.Asimpleenumerationofallpossibleinputcombinationsisunfeasibleanditishardtopredictorextrapolateexpecte
4、dbehaviorbygroupingsimilarelementsoftheinputspaceintoequivalenceclasses.Thequeryoptimizationprocessitselfisofhighalgorithmiccomplexity,andreliesoninexactcostestimationmodels.Moreover,queryoptimizersoughttosatisfyworkloadsandusagescenarioswithavarietyofdifferentrequirements
5、andexpectations,e.g.tooptimizeforthroughoutorforresponsetime.Overtime,thenumberofexistingcustomersthatneedtobesupportedincreases,afactthatintroducesconstraintsinadvancingqueryoptimizationtechnologywithoutdisturbingexistingcustomerexpectations.Whilenewoptimizationsmayimprov
6、equeryperformancebyordersofmagnitudeforsomeworkloads,thesameoptimizationsmaycauseperformanceregressions(orunnecessaryoverhead)tootherworkloads.Forthosereasons,alargepartofthevalidationprocessofthequeryoptimizerismeanttoprovideanunderstandingofthedifferenttradeoffsanddesign
7、choicesinrespecttotheirimpactacrossdifferentcustomerscenarios.Atthesametime,thevalidationprocessneedstoprovideanassessmentofregressionriskforcodechangesthatmayhavealargeimpactacrossalargenumberofworkloadandquerytypes.2KeyChallengesThegoalofqueryoptimizationistoproduceeffic
8、ientexecutionstrategiesfordeclarativequeries.Thisinvolvestheselectionofanoptimalexecution