Administração e Optimização de BDs 2º semestre

Transcription

1 DepartamentodeEngenhariaInformática 2008/2009 AdministraçãoeOptimizaçãodeBDs2ºsemestre AuladeLaboratório9 Inthislabclasswewillapproachthefollowingtopics: 1. IndextuningandSQLServer sdatabasetuningadvisor 2. Findingtherightindexesforyourworkload 3. HowtheDatabaseEngineTuningAdvisor(DTA)works 4. UsingthetheDatabaseEngineTuningAdvisor(DTA) 5. APracticalExerciseUsingtheDatabaseEngineTuningAdvisor(DTA) 6. ImplementingtheDatabaseEngineTuningAdvisor(DTA)recommendations 7. Otherresources 1. IndextuningandSQLServer sdatabasetuningadvisor Intermsoftuning,theoptionthatproducesmaximumgainswithleastimpactonexisting systems and processes is to examine your indexing strategy. However, the task of identifying the right indexes is not necessarily straightforward. It requires a sound knowledgeofthesortofqueriesthatwillberunagainstthedata,thedistributionofthat data,andthevolumeofdata,aswellasanunderstandingofwhattypeofindexwillsuit yourneedsbest.considerthefollowingquery: SelectA,COUNT(*)FROMTWHEREX<10GROUPBYA; Forthisqueryseveraldifferentphysicaldesignstructurescanreduceitsexecutioncost: (i)aclusteredindexon(x); (ii)tablerangepartitionedonx; (iii)anon clusteredindexwithkey(x)andincludingtheadditionalatríbutea; (iv)amaterializedviewthatmatchesthequery,andsoon. However,thesefeaturescanhavewidelyvaryingstorageandupdatecharacteristics.Thus in the presence of storage constraints or for a workload containing updates, making a global choice for a workload is difficult. For example, a clustered index on a table and horizontal partitioning of a table are both non redundant structures (incur negligible additional storage overhead) whereas non clustered indexes and materialized views can potentiallybestorageintensive(similarlytheirupdatecostscanbehigher).however,nonclustered indexes and materialized views can often be much more beneficial than a clusteredindexorahorizontallypartitionedtable.clearly,aphysicaldesigntoolthatcan giveanintegratedphysicaldesignrecommendationcangreatlyreduce/eliminatetheneed foradbatomakead hocdecisions. IST/DEI Pág.1de7

2 AdministraçãoeOptimizaçãodeBasesdeDados Whileunderstandingthebasicsisstillessential,SQLServer2008doesofferahelpinghand intheformofsomenewtools principallythedatabaseenginetuningadvisor thatcan help to determine, tune and monitor your indexes. In this class, we will see how to use thesetoolstogetanswerstothefollowingquestions: WhichindexesdoIneedformyqueries? HowdoImonitorindexusageandtheireffectiveness? HowdoIidentifyredundantindexesthatcouldnegativelyimpactperformanceof mydmloperations As the workload changes, how I do I identify the missing indexes that could enhanceperformanceformynewqueries? 2. Findingtherightindexesforyourworkload Determiningexactlytherightindexesforyoursystemcanbequiteataxingprocess.For example,youhavetoconsider: Which columns should be indexed, based on your knowledge of how the data is queried. Whethertochooseasingle columnindexoramultiplecolumníndex. Whetheryouneedaclusteredindexoranon clusteredíndex. Whether or not you could benefit from an index with included columns to avoid datapagelookups. How to utilize indexed(i.e., materialized) views, which the optimizer might access insteadoftheunderlyingtables. Moreover, once you've determined the prefect set of indexes, your job is not finished. Yourworkloadwillchangeovertime(newqueriesarewillbeadded,olderonesremoved from the application) and this might warrant revisiting existing indexes, analyzing their usage and making adjustments (modifying/dropping existing indexes or creating new ones).maintenanceofindexesiscriticaltoensuringoptimalperformanceinthelongrun. This lab class will focus on the usage of the Database Engine Tuning Advisor (DTA), a physical design tool providing an integrated"console" where DBAs can tune all physical designfeaturessupportedbytheserver. 3. HowtheDatabaseEngineTuningAdvisor(DTA)works TheDTAtakesintoaccountallaspectsofperformancethatthequeryoptimizercanmodel includingtheimpactofmultipleprocessors,amountofmemoryontheserver,andsoon. It is important to note, however, that query optimizers typically do not model all the aspects of query execution (e.g. impact of indexes on locking behavior, impact of data IST/DEI Pág.2de7

3 AdministraçãoeOptimizaçãodeBasesdeDados layout etc.). Thus, DTA s estimated improvement may be different from the actual improvementinexecutiontime. Takingasinputaworkloadtofine tune,i.e.asetofsqlstatementsthatexecuteagainst the database server, the DTA produces a set of physical design recommendations, consistingofindexes,materializedviews,andstrategiesforhorizontalrangepartitioning oftables,indexesandviews.thebasisofdta srecommendationsisa what if analysis provided by the SQL Server query optimizer, which allows the computation of an estimated cost as if a given configuration (i.e. the existence of some indexes) were materializedinthedatabase.similarlytotheactualevaluationofagivenqueryplan,the queryoptimizercomponentcandoanevaluationconsideringthe what if existenceofa givenphysicaldesignstructure. 4. UsingtheDatabaseEngineTuningAdvisor(DTA) The DTA is a new tool since SQL 2005, replacing the Index Tuning Wizard in earlier versionsofsqlserver.youcantuneasinglequeryortheentireworkloadtowhichyour server is subjected. A workload can be obtained by using SQL Server Profiler, a tool for logging events that execute on a server. Alternatively, a workload can be a SQL file containinganorganizationorindustrybenchmark.thedtacanalsotakeasinputeithera single or a set of databases, as many applications use more than one database simultaneously. Based on the options that you select, you can use the DTA to make recommendationsforseveralphysicaldesignstructures(pds),including: Clusteredindexes Non clusteredindexes Indexeswithincludedcolumns(toavoidbookmarklookups) Indexedviews Partitions ThefirststepistocollectaworkloadforDTAtoanalyze.Youcandothisoneoftwoways: UsingtheManagementStudio Ifyouneedtooptimizetheperformanceofasingle query, you can use Management Studio to provide an input to DTA. Simply type the queryinmanagementstudio,highlightitandthenrightclickonittochooseanalyzein DatabaseEngineTuningAdvisor.ThiswilllaunchDTAwithyourqueryastheinput. UsingtheProfiler Ifyouwanttodeterminetheoptimumindexsetfortheentire workload,youshouldcollectaprofilertraceusingthebuilt intuningtemplate. TofullyexploittheeffectivenessofDTA,youshouldalwaysusearepresentativeprofiler trace. For instance the indexes and partitioning considered by DTA are limited only to interesting column groups(i.e. those that appear in a large fraction of the workload by IST/DEI Pág.3de7

4 AdministraçãoeOptimizaçãodeBasesdeDados cost),inordertoimprovingscalabilitywithlittleimpactonquality.iftheprofilertraceis notrepresentativeofatrueworkload,importantquerieswilllikelybemissed. Youshouldmakesurethatyousubjectyourservertoallthequeriesthatwilltypicallybe runagainstthedata,whileyouarecollectingthetrace.thiscouldleadtoahugetracefile, butthatisnormal.ifyousimplycollectaprofilertraceovera5 10minuteperiod,youcan beprettysureitwon'tbetrulyrepresentativeofallthequeriesrunagainstyourdatabase. IntheSQLProfiler,theTUNINGtemplatecapturesonlyminimalevents,sothereshould not be any significant performance impact on your server. A technique for workload compression is also employed, particioning workloads with basis on the signature of eachquery(i.e.,twoquerieshavesamesignatureiftheyareidenticalinallaspectsexcept fortheconstantsreferencedinthequery). 8. APracticalExerciseUsingtheDatabaseEngineTuningAdvisor(DTA) As materials for this class, we've provided a profiler trace( Workload2Analyze.trc) that was captured with the TUNING template while running numerous queries ( Queries4Workload.sql ) against the AdventureWorks database. We would strongly recommendthatyouusethistrace,togetahands onperspectiveofdta. Having collected a truly representative profiler trace, you can use it as an input to DTA, which will then generate recommendations. You can perform one of the following two typesofanalysis. A.KeepmyexistingPhysicalDesignStructures(PDS)andtellmewhatelseIammissing Thistypeofanalysisiscommonandisusefulifyouhavepreviouslyestablishedthesetof indexesthatyoudeemtobemostusefulforyourgivenworkload,andareseekingfurther recommendations.toconductthisanalysis: 1. LaunchanewsessioninDTA. 2. Choosetheprofilertraceastheinputtothissession. 3. In the Select databases and tables to tune section select your target database (AdventureWorks). 4. In the Database for workload analysis dropdown, you can either use the same databaseortempdb. 5. AttheTuningOptionstab,selectthefollowingoptions: a. PhysicalDesignStructurestouseindatabase >IndexesandIndexedviews b. PhysicalDesignStructurestokeepindatabase >KeepallexistingPDS 6. Uncheckthecheckboxforlimittuningtime. 7. HitSTARTANALYSISandDTAwillstartconsumingyourworkload. IST/DEI Pág.4de7

5 AdministraçãoeOptimizaçãodeBasesdeDados OnceDTAisfinishedconsumingtheworkload,itwilllistitsrecommendationsunderthe Recommendationstab. B.IgnoremyexistingPhysicalDesignStructuresandtellmewhatqueryoptimizerneeds In scenario 1, DTA makes recommendations for any missing indexes. However, this doesn't necessarily mean your existing indexes are optimal for the query optimizer. You mayalsoconsiderconductingananalysiswherebydtaignoresallexistingphysicaldesign structures and recommends what it deems the best possible set of PDS for the given workload. In this way, you can validate your assumptions about what indexes are required. Toconductthisanalysis,followsteps1to6ofA,exceptthatatstep5b,choose"Donot keepanyexistingpds". Contrarytohowthismightsound,DTAwillnotactuallydropordeleteanyexistingPDSs. ThisisthebiggestadvantageofusingDTA,asitmeansyoucanusethetooltoperform what ifanalysiswithoutactuallyintroducinganychangestotheunderlyingschema. After consuming the workload, DTA presents, under the Recommendations tab, a set of recommendationsfortuningyourpds.agoodideaistofocusonthefollowingsections: Recommendation thisistheactionthatyouneedtotake.possiblevaluesinclude CreateorDrop. TargetofRecommendation thisistheproposednameofthepdstobecreated. ThenamingconventionistypicalofDTAandgenerallystartswith_dta*.However,I recommend that you change this name based on the naming convention in your database. Definition thisisthelistofcolumnsthatthisnewpdswillinclude.ifyouclickon thehyperlink,itwillopenupanewwindowwiththet SQLscripttoimplementthis recommendation. EstimatedImprovements thisistheestimatedpercentageimprovementthatyou canexpectinyourworkloadperformance,ifyouimplementalltherecommendations madebydta. Spaceusedbyrecommendation(MB) underthetuningsummarysectionofthe Reports tab, you can find out the extra space in MB that you would need, if you decidetoimplementtheserecommendations. Inbuiltanalysisreports Thereare15built inreportsunderthereportstab.however,thefollowingthreereports arethemostimportant. IST/DEI Pág.5de7

6 AdministraçãoeOptimizaçãodeBasesdeDados CurrentIndexUsageReport Startwiththisreporttoseehowyourexistingindexesare beingusedbythequeriesrunningagainstyourserver.eachindexthathasbeenusedbya queryislistedhere.eachreferencedindexhasapercentusagevaluewhichindicatesthe percentageofstatementsinyourworkloadthatreferencedthisindex.ifanindexisnot listedhere,itmeansthatithasnotbeenusedbyanyqueryinyourworkload.ifyouare certain that all the queries that run against your server have been captured by your profilertrace,thenyoucanusethisreporttoidentifyindexesthatarenotrequiredand possiblydeletethem. RecommendedIndexUsageReport Lookatthisreporttoidentifyhowindexusagewill changeiftherecommendedindexesareimplemented.ifyoucomparethesetworeports, you will see that the index usage of some of the current indexes has fallen while some new indexes have been included with a higher usage percentage, indicating a different executionplanforyourworkloadandimprovedperformance. StatementCostReport Thisreportlistsindividualstatementsinyourworkloadandthe estimated performance improvement for each one. Using this report, you can identify your poorly performing queries and see the sort of improvement you can expect if you implementtherecommendationsmadebydta.youwillfindthatsomestatementsdon't haveanyimprovements(percentimprovement=0).thisisbecauseeitherthestatement wasnottunedforsomereasonoritalreadyhasalltheindexesthatitneedstoperform optimally. 5. ImplementingtheDatabaseEngineTuningAdvisorRecommendations Bynow,wehavecollectedaworkloadusingProfiler,consumeditusingDTAandgotaset ofrecommendationstoimproveperformance.youthenhavethechoicetoeither: Save recommendations you can save the recommendations in an SQL script by navigating ACTIONS SAVE RECOMMENDATIONS. You can then manually run the scriptinmanagementstudiotocreatealltherecommendedpds. Apply recommendations using DTA if you are happy with the set of recommendationsthensimplynavigateactions APPLYRECOMMENDATIONS.You can also schedule a later time to apply these recommendations (during off peak hours,forexample) Performing what if analysis is a very useful feature of DTA. Suppose you don't want to apply all the recommendations that DTA provided. However, since the Estimated Improvement value can only be achieved if you apply all of these recommendations together,youarenotreallysurewhatkindofanimpactitwillhaveifyouonlychooseto applyasub setoftheserecommendations. IST/DEI Pág.6de7

7 AdministraçãoeOptimizaçãodeBasesdeDados To do a what if analysis, deselect the recommendations that you don't want to apply. Now,gotoACTIONS EVALUATERECOMMENDATIONS.Thiswilllaunchanothersession with the same options as the earlier one. However, now when you click on START ANALYSIS,DTAwillprovidedataonestimatedperformanceimprovements,basedonjust this sub set of the recommendations. Again, the key thing to remember is that DTA performsthis"what if"analysiswithoutactuallyimplementinganythinginthedatabase. 6. OtherResources SanjayAgrawal,SurajitChaudhuri,LuborKollar,ArunMarathe,VivekNarasayya, ManojSyamala(2004)DatabaseTuningAdvisorforMicrosoftSQLServer2005 o Usingthedatabasetuningadvisor o server 2005/sql server database tuning advisor.html IST/DEI Pág.7de7