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IndexClusteringFactorDeepDive

TonyHasler

UKOUGBirminghamDecember2021

AgendaActor'sreconstructionofareal-lifeperformanceproblemDiscussionofthe(in)accuracyofOracle'sclusteringfactorestimateandwhyitisimportantHowtoidentifyandcorrectclusteringfactorrelatedperformanceissuesACKNOWLEDGEMENT:

Chapter5of"CostBasedOracleFundamentals"byJonathanLewiswasmysourcefor99%oftheinformationinthistalk2TheCaseStudyAlargetablewasloadedwithadditionaldataoncepermonthandwasread-onlytherestofthetime

Atthetimeoftheissuethetableheldabouttwoyearsworthofdata

AcolumnonthetablecalledMONTH_DATEheldthedateofthefirstdayofthemonthoftheload

ThedatawasqueriedalongwithothertablesusingBusinessObjects3DEMO4TheconceptofindexclusteringSevenmoretableblocks10%Selectivitywithweakclustering10%SelectivitywithstrongclusteringSevenmoretableblocks5WhyClusteringFactorisimportantFulltablescansavoidaccesstotheindexstructuresFulltablescansusemulti-blockreadstothetabledatawhereasaccessviaanindexusessingleblockreadsEachblockisvisitedexactlyonceBUTIndexedaccessdoesn'tnecessarilyreadalltheblocksinthetableThepreviousexampleshowsthatselectivityisnotsufficienttodeterminewhethertheindexshouldbeused6CostofIndexedAccesstoaTableThefullformulais:cost=blevel+ceiling(leaf_blocks*effectiveindexselectivity)+ceiling(clustering_factor*effectiveindexselectivity)Thesimplifiedformula,ignoringcostoftraversingindexandassumingthatallpredicatesaremanagedbytheindex:cost=ceiling(clustering_factor*indexselectivity)7ClusteringFactorForaStronglyClusteredIndexIndexTableBlocks8ClusteringFactorForaWeaklyClusteredIndexIndexTableBlocks9HowtoInterprettheClusteringFactorofanIndexGenerallytheclusteringfactorwillbebetweenthenumberofblocksandthenumberofrowsinatableAlownumberisgoodandreflectsstrongclusteringAhighnumberisbadandreflectsweakclusteringTheClusteringFactormaybelowerthanthenumberofblocksifthereareemptyblocksinthetablebelowtheHWMand/ortherearemanyrowsthathavenullvaluesfortheindexedcolumn(s)TheClusteringFactorcanneverbegreaterthanthenumberofrowsinatableTheclusteringfactorforabitmapindexisjustacopyofthenumberofrowsandisnotused10DEMO11Whyisn'tmyindexbeingused?Anindexisunlikelytobeusediftheclusteringfactorisclosetothenumberofrowsunlesstheselectivityisverystrong(e.g.auniquescan)TheclusteringfactorofanindexwillnotchangeifyourebuildanindexTheclusteringfactorofanindexwillchangeifyourebuildthetablesothatthedataisphysicallyorderedbytheindexkeyYoucannothavealowclusteringfactorfortwoindependentindexesofthesametable(unlessyousetthembyhand)12ClusteringFactorKillers"Concurrent"insertswith:MultiplefreelistsMultiplefreelistgroups(RAC)ASSMReversekeyindexesInotherwordsstepstoreducecontentionalsoharmtheclusteringfactorButnoneofthisappliestoourtestcasesowhatwentwrong?13MultiColumnIndexesNon-uniquesinglecolumnindexesaresortedby:IndexedcolumnRowidAllindexentriesforthesamevalueoftheindexedcolumninthesameblockwillbeadjacentNon-uniquemulti-columnindexesaresortedby:LeadingindexedcolumnNextcolumn......RowidIndexentriesforthesamevalueoftheleadingindexedcolumninthesametableblockmaynotbeadjacentintheindexbutwillbeclose!14DEMO15TheBigTipLoweringtheclusteringfactorusingDBMS_STATS.SET_INDEX_STATSwillincreasethelikelihoodofyourindexbeingused

Onewaytoobtainadecentvalueistobuildanindexjustontheleadin

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