basic materialized views

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8()BasicMaterializedViewsThischapterdescribestheuseofmaterializedviews.Itcontainsthefollowingtopics:

OverviewofDataWarehousingwithMaterializedViews

TypesofMaterializedViews

CreatingMaterializedViews

RegisteringExistingMaterializedViews

ChoosingIndexesforMaterializedViews

DroppingMaterializedViews

AnalyzingMaterializedViewCapabilities

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OverviewofDataWarehousingwithMaterializedViews()()()Typically,dataflowsfromoneormoreonlinetransactionprocessing(OLTP)databaseintoadatawarehouseonamonthly,weekly,ordailybasis.Thedataisnormallyprocessedina()stagingfile(glossary.htm#i997125)before

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beingaddedtothedatawarehouse.Datawarehousescommonlyrangeinsizefromtensofgigabytestoafewterabytes.Usually,thevastmajorityofthedataisstoredinafewverylargefacttables.

Onetechniqueemployedindatawarehousestoimproveperformanceisthecreationofsummaries.Summariesarespecialtypesofaggregateviewsthatimprovequeryexecutiontimesbyprecalculatingexpensivejoinsandaggregationoperationspriortoexecutionandstoringtheresultsinatableinthedatabase.Forexample,youcancreateasummarytabletocontainthesumsofsalesbyregionandbyproduct.

ThesummariesoraggregatesthatarereferredtointhisbookandinliteratureondatawarehousingarecreatedinOracleDatabaseusingaschemaobjectcalled()amaterializedview(glossary.htm#i997008).Materializedviewscanperformanumberofroles,suchasimprovingqueryperformanceorprovidingreplicateddata.

Inthepast,organizationsusingsummariesspentasignificantamountoftimeandeffortcreatingsummariesmanually,identifyingwhichsummariestocreate,indexingthesummaries,updatingthem,andadvisingtheirusersonwhichonestouse.Theintroductionofsummarymanagementeasedtheworkloadofthedatabaseadministratorandmeanttheusernolongerneededtobeawareofthesummariesthathadbeendefined.Thedatabaseadministratorcreatesoneormorematerializedviews,whicharetheequivalentofasummary.Theenduserqueriesthetablesandviewsatthedetaildatalevel.ThequeryrewritemechanismintheOracleserverautomaticallyrewritestheSQLquerytousethesummarytables.Thismechanismreducesresponsetimeforreturningresultsfromthequery.Materializedviewswithinthedatawarehousearetransparenttotheenduserortothedatabaseapplication.

Althoughmaterializedviewsareusuallyaccessedthroughthequeryrewritemechanism,anenduserordatabaseapplicationcanconstructqueriesthatdirectlyaccessthematerializedviews.However,seriousconsiderationshouldbegiventowhetherusersshouldbeallowedtodothisbecauseanychangetothematerializedviewswillaffectthequeriesthatreferencethem.

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MaterializedViewsforDataWarehousesIndatawarehouses,youcanusematerializedviewstoprecomputeandstoreaggregateddatasuchasthesumofsales.Materializedviewsintheseenvironmentsareoftenreferredtoassummaries,becausetheystoresummarizeddata.Theycanalsobeusedtoprecomputejoinswithorwithoutaggregations.Amaterializedvieweliminatestheoverheadassociatedwithexpensivejoinsandaggregationsforalargeorimportantclassofqueries.

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MaterializedViewsforDistributedComputing

Indistributedenvironments,youcanusematerializedviewstoreplicatedataatdistributedsitesandtosynchronizeupdatesdoneatthosesiteswithconflictresolutionmethods.Thesereplicamaterializedviewsprovidelocalaccesstodatathatotherwisewouldhavetobeaccessedfromremotesites.Materializedviewsarealsousefulinremotedatamarts.SeeOracleDatabaseAdvancedReplication(../../server.111/b28326/toc.htm)andOracleDatabaseHeterogeneousConnectivityAdministrator'sGuide(../../server.111/b28277/toc.htm)fordetailsondistributedandmobilecomputing.

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MaterializedViewsforMobileComputingYoucanalsousematerializedviewstodownloadasubsetofdatafromcentralserverstomobileclients,withperiodicrefreshesandupdatesbetweenclientsandthecentralservers.

Thischapterfocusesontheuseofmaterializedviewsindatawarehouses.SeeOracleDatabaseAdvancedReplication(../../server.111/b28326/toc.htm)andOracleDatabaseHeterogeneousConnectivityAdministrator'sGuide(../../server.111/b28277/toc.htm)fordetailsondistributedandmobilecomputing.

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TheNeedforMaterializedViewsYoucanusematerializedviewstoincreasethespeedofqueriesonverylargedatabases.Queriestolargedatabasesofteninvolvejoinsbetweentables,aggregationssuchasSUM ,orboth.Theseoperationsareexpensiveintermsoftimeandprocessingpower.Thetypeofmaterializedviewyoucreatedetermineshowthematerializedviewisrefreshedandusedbyqueryrewrite.

Materializedviewsimprovequeryperformancebyprecalculatingexpensivejoinandaggregationoperationsonthedatabasepriortoexecutionandstoringtheresultsinthedatabase.Thequeryoptimizerautomaticallyrecognizeswhenanexistingmaterializedviewcanandshouldbeusedtosatisfyarequest.Itthentransparentlyrewritestherequesttousethematerializedview.Queriesgodirectlytothematerializedviewandnottotheunderlyingdetailtables.Ingeneral,rewritingqueriestousematerializedviewsratherthandetailtablesimprovesresponsetime.Figure81illustrateshowqueryrewriteworks.

()Figure81TransparentQueryRewrite

Descriptionof"Figure81TransparentQueryRewrite"(./img_text/dwhsg027.htm)

Whenusingqueryrewrite,creatematerializedviewsthatsatisfythelargestnumberofqueries.Forexample,ifyouidentify20queriesthatarecommonlyappliedtothedetailorfacttables,thenyoumightbeabletosatisfythemwithfiveorsixwellwrittenmaterializedviews.Amaterializedviewdefinitioncanincludeanynumberofaggregations(SUM ,COUNT(x) ,COUNT(*) ,COUNT(DISTINCTx) ,AVG ,VARIANCE ,STDDEV ,MIN ,andMAX ).Itcanalsoincludeanynumberofjoins.Ifyouareunsureofwhichmaterializedviewstocreate,OracleprovidestheSQLAccessAdvisor,whichisasetofadvisoryprocedure()sintheDBMS_ADVISOR packagetohelpindesigningandevaluatingmaterializedviewsforqueryrewrite.

Ifamaterializedviewistobeusedbyqueryrewrite,itmustbestoredinthesamedatabaseasthedetailtablesonwhichitrelies.Amaterializedviewcanbepartitioned,andyoucandefineamaterializedviewonapartitionedtable.Youcanalsodefineoneormoreindexesonthematerializedview.

Unlikeindexes,materializedviewscanbeaccesseddirectlyusingaSELECTstatement.However,itisrecommendedthatyoutrytoavoidwritingSQLstatementsthatdirectlyreferencethematerializedview,becausethenitisdifficulttochangethemwithoutaffectingtheapplication.Instead,letqueryrewritetransparentlyrewriteyourquerytousethematerializedview.

Notethatthetechniquesshowninthischapterillustratehowtousematerializedviewsindatawarehouses.MaterializedviewscanalsobeusedbyOracleReplication.SeeOracleDatabaseAdvancedReplication(../../server.111/b28326/toc.htm)forfurtherinformation.

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ComponentsofSummaryManagementSummary()managementconsistsof:

TableofContents(toc.htm)OracleDatabase(toc.htm)

Preface(preface.htm)What'sNewinOracleDatabase?(whatsnew.htm)DataWarehousingConcepts(concept.htm)LogicalDesigninDataWarehouses(logical.htm)PhysicalDesigninDataWarehouses(physical.htm)HardwareandI/OConsiderationsinDataWarehouses(hardware.htm)

PartitioninginDataWarehouses(parpart.htm)Indexes(indexes.htm)IntegrityConstraints(constra.htm)BasicMaterializedViews(basicmv.htm)AdvancedMaterializedViews(advmv.htm)Dimensions(dimen.htm)OverviewofExtraction,Transformation,andLoading(ettover.htm)ExtractioninDataWarehouses(extract.htm)TransportationinDataWarehouses(transport.htm)LoadingandTransformation(transform.htm)MaintainingtheDataWarehouse(refresh.htm)ChangeDataCapture(cdc.htm)BasicQueryRewrite(qrbasic.htm)AdvancedQueryRewrite(qradv.htm)SchemaModelingTechniques(schemas.htm)SQLforAggregationinDataWarehouses(aggreg.htm)SQLforAnalysisandReporting(analysis.htm)SQLforModeling(sqlmodel.htm)OLAPandDataMining(bi.htm)AdvancedBusinessIntelligenceQueries(advbi.htm)UsingParallelExecution(usingpe.htm)Glossary(glossary.htm)

CategoriesHome(../../index.htm)BookList(../../nav/portal_booklist.htm)DataDictionary(../../nav/catalog_views.htm)SQLKeywords(../../nav/sql_keywords.htm)Acronyms(../../nav/acronym_list.htm)InitializationParameters(../../nav/initora.htm)ErrorMessages(../../nav/lookup.htm?id=ERRMG)

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Mechanismstodefinematerializedviewsanddimensions.

Arefreshmechanismtoensurethatallmaterializedviewscontainthelatestdata.

Aqueryrewritecapabilitytotransparentlyrewriteaquerytouseamaterializedview.

TheSQLAccessAdvisor,whichrecommendsmaterializedviews,partitions,andindexestocreate.

TUNE_MVIEW ,whichshowsyouhowtomakeyourmaterializedviewfastrefreshableandusegeneralqueryrewrite.

Theuseofsummarymanagementfeaturesimposesnoschemarestrictions,andcanenablesomeexistingDSSdatabaseapplicationstoimproveperformancewithouttheneedtoredesignthedatabaseortheapplication.

Figure82illustratestheuseofsummarymanagementinthewarehousingcycle.Afterthedatahasbeentransformed,staged,andloadedintothedetaildatainthewarehouse,youcaninvokethesummarymanagementprocess.First,usetheSQLAccessAdvisortoplanhowyouwillusematerializedviews.Then,creatematerializedviewsanddesignhowquerieswillberewritten.IfyouarehavingproblemstryingtogetyourmaterializedviewstoworkthenuseTUNE_MVIEW toobtainanoptimizedmaterializedview.

()Figure82OverviewofSummaryManagement

Descriptionof"Figure82OverviewofSummaryManagement"(./img_text/dwhsg071.htm)

Understandingthesummarymanagementprocessduringtheearlieststagesofdatawarehousedesigncanyieldlargedividendslaterintheformofhigherperformance,lowersummaryadministrationcosts,andreducedstoragerequirements.

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DataWarehousingTerminologySomebasicdatawarehousingtermsaredefinedasfollows:

()()()()Dimensiontablesdescribethebusinessentitiesofanenterprise,representedashierarchical,categoricalinformationsuchastime,departments,locations,andproducts.Dimensiontablesaresometimescalled()lookupor()referencetables.

Dimensiontablesusuallychangeslowlyovertimeandarenotmodifiedonaperiodicschedule.Theyareusedinlongrunningdecisionsupportqueriestoaggregatethedatareturnedfromthequeryintoappropriatelevelsofthedimensionhierarchy.

Hierarchiesdescribethebusinessrelationshipsandcommonaccesspatternsinthedatabase.Ananalysisofthedimensions,combinedwithanunderstandingofthetypicalworkload,canbeusedtocreatematerializedviews.SeeChapter10,"Dimensions"(dimen.htm#g1011059)formoreinformation.

()()()Facttablesdescribethebusinesstransactionsofanenterprise.

ThevastmajorityofdatainadatawarehouseisstoredinafewverylargefacttablesthatareupdatedperiodicallywithdatafromoneormoreoperationalOLTPdatabases.

Facttablesinclud()efacts(alsocalledmeasures)suchassales,units,andinventory.

Asimplemeasureisanumericorcharactercolumnofonetablesuchasfact.sales .

Acomputedmeasureisanexpressioninvolvingmeasuresofonetable,forexample,fact.revenues fact.expenses .

Amultitablemeasureisacomputedmeasuredefinedonmultipletables,forexample,fact_a.revenues fact_b.expenses .

Facttablesalsocontainoneormoreforeign()keysthatorganizethebusinesstransactionsbytherelevantbusinessentitiessuchastime,product,andmarket.Inmostcases,theseforeignkeysarenonnull,formauniquecompoundkeyofthefacttable,andeachforeignkeyjoinswithexactlyonerowofa()dimensiontable(glossary.htm#i996883).

Amaterializedviewisaprecomputedtablecomprisingaggregatedandjoineddatafromfactandpossiblyfromdimensiontables.Amongbuildersofdatawarehouses,amaterializedviewisalsoknownasasummary(glossary.htm#i997158).

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()()MaterializedViewSchemaDesign

Summarymanagement()canperformmanyusefulfunctions,includingqueryrewriteandmaterializedviewrefresh,evenifyourdatawarehousedesigndoesnotfollowtheseguidelines.However,youwillrealizesignificantlygreaterqueryexecutionperformanceandmaterializedviewrefreshperformancebenefitsandyouwillrequirefewermaterializedviewsifyourschemadesigncomplieswiththeseguidelines.

Amaterializedviewdefinitionincludesanynumberofaggregates,aswellasanynumberofjoins.Inseveralways,amaterializedviewbehaveslikeanindex:

Thepurposeofamaterializedviewistoincreasequeryexecutionperformance.

TheexistenceofamaterializedviewistransparenttoSQLapplications,sothatadatabaseadministratorcancreateordropmaterializedviewsatanytimewithoutaffectingthevalidityofSQLapplications.

Amaterializedviewconsumesstoragespace.

Thecontentsofthematerializedviewmustbeupdatedwhentheunderlyingdetailtablesaremodified.

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SchemasandDimensionTablesInthecaseofnormalizedorpartiallynormalizeddimensiontables(adimensionthatisstoredinmorethanonetable),identifyhowthesetablesarejoined.Notewhetherthejoinsbetweenthedimensiontablescanguaranteethateachchildsiderowjoinswithoneandonlyoneparentsiderow.Inthecaseofdenormalizeddimensions,determinewhetherthechildsidecolumnsuniquelydeterminetheparentside(orattribute)columns.Theserelationshipscanbeenabledwithconstraints,usingtheNOVALIDATE andRELY optionsiftherelationshipsrepresentedbytheconstraintsareguaranteedbyothermeans.Notethatifthejoinsbetweenfactanddimensiontablesdonotsupporttheparentchildrelationshipdescribedpreviously,youstillgainsignificantperformanceadvantagesfromdefiningthedimensionwiththeCREATEDIMENSION statement.Anotheralternative,subjecttosomerestrictions,istouseouterjoinsinthematerializedviewdefinition(thatis,intheCREATEMATERIALIZED VIEW statement).

Youmustnotcreatedimensionsinanyschemathatdoesnotsatisfytheserelationships.Incorrectresultscanbereturnedfromqueriesotherwise.

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MaterializedViewSchemaDesignGuidelinesBeforestartingtodefineandusethevariouscomponentsofsummarymanagement,youshouldreviewyourschemadesigntoabidebythefollowingguidelineswhereverpossible.

Guidelines1and2aremoreimportantthanguideline3.Ifyourschemadesigndoesnotfollowguidelines1and2,itdoesnotthenmatterwhetheritfollowsguideline3.Guidelines1,2,and3affectbothqueryrewriteperformanceandmaterializedviewrefreshperformance.

()()Table81SchemaDesignGuidelines

SchemaGuideline Description

Guideline1

Dimensions

Dimensionsshouldeitherbedenormalized(eachdimensioncontainedinonetable)orthejoinsbetweentablesinanormalizedorpartiallynormalizeddimensionshouldguaranteethateachchildsiderowjoinswithexactlyoneparentsiderow.Thebenefitsofmaintainingthisconditionaredescribedin"CreatingDimensions"(dimen.htm#i1006266).

YoucanenforcethisconditionbyaddingFOREIGN KEY andNOTNULL constraintsonthechildsidejoinkeysandPRIMARY KEY constraintsontheparentsidejoinkeys.

Guideline2

Dimensions

Ifdimensionsaredenormalizedorpartiallydenormalized,hierarchicalintegritymustbemaintainedbetweenthekeycolumnsofthedimensiontable.Eachchildkeyvaluemustuniquelyidentifyitsparentkeyvalue,evenifthedimensiontableisdenormalized.HierarchicalintegrityinadenormalizeddimensioncanbeverifiedbycallingtheVALIDATE_DIMENSION procedureoftheDBMS_DIMENSIONpackage.

Guideline3

Dimensions

Factanddimensiontablesshouldsimilarlyguaranteethateachfacttablerowjoinswithexactlyonedimensiontablerow.Thisconditionmustbedeclared,andoptionallyenforced,byaddingFOREIGN KEY andNOT NULLconstraintsonthefactkeycolumn(s)andPRIMARY KEYconstraintsonthedimensionkeycolumn(s),orbyusingouterjoins.Inadatawarehouse,constraintsaretypicallyenabledwiththeNOVALIDATE andRELY clausestoavoidconstraintenforcementperformanceoverhead.SeeOracleDatabaseSQLLanguageReference(../../server.111/b28286/toc.htm)forfurtherdetails.

Guideline4

IncrementalLoads

IncrementalloadsofyourdetaildatashouldbedoneusingtheSQL*Loaderdirectpathoption,oranybulkloaderutilitythatusesOracle'sdirectpathinterface.ThisincludesINSERT ...ASSELECT withtheAPPEND orPARALLELhints,wherethehintscausethedirectloaderlogtobeusedduringtheinsert.SeeOracleDatabaseSQLLanguageReference(../../server.111/b28286/toc.htm)and"TypesofMaterializedViews"formoreinformation.

Guideline5

Partitions

Range/compositepartitionyourtablesbyamonotonicallyincreasingtimecolumnifpossible(preferablyoftypeDATE ).

Guideline6

Dimensions

Aftereachloadandbeforerefreshingyourmaterializedview,usetheVALIDATE_DIMENSION procedureoftheDBMS_DIMENSION packagetoincrementallyverifydimensionalintegrity.

Guideline7

TimeDimensions

Ifatimedimensionappearsinthematerializedviewasatimecolumn,partitionandindexthematerializedviewinthesamemannerasyouhavethefacttables.

Ifyouareconcernedwiththetimerequiredtoenableconstraintsandwhetheranyconstraintsmightbeviolated,thenusetheENABLE NOVALIDATE withtheRELY clausetoturnonconstraintcheckingwithoutvalidatinganyoftheexistingconstraints.Theriskwiththisapproachisthatincorrectqueryresultscouldoccurifanyconstraintsarebroken.Therefore,asthedesigner,youmustdeterminehowcleanthedataisandwhethertheriskofincorrectresultsistoogreat.

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LoadingDataintoDataWarehousesApopularandefficientwaytoloaddataintoadatawarehouseordatamartistouseSQL*LoaderwiththeDIRECT orPARALLEL option,DataPump,ortouseanotherloadertoolthatusestheOracledirectpathAPI.SeeOracleDatabaseUtilities(../../server.111/b28319/ldr_concepts.htm#SUTIL003)fortherestrictionsandconsiderationswhenusingSQL*LoaderwiththeDIRECT orPARALLELkeywords.

Loadingstrategiescanbeclassifiedasonephaseortwophase.Inonephaseloading,dataisloadeddirectlyintothetargettable,qualityassurancetestsareperformed,anderrorsareresolvedbyperformingDMLoperationspriortorefreshingmaterializedviews.Ifalargenumberofdeletionsarepossible,thenstorageutilizationcanbeadverselyaffected,buttemporaryspacerequirementsandloadtimeareminimized.

Inatwophaseloadingprocess:

Dataisfirstloadedintoatemporarytableinthewarehouse.

Qualityassuranceproceduresareappliedtothedata.

Referentialintegrityconstraintsonthetargettablearedisabled,andthelocalindexinthetargetpartitionismarkedunusable.

ThedataiscopiedfromthetemporaryareaintotheappropriatepartitionofthetargettableusingINSERT AS SELECT withthePARALLEL orAPPENDhint.Thetemporarytableisthendropped.Alternatively,ifthetargettableispartitioned,youcancreateanew(empty)partitioninthetargettableanduseALTERTABLE...EXCHANGEPARTITION toincorporatethetemporarytableintothetargettable.SeeOracleDatabaseSQLLanguageReference(../../server.111/b28286/toc.htm)formoreinformation.

Theconstraintsareenabled,usuallywiththeNOVALIDATE option.

Immediatelyafterloadingthedetaildataandupdatingtheindexesonthedetaildata,thedatabasecanbeopenedforoperation,ifdesired.YoucandisablequeryrewriteatthesystemlevelbyissuinganALTER SYSTEM SETQUERY_REWRITE_ENABLED =FALSE statementuntilallthematerializedviewsarerefreshed.

IfQUERY_REWRITE_INTEGRITY issettoSTALE_TOLERATED ,accesstothematerializedviewcanbeallowedatthesessionleveltoanyuserswhodonotrequirethematerializedviewstoreflectthedatafromthelatestloadbyissuinganALTER SESSION SET QUERY_REWRITE_ENABLED =TRUE statement.ThisscenariodoesnotapplywhenQUERY_REWRITE_INTEGRITY iseitherENFORCED orTRUSTED becausethesystemensuresinthesemodesthatonlymaterializedviewswithupdateddataparticipateinaqueryrewrite.

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OverviewofMaterializedViewManagementTasks

Themotivationforusingmaterializedviewsistoimproveperformance,buttheoverheadassociatedwithmaterializedviewmanagementcanbecomeasignificantsystemmanagementproblem.Whenreviewingorevaluatingsomeofthenecessarymaterializedviewmanagementactivities,considersomeofthefollowing:

Identifyingwhatmaterializedviewstocreateinitially.

Indexingthematerializedviews.

Ensuringthatallmaterializedviewsandmaterializedviewindexesarerefreshedproperlyeachtimethedatabaseisupdated.

Checkingwhichmaterializedviewshavebeenused.

Determininghoweffectiveeachmaterializedviewhasbeenonworkloadperformance.

Measuringthespacebeingusedbymaterializedviews.

Determiningwhichnewmaterializedviewsshouldbecreated.

Determiningwhichexistingmaterializedviewsshouldbedropped.

Archivingolddetailandmaterializedviewdatathatisnolongeruseful.

Aftertheinitialeffortofcreatingandpopulatingthedatawarehouseordatamart,themajoradministrationoverheadistheupdateprocess,whichinvolves:

Periodicextractionofincrementalchangesfromtheoperationalsystems.

Transformingthedata.

Verifyingthattheincrementalchangesarecorrect,consistent,andcomplete.

Bulkloadingthedataintothewarehouse.

Refreshingindexesandmaterializedviewssothattheyareconsistentwiththedetaildata.

Theupdateprocessmustgenerallybeperformedwithinalimitedperiodoftimeknownasthe()updatewindow(glossary.htm#i997213).Theupdatewindowdependsonthe()updatefrequency(glossary.htm#i997216)(suchasdailyorweekly)andthenatureofthebusiness.Foradailyupdatefrequency,anupdatewindowoftwotosixhoursmightbetypical.

Youneedtoknowyourupdatewindowforthefollowingactivities:

Loadingthedetaildata

Updatingorrebuildingtheindexesonthedetaildata

Performingqualityassurancetestsonthedata

Refreshingthematerializedviews

Updatingtheindexesonthematerializedviews

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TypesofMaterializedViews()TheSELECT clauseinthematerializedviewcreationstatementdefinesthedatathatthematerializedviewistocontain.Onlyafewrestrictionslimitwhatcanbespecified.Anynumberoftablescanbejoinedtogether.Besidestables,otherelementssuchasviews,inlineviews(subqueriesintheFROM clauseofaSELECT statement),subqueries,andmaterializedviewscanallbejoinedorreferencedintheSELECT clause.Youcannot,however,defineamaterializedviewwithasubqueryintheSELECT listofthedefiningquery.Youcan,however,includesubquerieselsewhereinthedefiningquery,suchasintheWHERE clause.

Thetypesofmaterializedviewsare:

MaterializedViewswithAggregates

MaterializedViewsContainingOnlyJoins

NestedMaterializedViews

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MaterializedViewswith()AggregatesIn()datawarehouses,materializedviewsnormallycontainaggregatesasshowninExample81.Forfastrefreshtobepossible,theSELECT listmustcontainalloftheGROUP BY columns(ifpresent),andtheremustbea

COUNT(*) andaCOUNT(column) onanyaggregatedcolumns.Also,materializedviewlogsmustbepresentonalltablesreferencedinthequerythatdefinesthematerializedview.Thevalidaggregatefunctionsare:SUM ,COUNT(x) ,COUNT(*) ,AVG ,VARIANCE ,STDDEV ,MIN ,andMAX ,andtheexpressiontobeaggregatedcanbeanySQLvalueexpression.See"RestrictionsonFastRefreshonMaterializedViewswithAggregates".

FastrefreshforamaterializedviewcontainingjoinsandaggregatesispossibleafteranytypeofDMLtothebasetables(directloadorconventionalINSERT ,UPDATE ,orDELETE ).ItcanbedefinedtoberefreshedON COMMIT orONDEMAND .AREFRESHON COMMIT materializedviewwillberefreshedautomaticallywhenatransactionthatdoesDMLtooneofthematerializedview'sdetailtablescommits.Thetimetakentocompletethecommitmaybeslightlylongerthanusualwhenthismethodischosen.Thisisbecausetherefreshoperationisperformedaspartofthecommitprocess.Therefore,thismethodmaynotbesuitableifmanyusersareconcurrentlychangingthetablesuponwhichthematerializedviewisbased.

Herearesomeexamplesofmaterializedviewswithaggregates.Notethatmaterializedviewlogsareonlycreatedbecausethismaterializedviewwillbefastrefreshed.

()Example81Example1:CreatingaMaterializedView

CREATEMATERIALIZEDVIEWLOGONproductsWITHSEQUENCE,ROWID(prod_id,prod_name,prod_desc,prod_subcategory,prod_subcategory_desc,prod_category,prod_category_desc,prod_weight_class,prod_unit_of_measure,prod_pack_size,supplier_id,prod_status,prod_list_price,prod_min_price)INCLUDINGNEWVALUES;

CREATEMATERIALIZEDVIEWLOGONsalesWITHSEQUENCE,ROWID(prod_id,cust_id,time_id,channel_id,promo_id,quantity_sold,amount_sold)INCLUDINGNEWVALUES;

CREATEMATERIALIZEDVIEWproduct_sales_mvPCTFREE0TABLESPACEdemoSTORAGE(INITIAL8kNEXT8kPCTINCREASE0)BUILDIMMEDIATEREFRESHFASTENABLEQUERYREWRITEASSELECTp.prod_name,SUM(s.amount_sold)ASdollar_sales,COUNT(*)AScnt,COUNT(s.amount_sold)AScnt_amtFROMsaless,productspWHEREs.prod_id=p.prod_idGROUPBYp.prod_name;

Thisexamplecreatesamaterializedviewproduct_sales_mv thatcomputestotalnumberandvalueofsalesforaproduct.Itisderivedbyjoiningthetablessales andproducts onthecolumnprod_id .Thematerializedviewispopulatedwithdataimmediatelybecausethebuildmethodisimmediateanditisavailableforusebyqueryrewrite.Inthisexample,thedefaultrefreshmethodisFAST ,whichisallowedbecausetheappropriatematerializedviewlogshavebeencreatedontablesproducts andsales .

()Example82Example2:CreatingaMaterializedView

CREATEMATERIALIZEDVIEWproduct_sales_mvPCTFREE0TABLESPACEdemoSTORAGE(INITIAL16kNEXT16kPCTINCREASE0)BUILDDEFERREDREFRESHCOMPLETEONDEMANDENABLEQUERYREWRITEASSELECTp.prod_name,SUM(s.amount_sold)ASdollar_salesFROMsaless,productspWHEREs.prod_id=p.prod_idGROUPBYp.prod_name;

Thisexamplecreatesamaterializedviewproduct_sales_mv thatcomputesthesumofsalesbyprod_name .Itisderivedbyjoiningthetablessales andproducts onthecolumnprod_id .Thematerializedviewdoesnotinitiallycontainanydata,becausethebuildmethodisDEFERRED .Acompleterefreshisrequiredforthefirstrefreshofabuilddeferredmaterializedview.Whenitisrefreshedandoncepopulated,thismaterializedviewcanbeusedbyqueryrewrite.

()Example83Example3:CreatingaMaterializedView

CREATEMATERIALIZEDVIEWLOGONsalesWITHSEQUENCE,ROWID(prod_id,cust_id,time_id,channel_id,promo_id,quantity_sold,amount_sold)INCLUDINGNEWVALUES;

CREATEMATERIALIZEDVIEWsum_salesPARALLELBUILDIMMEDIATEREFRESHFASTONCOMMITASSELECTs.prod_id,s.time_id,COUNT(*)AScount_grp,SUM(s.amount_sold)ASsum_dollar_sales,COUNT(s.amount_sold)AScount_dollar_sales,SUM(s.quantity_sold)ASsum_quantity_sales,COUNT(s.quantity_sold)AScount_quantity_salesFROMsalessGROUPBYs.prod_id,s.time_id;

Thisexamplecreatesamaterializedviewthatcontainsaggregatesonasingletable.Becausethematerializedviewloghasbeencreatedwithallreferencedcolumnsinthematerializedview'sdefiningquery,thematerializedviewisfastrefreshable.IfDMLisappliedagainstthesales table,thenthechangeswillbereflectedinthematerializedviewwhenthecommitisissued.

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RequirementsforUsingMaterializedViewswithAggregatesTable82illustratestheaggregaterequirementsformaterializedviews.IfaggregateX ispresent,aggregateY isrequiredandaggregateZ isoptional.

()()Table82RequirementsforMaterializedViewswithAggregates

X Y Z

COUNT(expr) - -

MIN(expr)

MAX(expr)

SUM(expr) COUNT(expr) -

SUM(col) ,colhasNOT NULLconstraint

AVG(expr) COUNT(expr) SUM(expr)

STDDEV(expr) COUNT(expr)SUM(expr) SUM(expr*expr)

VARIANCE(expr) COUNT(expr)SUM(expr) SUM(expr*expr)

NotethatCOUNT(*) mustalwaysbepresenttoguaranteealltypesoffastrefresh.Otherwise,youmaybelimitedtofastrefreshafterinsertsonly.OraclerecommendsthatyouincludetheoptionalaggregatesincolumnZ inthematerializedviewinordertoobtainthemostefficientandaccuratefastrefreshoftheaggregates.

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MaterializedViews()ContainingOnlyJoinsSomematerializedviewscontainonlyjoinsandnoaggregates,suchasinExample84,whereamaterializedviewiscreatedthatjoinsthesales tabletothetimes andcustomers tables.Theadvantageofcreatingthistypeofmaterializedviewisthatexpensivejoinswillbeprecalculated.

FastrefreshforamaterializedviewcontainingonlyjoinsispossibleafteranytypeofDMLtothebasetables(directpathorconventionalINSERT ,UPDATE ,orDELETE ).

AmaterializedviewcontainingonlyjoinscanbedefinedtoberefreshedONCOMMIT orON DEMAND .IfitisON COMMIT ,therefreshisperformedatcommittimeofthetransactionthatdoesDMLonthematerializedview'sdetailtable.

IfyouspecifyREFRESH FAST ,Oracleperformsfurtherverificationofthequerydefinitiontoensurethatfastrefreshcanbeperformedifanyofthedetailtableschange.Theseadditionalchecksare:

AmaterializedviewlogmustbepresentforeachdetailtableunlessthetablesupportsPCT.Also,whenamaterializedviewlogisrequired,theROWIDcolumnmustbepresentineachmaterializedviewlog.

TherowidsofallthedetailtablesmustappearintheSELECT listofthematerializedviewquerydefinition.

Ifsomeoftheserestrictionsarenotmet,youcancreatethematerializedviewasREFRESH FORCE totakeadvantageoffastrefreshwhenitispossible.Ifoneofthetablesdidnotmeetallofthecriteria,buttheothertablesdid,thematerializedviewwouldstillbefastrefreshablewithrespecttotheothertablesforwhichallthecriteriaaremet.

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MaterializedJoinViewsFROMClauseConsiderations

Ifthematerializedviewcontainsonlyjoins,theROWID columnsforeachtable(andeachinstanceofatablethatoccursmultipletimesintheFROM list)mustbepresentintheSELECT listofthematerializedview.

IfthematerializedviewhasremotetablesintheFROM clause,alltablesintheFROM clausemustbelocatedonthatsamesite.Further,ON COMMIT refreshisnotsupportedformaterializedviewwithremotetables.Materializedviewlogs

mustbepresentontheremotesiteforeachdetailtableofthematerializedviewandROWID columnsmustbepresentintheSELECT listofthematerializedview,asshowninthefollowingexample.

()Example84MaterializedViewContainingOnlyJoins

CREATEMATERIALIZEDVIEWLOGONsalesWITHROWID;CREATEMATERIALIZEDVIEWLOGONtimesWITHROWID;CREATEMATERIALIZEDVIEWLOGONcustomersWITHROWID;CREATEMATERIALIZEDVIEWdetail_sales_mvPARALLELBUILDIMMEDIATEREFRESHFASTASSELECTs.rowid"sales_rid",t.rowid"times_rid",c.rowid"customers_rid",c.cust_id,c.cust_last_name,s.amount_sold,s.quantity_sold,s.time_idFROMsaless,timest,customerscWHEREs.cust_id=c.cust_id(+)ANDs.time_id=t.time_id(+);

Alternatively,ifthepreviousexampledidnotincludethecolumnstimes_ridandcustomers_rid ,andiftherefreshmethodwasREFRESH FORCE ,thenthismaterializedviewwouldbefastrefreshableonlyifthesalestablewasupdatedbutnotifthetablestimes orcustomers wereupdated.

CREATEMATERIALIZEDVIEWdetail_sales_mvPARALLELBUILDIMMEDIATEREFRESHFORCEASSELECTs.rowid"sales_rid",c.cust_id,c.cust_last_name,s.amount_sold,s.quantity_sold,s.time_idFROMsaless,timest,customerscWHEREs.cust_id=c.cust_id(+)ANDs.time_id=t.time_id(+);

()

NestedMaterializedViews()()Anestedmaterializedviewisamaterializedviewwhosedefinitionisbasedonanothermaterializedview.Anestedmaterializedviewcanreferenceotherrelationsinthedatabaseinadditiontoreferencingmaterializedviews.

()

WhyUseNestedMaterializedViews?Inadatawarehouse,youtypicallycreatemanyaggregateviewsonasinglejoin(forexample,rollupsalongdifferentdimensions).Incrementallymaintainingthesedistinctmaterializedaggregateviewscantakealongtime,becausetheunderlyingjoinhastobeperformedmanytimes.

Usingnestedmaterializedviews,youcancreatemultiplesingletablematerializedviewsbasedonajoinsonlymaterializedviewandthejoinisperformedjustonce.Inaddition,optimizationscanbeperformedforthisclassofsingletableaggregatematerializedviewandthusrefreshisveryefficient.

()Example85NestedMaterializedView

Youcancreateanestedmaterializedviewonmaterializedviews,butallparentandbasematerializedviewsmustcontainjoinsoraggregates.Ifthedefiningqueriesforamaterializedviewdonotcontainjoinsoraggregates,itcannotbenested.Alltheunderlyingobjects(materializedviewsortables)onwhichthematerializedviewisdefinedmusthaveamaterializedviewlog.Alltheunderlyingobjectsaretreatedasiftheyweretables.Inaddition,youcanusealltheexistingoptionsformaterializedviews.

Usingthetablesandtheircolumnsfromthesh sampleschema,thefollowingmaterializedviewsillustratehownestedmaterializedviewscanbecreated.

CREATEMATERIALIZEDVIEWLOGONsalesWITHROWID;CREATEMATERIALIZEDVIEWLOGONcustomersWITHROWID;CREATEMATERIALIZEDVIEWLOGONtimesWITHROWID;

/*creatematerializedviewjoin_sales_cust_timeasfastrefreshableatCOMMITtime*/CREATEMATERIALIZEDVIEWjoin_sales_cust_timeREFRESHFASTONCOMMITASSELECTc.cust_id,c.cust_last_name,s.amount_sold,t.time_id,t.day_number_in_week,s.rowidsrid,t.rowidtrid,c.rowidcridFROMsaless,customersc,timestWHEREs.time_id=t.time_idANDs.cust_id=c.cust_id;

Tocreateanestedmaterializedviewonthetablejoin_sales_cust_time ,youwouldhavetocreateamaterializedviewlogonthetable.Becausethiswillbeasingletableaggregatematerializedviewonjoin_sales_cust_time ,youneedtologallthenecessarycolumnsandusetheINCLUDING NEWVALUES clause.

/*creatematerializedviewlogonjoin_sales_cust_time*/CREATEMATERIALIZEDVIEWLOGONjoin_sales_cust_timeWITHROWID(cust_last_name,day_number_in_week,amount_sold)INCLUDINGNEWVALUES;

/*createthesingle-tableaggregatematerializedviewsum_sales_cust_timeonjoin_sales_cust_timeasfastrefreshableatCOMMITtime*/CREATEMATERIALIZEDVIEWsum_sales_cust_timeREFRESHFASTONCOMMITASSELECTCOUNT(*)cnt_all,SUM(amount_sold)sum_sales,COUNT(amount_sold)cnt_sales,cust_last_name,day_number_in_weekFROMjoin_sales_cust_timeGROUPBYcust_last_name,day_number_in_week;

()

NestingMaterializedViewswithJoinsandAggregatesSometypesofnestedmaterializedviewscannotbefastrefreshed.UseEXPLAIN_MVIEW toidentifythosetypesofmaterializedviews.Youcanrefreshatreeofnestedmaterializedviewsintheappropriatedependencyorderbyspecifyingthenested=TRUE parameterwiththeDBMS_MVIEW.REFRESHparameter.Forexample,ifyoucallDBMS_MVIEW.REFRESH('SUM_SALES_CUST_TIME',nested=>TRUE) ,theREFRESH procedurewillfirstrefreshthejoin_sales_cust_timematerializedview,andthenrefreshthesum_sales_cust_time materializedview.

()

NestedMaterializedViewUsageGuidelinesYoushouldkeepthefollowinginmindwhendecidingwhethertousenestedmaterializedviews:

Ifyouwanttousefastrefresh,youshouldfastrefreshallthematerializedviewsalonganychain.

Ifyouwantthehighestlevelmaterializedviewtobefreshwithrespecttothedetailtables,youneedtoensurethatallmaterializedviewsinatreearerefreshedinthecorrectdependencyorderbeforerefreshingthehighestlevel.Youcanautomaticallyrefreshintermediatematerializedviewsinanestedhierarchyusingthenested=TRUE parameter,asdescribedin"NestingMaterializedViewswithJoinsandAggregates".Ifyoudonotspecifynested=TRUE andthematerializedviewsunderthehighestlevelmaterializedviewarestale,refreshingonlythehighestlevelwillsucceed,butmakesitfreshonlywithrespecttoitsunderlyingmaterializedview,notthedetailtablesatthebaseofthetree.

Whenrefreshingmaterializedviews,youneedtoensurethatallmaterializedviewsinatreearerefreshed.Ifyouonlyrefreshthehighestlevelmaterializedview,thematerializedviewsunderitwillbestaleandyoumustexplicitlyrefreshthem.IfyouusetheREFRESH procedurewiththenestedparametervaluesettoTRUE ,onlyspecifiedmaterializedviewsandtheirchildmaterializedviewsinthetreearerefreshed,andnottheirtoplevelmaterializedviews.UsetheREFRESH_DEPENDENT procedurewiththenestedparametervaluesettoTRUE ifyouwanttoensurethatallmaterializedviewsinatreearerefreshed.

Freshnessofamaterializedviewiscalculatedrelativetotheobjectsdirectlyreferencedbythematerializedview.Whenamaterializedviewreferencesanothermaterializedview,thefreshnessofthetopmostmaterializedviewiscalculatedrelativetochangesinthematerializedviewitdirectlyreferences,notrelativetochangesinthetablesreferencedbythematerializedviewitreferences.

()

RestrictionsWhenUsingNestedMaterializedViews()()Youcannotcreatebothamaterializedviewandaprebuiltmaterializedviewonthesametable.Forexample,Ifyouhaveatablecosts withamaterializedviewcost_mv basedonit,youcannotthencreateaprebuiltmaterializedviewontablecosts .Theresultwouldmakecost_mv anestedmaterializedviewandthismethodofconversionisnotsupported.

()

CreatingMaterializedViews()AmaterializedviewcanbecreatedwiththeCREATE MATERIALIZED VIEWstatementorusingEnterpriseManager.Example86illustratescreatinganmaterializedviewcalledcust_sales_mv .

()Example86CreatingaMaterializedView

CREATEMATERIALIZEDVIEWcust_sales_mvPCTFREE0TABLESPACEdemoSTORAGE(INITIAL16kNEXT16kPCTINCREASE0)PARALLELBUILDIMMEDIATEREFRESHCOMPLETEENABLEQUERYREWRITEASSELECTc.cust_last_name,SUM(amount_sold)ASsum_amount_soldFROMcustomersc,salessWHEREs.cust_id=c.cust_idGROUPBYc.cust_last_name;

Itisnotuncommoninadatawarehousetohavealreadycreatedsummaryoraggregationtables,andyoumightnotwishtorepeatthisworkbybuildinganewmaterializedview.Inthiscase,thetablethatalreadyexistsinthedatabasecanberegisteredasa()()prebuiltmaterializedview.Thistechniqueisdescribedin"RegisteringExistingMaterializedViews".

Onceyouhaveselectedthematerializedviewsyouwanttocreate,followthesestepsforeachmaterializedview.

1. Designthematerializedview.Existinguserdefinedmaterializedviewsdonotrequirethisstep.Ifthematerializedviewcontainsmanyrows,then,ifappropriate,thematerializedviewshouldbepartitioned(ifpossible)andshouldmatchthepartitioningofthelargestormostfrequentlyupdateddetailorfacttable(ifpossible).Refreshperformancebenefitsfrompartitioning,becauseitcantakeadvantageofparallelDMLcapabilitiesandpossiblePCTbasedrefresh.

2. UsetheCREATE MATERIALIZED VIEW statementtocreateand,optionally,populatethematerializedview.Ifauserdefinedmaterializedviewalreadyexists,thenusetheON PREBUILT TABLE clauseintheCREATEMATERIALIZED VIEW statement.Otherwise,usetheBUILD IMMEDIATEclausetopopulatethematerializedviewimmediately,ortheBUILDDEFERRED clausetopopulatethematerializedviewlater.ABUILDDEFERRED materializedviewisdisabledforusebyqueryrewriteuntilthefirstCOMPLETE REFRESH ,afterwhichitwillbeautomaticallyenabled,providedtheENABLE QUERY REWRITE clausehasbeenspecified.

SeeAlso:()()()OracleDatabaseSQLLanguageReference(../../server.111/b28286/toc.htm)fordescriptionsoftheSQLstatementsCREATE MATERIALIZED VIEW , ALTER MATERIALIZEDVIEW ,and DROP MATERIALIZED VIEW

()

CreatingMaterializedViewswithColumnAliasListsCurrently,whenamaterializedviewiscreated,ifitsdefiningquerycontainssamenamecolumnsintheSELECT list,thenameconflictsneedtoberesolvedbyspecifyinguniquealiasesforthosecolumns.Otherwise,theCREATEMATERIALIZED VIEW statementwillfailwiththeerrormessagesofcolumnsambiguouslydefined.However,thestandardmethodofattachingaliasesintheSELECT clausefornameresolutionrestrictstheuseofthefulltextmatchqueryrewriteanditwilloccuronlywhenthetextofthematerializedview'sdefiningqueryandthetextofuserinputqueryareidentical.Thus,iftheuserspecifiesselectaliasesinthematerializedview'sdefiningquerywhilethereisnoaliasinthequery,thefulltextmatchcomparisonwillfail.ThisisparticularlyaproblemforqueriesfromDiscoverer,whichmakesextensiveuseofcolumnaliases.

Thefollowingisanexampleoftheproblem.sales_mv iscreatedwithcolumnaliasesintheSELECT clausebuttheinputqueryQ1 doesnothavethealiases.Thefulltextmatchrewritewillfail.Thematerializedviewisasfollows:

CREATEMATERIALIZEDVIEWsales_mvENABLEQUERYREWRITEASSELECTs.time_idsales_tid,c.time_idcosts_tidFROMsaless,productsp,costscWHEREs.prod_id=p.prod_idANDc.prod_id=p.prod_idANDp.prod_nameIN(SELECTprod_nameFROMproducts);

InputquerystatementQ1 isasfollows:

SELECTs.time_id,c.time_idFROMsaless,productsp,costscWHEREs.prod_id=p.prod_idANDc.prod_id=p.prod_idANDp.prod_nameIN(SELECTprod_nameFROMproducts);

Eventhoughthematerializedview'sdefiningqueryisalmostidenticalandlogicallyequivalenttotheuser'sinputquery,queryrewritedoesnothappenbecauseofthefailureoffulltextmatchthatistheonlyrewritepossibilityforsomequeries(forexample,asubqueryintheWHERE clause).

YoucanaddacolumnaliaslisttoaCREATE MATERIALIZED VIEW statement.ThecolumnaliaslistexplicitlyresolvesanycolumnnameconflictwithoutattachingaliasesintheSELECT clauseofthematerializedview.Thesyntaxofthematerializedviewcolumnaliaslistisillustratedinthefollowingexample:

CREATEMATERIALIZEDVIEWsales_mv(sales_tid,costs_tid)ENABLEQUERYREWRITEASSELECTs.time_id,c.time_idFROMsaless,productsp,costscWHEREs.prod_id=p.prod_idANDc.prod_id=p.prod_idANDp.prod_nameIN(SELECTprod_nameFROMproducts);

Inthisexample,thedefiningqueryofsales_mv nowmatchesexactlywiththeuserqueryQ1 ,sofulltextmatchrewritewilltakeplace.

NotethatwhenaliasesarespecifiedinboththeSELECT clauseandthenewaliaslistclause,thealiaslistclausesupersedestheonesintheSELECTclause.

()

NamingMaterializedViews()ThenameofamaterializedviewmustconformtostandardOraclenamingconventions.However,ifthematerializedviewisbasedonauserdefinedprebuilttable,thenthenameofthematerializedviewmustexactlymatchthattablename.

Ifyoualreadyhaveanamingconventionfortablesandindexes,youmightconsiderextendingthisnamingschemetothematerializedviewssothattheyareeasilyidentifiable.Forexample,insteadofnamingthematerializedviewsum_of_sales ,itcouldbecalledsum_of_sales_mv todenotethatthisisamaterializedviewandnotatableorview.

()

StorageAndTableCompression()Unlessthematerializedviewisbasedon()()()()auserdefinedprebuilttable,itrequiresandoccupiesstoragespaceinsidethedatabase.Therefore,thestorageneedsforthematerializedviewshouldbespecifiedintermsofthetablespacewhereitistoresideandthesizeoftheextents.

Ifyoudonotknowhowmuchspacethematerializedviewwillrequire,thentheDBMS_MVIEW.ESTIMATE_SIZE packagecanestimatethenumberofbytesrequiredtostorethisuncompressedmaterializedview.Thisinformationcanthenassistthedesignteamindeterminingthetablespaceinwhichthematerializedviewshouldreside.

Youshouldusetablecompressionwithhighlyredundantdata,suchastableswithmanyforeignkeys.ThisisparticularlyusefulformaterializedviewscreatedwiththeROLLUP clause.Tablecompressionreducesdiskuseandmemoryuse

(specifically,thebuffercache),oftenleadingtoabetterscaleupforreadonlyoperations.Tablecompressioncanalsospeedupqueryexecutionattheexpenseofupdatecost.

SeeAlso:OracleDatabaseSQLLanguageReference(../../server.111/b28286/statements_10.htm#SQLRF017)foracompletedescriptionof STORAGE semantics,OracleDatabasePerformanceTuningGuide(../../server.111/b28274/toc.htm),andOracleDatabaseVLDBandPartitioningGuide(../../server.111/b32024/part_avail.htm#VLDBG004)fortablecompressionexamples

()

BuildMethods()()Twobuildmethodsareavailableforcreatingthematerializedview,asshowninTable83.IfyouselectBUILD IMMEDIATE ,thematerializedviewdefinitionisaddedtotheschemaobjectsinthedatadictionary,andthenthefactordetailtablesarescannedaccordingtotheSELECT expressionandtheresultsarestoredinthematerializedview.Dependingonthesizeofthetablestobescanned,thisbuildprocesscantakeaconsiderableamountoftime.

AnalternativeapproachistousetheBUILD DEFERRED clause,whichcreatesthematerializedviewwithoutdata,therebyenablingittobepopulatedatalaterdateusingtheDBMS_MVIEW.REFRESH packagedescribedinChapter15,"MaintainingtheDataWarehouse"(refresh.htm#g1018554).

()()Table83BuildMethods

BuildMethod Description

BUILD IMMEDIATE Createthematerializedviewandthenpopulateitwithdata.

BUILD DEFERRED Createthematerializedviewdefinitionbutdonotpopulateitwithdata.

()

EnablingQueryRewriteBefore()creatingamaterializedview,youcanverifywhattypesofqueryrewritearepossiblebycallingtheprocedureDBMS_MVIEW.EXPLAIN_MVIEW ,oruseDBMS_ADVISOR.TUNE_MVIEW tooptimizethematerializedviewsothatmanytypesofqueryrewritearepossible.Oncethematerializedviewhasbeencreated,youcanuseDBMS_MVIEW.EXPLAIN_REWRITE tofindoutif(orwhynot)itwillrewriteaspecificquery.

Eventhoughamaterializedviewisdefined,itwillnotautomaticallybeusedbythequeryrewritefacility.Eventhoughqueryrewriteisenabledbydefault,youalsomustspecifytheENABLE QUERY REWRITE clauseifthematerializedviewistobeconsideredavailableforrewritingqueries.

IfthisclauseisomittedorspecifiedasDISABLE QUERY REWRITE whenthematerializedviewiscreated,thematerializedviewcansubsequentlybeenabledforqueryrewritewiththeALTER MATERIALIZED VIEW statement.

IfyoudefineamaterializedviewasBUILD DEFERRED ,itisnoteligibleforqueryrewriteuntilitispopulatedwithdatathroughacompleterefresh.

()

QueryRewriteRestrictions()()()()Queryrewriteisnotpossiblewithallmaterializedviews.Ifqueryrewriteisnotoccurringwhenexpected,DBMS_MVIEW.EXPLAIN_REWRITE canhelpprovidereasonswhyaspecificqueryisnoteligibleforrewrite.Ifthisshowsthatnotalltypesofqueryrewritearepossible,usetheprocedureDBMS_ADVISOR.TUNE_MVIEW toseeifthematerializedviewcanbedefineddifferentlysothatqueryrewriteispossible.Also,checktoseeifyourmaterializedviewsatisfiesallofthefollowingconditions.

()

MaterializedViewRestrictions()Youshouldkeepinmindthefollowingrestrictions:

Thedefiningqueryofthematerializedviewcannotcontainanynonrepeatableexpressions(ROWNUM ,SYSDATE ,nonrepeatablePL/SQLfunctions,andsoon).

ThequerycannotcontainanyreferencestoRAW orLONG RAW datatypesorobjectREFs.

IfthematerializedviewwasregisteredasPREBUILT ,theprecisionofthecolumnsmustagreewiththeprecisionofthecorrespondingSELECTexpressionsunlessoverriddenbytheWITH REDUCED PRECISION clause.

()

GeneralQueryRewriteRestrictio()()nsYoushouldkeepinmindthefollowingrestrictions:

Aquerycanreferencebothlocalandremotetables.Suchaquerycanberewrittenaslongasaneligiblematerializedviewreferencingthesametablesisavailablelocally.

NeitherthedetailtablesnorthematerializedviewcanbeownedbySYS .

IfacolumnorexpressionispresentintheGROUP BY clauseofthematerializedview,itmustalsobepresentintheSELECT list.

Aggregatefunctionsmustoccuronlyastheoutermostpartoftheexpression.Thatis,aggregatessuchasAVG(AVG(x)) orAVG(x)+AVG(x) arenotallowed.

CONNECT BY clausesarenotallowed.

()

RefreshOptions()Whenyoudefineamaterializedview,youcanspecifythreerefreshoptions:howtorefresh,whattypeofrefresh,andcantrustedconstraintsbeused.Ifunspecified,thedefaultsareassumedasON DEMAND ,FORCE ,andENFORCEDconstraintsrespectively.

ThetworefreshexecutionmodesareON COMMIT andON DEMAND .Dependingonthematerializedviewyoucreate,someoftheoptionsmaynotbeavailable.Table84describestherefreshmodes.

()()Table84RefreshModes

RefreshMode Description

ONCOMMIT()

Refreshoccursautomaticallywhenatransactionthatmodifiedoneofthematerializedview'sdetailtablescommits.Thiscanbespecifiedaslongasthematerializedviewisfastrefreshable(inotherwords,notcomplex).TheON COMMIT privilegeisnecessarytousethismode.

ONDEMAND()

RefreshoccurswhenausermanuallyexecutesoneoftheavailablerefreshprocedurescontainedintheDBMS_MVIEWpackage(REFRESH ,REFRESH_ALL_MVIEWS ,REFRESH_DEPENDENT ).

WhenamaterializedviewismaintainedusingtheON COMMIT method,thetimerequiredtocompletethecommitmaybeslightlylongerthanusual.Thisisbecausetherefreshoperationisperformedaspartofthecommitprocess.Thereforethismethodmaynotbesuitableifmanyusersareconcurrentlychangingthetablesuponwhichthematerializedviewisbased.

Ifyouanticipateperforminginsert,updateordeleteoperationsontablesreferencedbyamaterializedviewconcurrentlywiththerefreshofthatmaterializedview,andthatmaterializedviewincludesjoinsandaggregation,OraclerecommendsyouuseON COMMIT fastrefreshratherthanON DEMANDfastrefresh.

Ifyouthinkthematerializedviewdidnotrefresh,checkthealertlogortracefile.

IfamaterializedviewfailsduringrefreshatCOMMIT time,youmustexplicitlyinvoketherefreshprocedureusingtheDBMS_MVIEW packageafteraddressingtheerrorsspecifiedinthetracefiles.Untilthisisdone,thematerializedviewwillnolongerberefreshedautomaticallyatcommittime.

Youcanspecifyhowyouwantyour()materializedviewstoberefreshedfromthedetailtablesbyselectingoneoffouroptions:COMPLETE ,FAST ,FORCE ,andNEVER .Table85describestherefreshoptions.

()()Table85RefreshOptions

RefreshOption Description

COMPLETE() Refreshesbyrecalculatingthematerializedview'sdefiningquery.

FAST()

Appliesincrementalchangestorefreshthematerializedviewusingtheinformationloggedinthematerializedviewlogs,orfromaSQL*Loaderdirectpathorapartitionmaintenanceoperation.

FORCE() AppliesFAST refreshifpossibleotherwise,itappliesCOMPLETE refresh.

NEVER() Indicatesthatthematerializedviewwillnotberefreshedwithrefreshmechanisms.

Whetherthefastrefreshoptionisavailabledependsuponthetypeofmaterializedview.YoucancalltheprocedureDBMS_MVIEW.EXPLAIN_MVIEWtodeterminewhetherfastrefreshispossible.

YoucanalsospecifyifitisacceptabletousetrustedconstraintsandQUERY_REWRITE_INTEGRITY=TRUSTED duringrefresh.AnynonvalidatedRELY constraintisatrustedconstraint.Forexample,nonvalidatedforeignkey/primarykeyrelationships,functionaldependenciesdefinedindimensionsoramaterializedviewintheUNKNOWN state.Ifqueryrewriteisenabledduringrefresh,thesecanimprovetheperformanceofrefreshbyenablingmoreperformantqueryrewrites.AnymaterializedviewthatcanuseTRUSTEDconstraintsforrefreshisleftinastateoftrustedfreshness(theUNKNOWN state)afterrefresh.

ThisisreflectedinthecolumnSTALENESS intheviewUSER_MVIEWS .ThecolumnUNKNOWN_TRUSTED_FD inthesameviewisalsosettoY ,whichmeansyes.

YoucandefinethispropertyofthematerializedvieweitherduringcreatetimebyspecifyingREFRESH USING TRUSTED [ENFORCED] CONSTRAINTS orbyusingALTER MATERIALIZED VIEW DDL.

()()Table86Constraints

ConstraintstoUse Description

TRUSTEDCONSTRAINTS RefreshcanusetrustedconstraintsandQUERY_REWRITE_INTEGRITY =TRUSTEDduringrefresh.Thisallowsuseofnon

validatedRELY constraintsandrewriteagainstmaterializedviewsinUNKNOWN orFRESH stateduringrefresh.

ENFORCEDCONSTRAINTS RefreshcanusevalidatedconstraintsandQUERY_REWRITE_INTEGRITY=ENFORCEDduringrefresh.Thisallowsuseofonlyvalidated,enforcedconstraintsandrewriteagainstmaterializedviewsinFRESH stateduringrefresh.

()

GeneralRestrictionson()()FastRefreshThedefiningqueryofthematerializedviewisrestrictedasfollows:

ThematerializedviewmustnotcontainreferencestononrepeatingexpressionslikeSYSDATE andROWNUM .

ThematerializedviewmustnotcontainreferencestoRAW orLONG RAWdatatypes.

ItcannotcontainaSELECT listsubquery.

Itcannotcontainanalyticalfunctions(forexample,RANK )intheSELECTclause.

ItcannotcontainaMODEL clause.

ItcannotcontainaHAVING clausewithasubquery.

ItcannotcontainnestedqueriesthathaveANY ,ALL ,orNOT EXISTS .

Itcannotcontaina[STARTWITH]CONNECTBY clause.

Itcannotcontainmultipledetailtablesatdifferentsites.

ON COMMIT materializedviewscannothaveremotedetailtables.

Nestedmaterializedviewsmusthaveajoinoraggregate.

()

RestrictionsonFastRefreshonMaterializedViewswithJoinsOnlyDefiningqueriesformaterializedviewswithjoinsonlyandnoaggregateshavethefollowingrestrictionsonfastrefresh:

Allrestrictionsfrom"GeneralRestrictionsonFastRefresh".

TheycannothaveGROUP BY clausesoraggregates.

RowidsofallthetablesintheFROM listmustappearintheSELECT listofthequery.

MaterializedviewlogsmustexistwithrowidsforallthebasetablesintheFROM listofthequery.

YoucannotcreateafastrefreshablematerializedviewfrommultipletableswithsimplejoinsthatincludeanobjecttypecolumnintheSELECTstatement.

()

RestrictionsonFastRefreshonMaterializedViewswithAggregatesDefiningqueriesformaterializedviewswithaggregatesorjoinshavethefollowingrestrictionsonfastrefresh:

Allrestrictionsfrom"GeneralRestrictionsonFastRefresh".

FastrefreshissupportedforbothON COMMIT andON DEMAND materializedviews,howeverthefollowingrestrictionsapply:

Alltablesinthematerializedviewmusthavematerializedviewlogs,andthematerializedviewlogsmust:

Containallcolumnsfromthetablereferencedinthematerializedview.However,noneofthesecolumnsinthebasetablecanbeencrypted.

SpecifywithROWID andINCLUDING NEW VALUES .

SpecifytheSEQUENCE clauseifthetableisexpectedtohaveamixofinserts/directloads,deletes,andupdates.

OnlySUM ,COUNT ,AVG ,STDDEV ,VARIANCE ,MIN andMAX aresupportedforfastrefresh.

COUNT(*) mustbespecified.

Aggregatefunctionsmustoccuronlyastheoutermostpartoftheexpression.Thatis,aggregatessuchasAVG(AVG(x)) orAVG(x)+AVG(x) arenotallowed.

ForeachaggregatesuchasAVG(expr) ,thecorrespondingCOUNT(expr)mustbepresent.OraclerecommendsthatSUM(expr) bespecified.SeeTable82forfurtherdetails.

IfVARIANCE(expr) orSTDDEV(expr )isspecified,COUNT(expr) andSUM(expr) mustbespecified.OraclerecommendsthatSUM(expr*expr) bespecified.SeeTable82forfurtherdetails.

TheSELECT columninthedefiningquerycannotbeacomplexexpressionwithcolumnsfrommultiplebasetables.Apossibleworkaroundtothisistouseanestedmaterializedview.

TheSELECT listmustcontainallGROUP BY columns.

Ifthematerializedviewhasoneofthefollowing,thenfastrefreshissupportedonlyonconventionalDMLinsertsanddirectloads.

MaterializedviewswithMIN orMAX aggregates

MaterializedviewswhichhaveSUM(expr) butnoCOUNT(expr)

MaterializedviewswithoutCOUNT(*)

Suchamaterializedviewiscalledaninsertonlymaterializedview.

AmaterializedviewwithMAX orMIN isfastrefreshableafterdeleteormixedDMLstatementsifitdoesnothaveaWHERE clause.

MaterializedviewswithnamedviewsorsubqueriesintheFROM clausecanbefastrefreshedprovidedtheviewscanbecompletelymerged.Forinformationonwhichviewswillmerge,refertotheOracleDatabasePerformanceTuningGuide(../../server.111/b28274/toc.htm).

Iftherearenoouterjoins,youmayhavearbitraryselectionsandjoinsintheWHERE clause.

MaterializedaggregateviewswithouterjoinsarefastrefreshableafterconventionalDMLanddirectloads,providedonlytheoutertablehasbeenmodified.Also,uniqueconstraintsmustexistonthejoincolumnsoftheinnerjointable.Ifthereareouterjoins,allthejoinsmustbeconnectedbyANDsandmustusetheequality(= )operator.

FormaterializedviewswithCUBE ,ROLLUP ,groupingsets,orconcatenationofthem,thefollowingrestrictionsapply:

TheSELECT listshouldcontaingroupingdistinguisherthatcaneitherbeaGROUPING_ID functiononallGROUP BY expressionsorGROUPINGfunctionsoneforeachGROUP BY expression.Forexample,iftheGROUPBY clauseofthematerializedviewis"GROUP BY CUBE(a,b) ",thentheSELECT listshouldcontaineither"GROUPING_ID(a,b) "or"GROUPING(a) AND GROUPING(b) "forthematerializedviewtobefastrefreshable.

GROUP BY shouldnotresultinanyduplicategroupings.Forexample,"GROUPBYa,ROLLUP(a,b) "isnotfastrefreshablebecauseitresultsinduplicategroupings"(a),(a,b),AND(a) ".

()

RestrictionsonFastRefreshonMaterializedViewswithUNIONALL

MaterializedviewswiththeUNION ALL setoperatorsupporttheREFRESHFAST optionifthefollowingconditionsaresatisfied:

ThedefiningquerymusthavetheUNION ALL operatoratthetoplevel.

TheUNION ALL operatorcannotbeembeddedinsideasubquery,withoneexception:TheUNION ALL canbeinasubqueryintheFROM clauseprovidedthedefiningqueryisoftheformSELECT*FROM (vieworsubquerywithUNION ALL )asinthefollowingexample:

CREATEVIEWview_with_unionallAS(SELECTc.rowidcrid,c.cust_id,2umarkerFROMcustomerscWHEREc.cust_last_name='Smith'UNIONALLSELECTc.rowidcrid,c.cust_id,3umarkerFROMcustomerscWHEREc.cust_last_name='Jones');

CREATEMATERIALIZEDVIEWunionall_inside_view_mvREFRESHFASTONDEMANDASSELECT*FROMview_with_unionall;

Notethattheviewview_with_unionall satisfiestherequirementsforfastrefresh.

EachqueryblockintheUNION ALL querymustsatisfytherequirementsofafastrefreshablematerializedviewwithaggregatesorafastrefreshablematerializedviewwithjoins.

Theappropriatematerializedviewlogsmustbecreatedonthetablesasrequiredforthecorrespondingtypeoffastrefreshablematerializedview.

NotethattheOracleDatabasealsoallowsthespecialcaseofasingletablematerializedviewwithjoinsonlyprovidedtheROWID columnhasbeenincludedintheSELECT listandinthematerializedviewlog.Thisisshowninthedefiningqueryoftheviewview_with_unionall .

TheSELECT listofeachquerymustincludeaUNION ALL marker,andtheUNION ALL columnmusthaveadistinctconstantnumericorstringvalueineachUNION ALL branch.Further,themarkercolumnmustappearinthesameordinalpositionintheSELECT listofeachqueryblock.See"UNIONALLMarker"(qradv.htm#BABEAHIA)formoreinformationregardingUNION ALLmarkers.

Somefeaturessuchasouterjoins,insertonlyaggregatematerializedviewqueriesandremotetablesarenotsupportedformaterializedviewswithUNION ALL .

Thecompatibilityinitializationparametermustbesetto9.2.0orhighertocreateafastrefreshablematerializedviewwithUNION ALL .

()

AchievingRefreshGoals

InadditiontotheEXPLAIN_MVIEW procedure,whichisdiscussedthroughoutthischapter,youcanusetheDBMS_ADVISOR.TUNE_MVIEW proceduretooptimizeaCREATE MATERIALIZED VIEW statementtoachieveREFRESH

FAST andENABLE QUERY REWRITE goals.

()

RefreshingNestedMaterializedViewsAnestedmaterializedviewisconsideredtobefreshaslongasitsdataissynchronizedwiththedatainitsdetailtables,evenifsomeofitsdetailtablescouldbestalematerializedviews.

Youcanrefreshnestedmaterializedviewsintwoways:DBMS_MVIEW.REFRESH withthenested flagsettoTRUE andREFRESH_DEPENDENT withthenested flagsettoTRUE onthebasetables.IfyouuseDBMS_MVIEW.REFRESH ,theentirematerializedviewchainisrefreshedandthecoveragestartingfromthespecifiedmaterializedviewintopdownfashion.Thatis,thespecifiedmaterializedviewandallitschildmaterializedviewsinthedependencyhierarchyarerefreshedinorder.WithDBMS_MVIEW.REFRESH_DEPENDENT ,theentirechainisrefreshedfromthebottomup.Thatis,alltheparentmaterializedviewsinthedependencyhierarchystartingfromthespecifiedtablearerefreshedinorder.

()Example87ExampleofRefreshingaNestedMaterializedView

Thefollowingstatementshowsanexampleofrefreshinganestedmaterializedview:

DBMS_MVIEW.REFRESH('SALES_MV,COST_MV',nested=>TRUE);

Thisstatementwillfirstrefreshallchildmaterializedviewsofsales_mv andcost_mv basedonthedependencyanalysisandthenrefreshthetwospecifiedmaterializedviews.

YoucanquerytheSTALE_SINCE columninthe*_MVIEWS viewstofindoutwhenamaterializedviewbecamestale.

()

ORDERBYClause()AnORDER BY clauseisallowedintheCREATE MATERIALIZED VIEWstatement.Itisusedonlyduringtheinitialcreationofthematerializedview.Itisnotusedduringafullrefreshorafastrefresh.

Toimprovetheperformanceofqueriesagainstlargematerializedviews,storetherowsinthematerializedviewintheorderspecifiedintheORDER BYclause.Thisinitialorderingprovidesphysicalclusteringofthedata.Ifindexesarebuiltonthecolumnsbywhichthematerializedviewisordered,accessingtherowsofthematerializedviewusingtheindexoftenreducesthetimefordiskI/Oduetothephysicalclustering.

TheORDER BY clauseisnotconsideredpartofthematerializedviewdefinition.Asaresult,thereisnodifferenceinthemannerinwhichOracleDatabasedetectsthevarioustypesofmaterializedviews(forexample,materializedjoinviewswithnoaggregates).Forthesamereason,queryrewriteisnotaffectedbytheORDER BY clause.ThisfeatureissimilartotheCREATE TABLE ...ORDER BY capability.

()

MaterializedViewLogsMaterializedview()()logsarerequiredifyouwanttousefastrefresh,withtheexceptionofPCTrefresh.Thatis,ifadetailtablesupportsPCTforamaterializedview,thematerializedviewlogonthatdetailtableisnotrequiredinordertodofastrefreshonthatmaterializedview.Asageneralrule,though,youshouldcreatematerializedviewlogsifyouwanttousefastrefresh.MaterializedviewlogsaredefinedusingaCREATE MATERIALIZED VIEW LOGstatementonthebasetablethatistobechanged.Theyarenotcreatedonthematerializedviewunlessthereisanothermaterializedviewontopofthatmaterializedview,whichisthecasewithnestedmaterializedviews.Forfastrefreshofmaterializedviews,thedefinitionofthematerializedviewlogsmustnormallyspecifytheROWID clause.Inaddition,foraggregatematerializedviews,itmustalsocontaineverycolumninthetablereferencedinthematerializedview,theINCLUDING NEW VALUES clauseandtheSEQUENCEclause.

Anexampleofamaterializedviewlogisshownasfollowswhereoneiscreatedonthetablesales :

CREATEMATERIALIZEDVIEWLOGONsalesWITHROWID(prod_id,cust_id,time_id,channel_id,promo_id,quantity_sold,amount_sold)INCLUDINGNEWVALUES;

Alternatively,amaterializedviewlogcanbeamendedtoincludetherowid,asinthefollowing:

ALTERMATERIALIZEDVIEWLOGONsalesADDROWID(prod_id,cust_id,time_id,channel_id,promo_id,quantity_sold,amount_sold)INCLUDINGNEWVALUES;

OraclerecommendsthatthekeywordSEQUENCE beincludedinyourmaterializedviewlogstatementunlessyouaresurethatyouwillneverperformamixedDMLoperation(acombinationofINSERT ,UPDATE ,orDELETEoperationsonmultipletables).TheSEQUENCE columnisrequiredinthe

materializedviewlogtosupportfastrefreshwithacombinationofINSERT ,UPDATE ,orDELETE statementsonmultipletables.Youcan,however,addtheSEQUENCE numbertothematerializedviewlogafterithasbeencreated.

TheboundaryofamixedDMLoperationisdeterminedbywhetherthematerializedviewisON COMMIT orON DEMAND .

ForON COMMIT ,themixedDMLstatementsoccurwithinthesametransactionbecausetherefreshofthematerializedviewwilloccuruponcommitofthistransaction.

ForON DEMAND ,themixedDMLstatementsoccurbetweenrefreshes.Thefollowingexampleofamaterializedviewlogillustrateswhereoneiscreatedonthetablesales thatincludestheSEQUENCE keyword:

CREATEMATERIALIZEDVIEWLOGONsalesWITHSEQUENCE,ROWID(prod_id,cust_id,time_id,channel_id,promo_id,quantity_sold,amount_sold)INCLUDINGNEWVALUES;

()

UsingtheFORCEOptionwithMaterializedViewLogs

IfyouspecifyFORCE andanyitemsspecifiedwiththeADD clausehavealreadybeenspecifiedforthematerializedviewlog,Oracledoesnotreturnanerror,butsilentlyignorestheexistingelementsandaddstothematerializedviewloganyitemsthatdonotalreadyexistinthelog.Forexample,ifyouusedafiltercolumnsuchascust_id andthiscolumnalreadyexisted,OracleDatabaseignorestheredundancyanddoesnotreturnanerror.

()

UsingOracleEnterpriseManagerAmaterializedviewcanalsobecreatedusingEnterpriseManagerbyselectingthematerializedviewobjecttype.Thereisnodifferenceintheinformationrequiredifthisapproachisused.

()

UsingMaterializedViewswithNLSParametersWhenusingcertainmaterializedviews,youmustensurethatyourNLSparametersarethesameaswhenyoucreatedthematerializedview.Materializedviewswiththisrestrictionareasfollows:

Inthecaseofaprebuilttable,ifithasanexistingcomment,thecommentwillbeinheritedbythematerializedviewafterithasbeencreated.Theexistingcommentwillbeprefixedwith'(fromtable)' .Forexample,tablesales_summary wascreatedtocontainsalessummaryinformation.Anexistingcomment'Salessummarydata' wasassociatedwiththetable.Amaterializedviewofthesamenameiscreatedtousetheprebuilttableasitscontainertable.Afterthematerializedviewcreation,thecommentbecomes'(fromtable)Salessummarydata' .

However,iftheprebuilttable,sales_summary ,doesnothaveanycomment,thefollowingcommentisadded:'Salessummarydata' .Then,ifwedropthematerializedview,thecommentwillbepassedtotheprebuilttablewiththecomment:'(frommaterializedview)Salessummarydata' .

()

RegisteringExistingMaterializedViews()Somedatawarehouseshaveimplementedmaterializedviewsinordinaryusertables.Althoughthissolutionprovidestheperformancebenefitsofmaterializedviews,itdoesnot:

ProvidequeryrewritetoallSQLapplications.

Enablematerializedviewsdefinedinoneapplicationtobetransparentlyaccessedinanotherapplication.

Generallysupportfastparallelorfastmaterializedviewrefresh.

Becauseoftheselimitations,andbecauseexistingmaterializedviewscanbeextremelylargeandexpensivetorebuild,youshouldregisteryourexistingmaterializedviewtableswheneverpossible.YoucanregisterauserdefinedmaterializedviewwiththeCREATE MATERIALIZED VIEW ...ON PREBUILTTABLE statement.Onceregistered,thematerializedviewcanbeusedforqueryrewritesormaintainedbyoneoftherefreshmethods,orboth.

Thecontentsofthetablemustreflectthematerializationofthedefiningqueryatthetimeyouregisteritasamaterializedview,andeachcolumninthedefiningquerymustcorrespondtoacolumninthetablethathasamatchingdatatype.However,youcanspecifyWITH REDUCED PRECISION toallowtheprecisionofcolumnsinthedefiningquerytobedifferentfromthatofthetablecolumns.

Thetableandthematerializedviewmusthavethesamename,butthetableretainsitsidentityasatableandcancontaincolumnsthatarenotreferencedinthedefiningqueryofthematerializedview.Theseextracolumnsareknownasunmanagedcolumns.Ifrowsareinsertedduringarefreshoperation,eachunmanagedcolumnoftherowissettoitsdefaultvalue.Therefore,theunmanagedcolumnscannothaveNOT NULL constraintsunlesstheyalsohavedefaultvalues.

MaterializedviewsbasedonprebuilttablesareeligibleforselectionbyqueryrewriteprovidedtheparameterQUERY_REWRITE_INTEGRITY issettoSTALE_TOLERATED orTRUSTED .SeeChapter17,"BasicQueryRewrite"(qrbasic.htm#g1045551)fordetailsaboutintegritylevels.

()()Whenyoudropamaterializedviewthatwascreatedonaprebuilttable,thetablestillexistsonlythematerializedviewisdropped.

Thefollowingexampleillustratesthetwostepsrequiredtoregisterauserdefinedtable.First,thetableiscreated,thenthematerializedviewisdefinedusingexactlythesamenameasthetable.Thismaterializedviewsum_sales_tab_mv iseligibleforuseinqueryrewrite.

CREATETABLEsum_sales_tabPCTFREE0TABLESPACEdemoSTORAGE(INITIAL16kNEXT16kPCTINCREASE0)ASSELECTs.prod_id,SUM(amount_sold)ASdollar_sales,SUM(quantity_sold)ASunit_salesFROMsalessGROUPBYs.prod_id;

CREATEMATERIALIZEDVIEWsum_sales_tab_mvONPREBUILTTABLEWITHOUTREDUCEDPRECISIONENABLEQUERYREWRITEASSELECTs.prod_id,SUM(amount_sold)ASdollar_sales,SUM(quantity_sold)ASunit_salesFROMsalessGROUPBYs.prod_id;

Youcouldhavecompressedthistabletosavespace.See"StorageAndTableCompression"fordetailsregardingtablecompression.

Insomecases,userdefinedmaterializedviewsarerefreshedonaschedulethatislongerthantheupdatecycle.Forexample,amonthlymaterializedviewmightbeupdatedonlyattheendofeachmonth,andthematerializedviewvaluesalwaysrefertocompletetimeperiods.Reportswrittendirectlyagainstthesematerializedviewsimplicitlyselectonlydatathatisnotinthecurrent(incomplete)timeperiod.Ifauserdefinedmaterializedviewalreadycontainsatimedimension:

Itshouldberegisteredandthenfastrefreshedeachupdatecycle.

Youcancreateaviewthatselectsthecompletetimeperiodofinterest.

Thereportsshouldbemodifiedtorefertotheviewinsteadofreferringdirectlytotheuserdefinedmaterializedview.

Iftheuserdefinedmaterializedviewdoesnotcontainatimedimension,then:

Createanewmaterializedviewthatdoesincludethetimedimension(ifpossible).

Theviewshouldaggregateoverthetimecolumninthenewmaterializedview.

()

ChoosingIndexesforMaterializedViewsThetwomostcommonoperationsonamaterializedviewarequeryexecutionandfastrefresh,andeachoperationhasdifferentperformancerequirements.Queryexecutionmightneedtoaccessanysubsetofthematerializedviewkeycolumns,andmightneedtojoinandaggregateoverasubsetofthosecolumns.Consequently,queryexecutionusuallyperformsbestifasinglecolumnbitmapindexisdefinedoneachmaterializedviewkeycolumn.

Inthecaseofmaterializedviewscontainingonlyjoinsusingfastrefresh,Oraclerecommendsthatindexesbecreatedonthecolumnsthatcontaintherowidstoimprovetheperformanceoftherefreshoperation.

Ifamaterializedviewusingaggregatesisfastrefreshable,thenanindexappropriateforthefastrefreshprocedureiscreatedunlessUSING NO INDEXisspecifiedintheCREATE MATERIALIZED VIEW statement.

Ifthematerializedviewispartitioned,then,afterdoingapartitionmaintenanceoperationonthematerializedview,theindexesbecomeunusable,andtheyneedtoberebuiltforfastrefreshtowork.

SeeOracleDatabasePerformanceTuningGuide(../../server.111/b28274/advisor.htm#PFGRF008)forinformationonusingtheSQLAccessAdvisortodeterminewhatindexesareappropriateforyourmaterializedview.

()

DroppingMaterializedViewsUsetheDROP MATERIALIZED VIEW statementtodropa()()materializedview.Forexample.thefollowingstatement:

DROPMATERIALIZEDVIEWsales_sum_mv;

Thisstatementdropsthematerializedviewsales_sum_mv .Ifthematerializedviewwasprebuiltonatable,thenthetableisnotdropped,butitcannolongerbemaintainedwiththerefreshmechanismorusedbyqueryrewrite.Alternatively,youcandropamaterializedviewusingOracleEnterpriseManager.

()

AnalyzingMaterializedViewCapabilitiesYoucanusetheDBMS_MVIEW .EXPLAIN_MVIEW proceduretolearnwhatispossiblewithamaterializedvieworpotentialmaterializedview.Inparticular,thisprocedureenablesyoutodetermine:

Ifamaterializedviewisfastrefreshable

Whattypesofqueryrewriteyoucanperformwiththismaterializedview

WhetherPCTrefreshispossible

Usingthisprocedureisstraightforward.YousimplycallDBMS_MVIEW.EXPLAIN_MVIEW ,passinginasasingleparametertheschemaandmaterializedviewnameforanexistingmaterializedview.Alternatively,youcanspecifytheSELECT stringforapotentialmaterializedvieworthecompleteCREATE MATERIALIZED VIEW statement.ThematerializedvieworpotentialmaterializedviewisthenanalyzedandtheresultsarewrittenintoeitheratablecalledMV_CAPABILITIES_TABLE ,whichisthedefault,ortoanarraycalledMSG_ARRAY .

Notethatyoumustruntheutlxmv.sql scriptpriortocallingEXPLAIN_MVIEW exceptwhenyouareplacingtheresultsinMSG_ARRAY .Thescriptisfoundintheadmin directory.ItistocreatetheMV_CAPABILITIES_TABLE inthecurrentschema.AnexplanationofthevariouscapabilitiesisinTable87,andallthepossiblemessagesarelistedinTable88.

()

UsingtheDBMS_MVIEW.EXPLAIN_MVIEWProcedureTheEXPLAIN_MVIEW proc()edurehasthefollowingparameters:

stmt_id

Anoptionalparameter.AclientsupplieduniqueidentifiertoassociateoutputrowswithspecificinvocationsofEXPLAIN_MVIEW .

mv

ThenameofanexistingmaterializedvieworthequerydefinitionortheentireCREATE MATERIALIZED VIEW statementofapotentialmaterializedviewyouwanttoanalyze.

msg-array

ThePL/SQLVARRAY thatreceivestheoutput.

EXPLAIN_MVIEW analyzesthespecifiedmaterializedviewintermsofitsrefreshandrewritecapabilitiesandinsertsitsresults(intheformofmultiplerows)intoMV_CAPABILITIES_TABLE orMSG_ARRAY .

SeeAlso:OracleDatabasePL/SQLPackagesandTypesReference(../../appdev.111/b28419/d_mview.htm#ARPLS027)forfurtherinformationaboutthe DBMS_MVIEW package

()

DBMS_MVIEW.EXPLAIN_MVIEWDeclarations

Thefollowing()PL/SQLdeclarationsthataremadeforyouintheDBMS_MVIEWpackageshowtheorderanddatatypesoftheseparametersforexplaininganexistingmaterializedviewandapotentialmaterializedviewwithoutputtoatableandtoaVARRAY .

ExplainanexistingorpotentialmaterializedviewwithoutputtoMV_CAPABILITIES_TABLE :

DBMS_MVIEW.EXPLAIN_MVIEW(mvINVARCHAR2,stmt_idINVARCHAR2:=NULL);

ExplainanexistingorpotentialmaterializedviewwithoutputtoaVARRAY :

DBMS_MVIEW.EXPLAIN_MVIEW(mvINVARCHAR2,msg_arrayOUTSYS.ExplainMVArrayType);

()

UsingMV_CAPABILITIES_TABLE

OneofthesimplestwaystouseDBMS_MVIEW.EXPLAIN_MVIEW is()withtheMV_CAPABILITIES_TABLE ,whichhasthefollowingstructure:

CREATETABLEMV_CAPABILITIES_TABLE(STATEMENT_IDVARCHAR(30),--Client-supplieduniquestatementidentifierMVOWNERVARCHAR(30),--NULLforSELECTbasedEXPLAIN_MVIEWMVNAMEVARCHAR(30),--NULLforSELECTbasedEXPLAIN_MVIEWCAPABILITY_NAMEVARCHAR(30),--Adescriptivenameoftheparticular--capability:--REWRITE--Candoatleastfulltextmatch--rewrite--REWRITE_PARTIAL_TEXT_MATCH--Candoatleatfullandpartial--textmatchrewrite--REWRITE_GENERAL--Candoallformsofrewrite--REFRESH--Candoatleastcompleterefresh--REFRESH_FROM_LOG_AFTER_INSERT--Candofastrefreshfromanmvlog--orchangecapturetableatleast--whenupdateoperationsare--restrictedtoINSERT--REFRESH_FROM_LOG_AFTER_ANY--candofastrefreshfromanmvlog--orchangecapturetableafterany--combinationofupdates--PCT--CandoEnhancedUpdateTrackingon--thetablenamedintheRELATED_NAME--column.EUTisneededforfast--refreshafterpartitioned--maintenanceoperationsonthetable

--namedintheRELATED_NAMEcolumn--andtodonon-staletolerated--rewritewhenthemvispartially--stalewithrespecttothetable--namedintheRELATED_NAMEcolumn.--EUTcanalsosometimesenablefast--refreshofupdatestothetable--namedintheRELATED_NAMEcolumn--whenfastrefreshfromanmvlog--orchangecapturetableisnot--possible.--SeeTable8-7POSSIBLECHARACTER(1),--T=capabilityispossible--F=capabilityisnotpossibleRELATED_TEXTVARCHAR(2000),--Owner.table.column,aliasname,andsoon--relatedtothismessage.Thespecific--meaningofthiscolumndependsonthe--NSGNOcolumn.Seethedocumentationfor--DBMS_MVIEW.EXPLAIN_MVIEW()fordetails.RELATED_NUMNUMBER,--Whenthereisanumericvalue--associatedwitharow,itgoeshere.MSGNOINTEGER,--Whenavailable,QSMmessage#explaining--whydisabledormoredetailswhen--enabled.MSGTXTVARCHAR(2000),--TextassociatedwithMSGNO.SEQNUMBER);--UsefulinORDERBYclausewhen--selectingfromthistable.

Youcanusetheutlxmv.sql scriptfoundintheadmin directorytocreateMV_CAPABILITIES_TABLE .

()Example88DBMS_MVIEW.EXPLAIN_MVIEW

First,createthematerializedview.Alternatively,youcanuseEXPLAIN_MVIEWonapotentialmaterializedviewusingitsSELECT statementorthecompleteCREATE MATERIALIZED VIEW statement.

CREATEMATERIALIZEDVIEWcal_month_sales_mvBUILDIMMEDIATEREFRESHFORCEENABLEQUERYREWRITEASSELECTt.calendar_month_desc,SUM(s.amount_sold)ASdollarsFROMsaless,timestWHEREs.time_id=t.time_idGROUPBYt.calendar_month_desc;

Then,youinvokeEXPLAIN_MVIEW withthematerializedviewtoexplain.YouneedtousetheSEQ columninanORDER BY clausesotherowswilldisplayinalogicalorder.Ifacapabilityisnotpossible,N willappearintheP columnandanexplanationintheMSGTXT column.Ifacapabilityisnotpossibleformorethanonereason,arowisdisplayedforeachreason.

EXECUTEDBMS_MVIEW.EXPLAIN_MVIEW('SH.CAL_MONTH_SALES_MV');

SELECTcapability_name,possible,SUBSTR(related_text,1,8)ASrel_text,SUBSTR(msgtxt,1,60)ASmsgtxtFROMMV_CAPABILITIES_TABLEORDERBYseq;

CAPABILITY_NAMEPREL_TEXTMSGTXT------------------------------PCTNREFRESH_COMPLETEYREFRESH_FASTNREWRITEYPCT_TABLENSALESnopartitionkeyorPMARKERinselectlistPCT_TABLENTIMESrelationisnotapartitionedtableREFRESH_FAST_AFTER_INSERTNSH.TIMESmvlogmusthavenewvaluesREFRESH_FAST_AFTER_INSERTNSH.TIMESmvlogmusthaveROWIDREFRESH_FAST_AFTER_INSERTNSH.TIMESmvlogdoesnothaveallnecessarycolumnsREFRESH_FAST_AFTER_INSERTNSH.SALESmvlogmusthavenewvaluesREFRESH_FAST_AFTER_INSERTNSH.SALESmvlogmusthaveROWIDREFRESH_FAST_AFTER_INSERTNSH.SALESmvlogdoesnothaveallnecessarycolumnsREFRESH_FAST_AFTER_ONETAB_DMLNDOLLARSSUM(expr)withoutCOUNT(expr)REFRESH_FAST_AFTER_ONETAB_DMLNseethereasonwhyREFRESH_FAST_AFTER_INSERTisdisabledREFRESH_FAST_AFTER_ONETAB_DMLNCOUNT(*)isnotpresentintheselectlistREFRESH_FAST_AFTER_ONETAB_DMLNSUM(expr)withoutCOUNT(expr)REFRESH_FAST_AFTER_ANY_DMLNseethereasonwhyREFRESH_FAST_AFTER_ONETAB_DMLisdisabledREFRESH_FAST_AFTER_ANY_DMLNSH.TIMESmvlogmusthavesequenceREFRESH_FAST_AFTER_ANY_DMLNSH.SALESmvlogmusthavesequenceREFRESH_PCTNPCTisnotpossibleonanyofthedetailtablesinthematerializedviewREWRITE_FULL_TEXT_MATCHYREWRITE_PARTIAL_TEXT_MATCHYREWRITE_GENERALYREWRITE_PCTNPCTisnotpossibleonanydetailtables

SeeAlso:

Chapter15,"MaintainingtheDataWarehouse"(refresh.htm#g1018554)andChapter18,"AdvancedQueryRewrite"(qradv.htm#g1045552)forfurtherdetailsaboutPCT

()

MV_CAPABILITIES_TABLE.CAPABILITY_NAMEDetails

Table87listsexplanationsforvaluesintheCAPABILITY_NAME column.

()()Table87CAPABILITY_NAMEColumnDetails

CAPABILITY_NAME Description

PCT Ifthiscapabilityispossible,PartitionChangeTracking(PCT)ispossibleonatleastonedetailrelation.Ifthiscapabilityisnotpossible,PCTisnotpossiblewithanydetailrelationreferencedbythematerializedview.

REFRESH_COMPLETE Ifthiscapabilityispossible,completerefreshofthematerializedviewispossible.

REFRESH_FAST Ifthiscapabilityispossible,fastrefreshispossibleatleastundercertaincircumstances.

REWRITE Ifthiscapabilityispossible,atleastfulltextmatchqueryrewriteispossible.Ifthiscapabilityisnotpossible,noformofqueryrewriteispossible.

PCT_TABLE Ifthiscapabilityispossible,itispossiblewithrespecttoaparticularpartitionedtableinthetoplevelFROM list.Whenpossible,PCTappliestothepartitionedtablenamedintheRELATED_TEXT column.

PCTisneededtosupportfastfreshafterpartitionmaintenanceoperationsonthetablenamedintheRELATED_TEXT column.

PCTmayalsosupportfastrefreshwithregardtoupdatestothetablenamedintheRELATED_TEXTcolumnwhenfastrefreshfromamaterializedviewlogisnotpossible.

PCTisalsoneededtosupportqueryrewriteinthepresenceofpartialstalenessofthematerializedviewwithregardtothetablenamedintheRELATED_TEXT column.

Whendisabled,PCTdoesnotapplytothetablenamedintheRELATED_TEXT column.Inthiscase,fastrefreshisnotpossibleafterpartitionmaintenanceoperationsonthetablenamedintheRELATED_TEXT column.Inaddition,PCTbasedrefreshofupdatestothetablenamedintheRELATED_TEXT columnisnotpossible.Finally,queryrewritecannotbesupportedinthepresenceofpartialstalenessofthematerializedviewwithregardtothetablenamedintheRELATED_TEXT column.

PCT_TABLE_REWRITE Ifthiscapabilityispossible,itispossiblewithrespecttoaparticularpartitionedtableinthetoplevelFROM list.Whenpossible,PCTappliestothepartitionedtablenamedintheRELATED_TEXT column.

Thiscapabilityisneededtosupportqueryrewriteagainstthismaterializedviewinpartialstale

statewithregardtothetablenamedintheRELATED_TEXTcolumn.

Whendisabled,queryrewritecannotbesupportedifthismaterializedviewisinpartialstalestatewithregardtothetablenamedintheRELATED_TEXTcolumn.

REFRESH_FAST_AFTER_INSERT Ifthiscapabilityispossible,fastrefreshfromamaterializedviewlogispossibleatleastinthecasewheretheupdatesarerestrictedtoINSERT operationscompleterefreshisalsopossible.Ifthiscapabilityisnotpossible,noformoffastrefreshfromamaterializedviewlogispossible.

REFRESH_FAST_AFTER_ONETAB_DML Ifthiscapabilityispossible,fastrefreshfromamaterializedviewlogispossibleregardlessofthetypeofupdateoperation,providedallupdateoperationsareperformedonasingletable.Ifthiscapabilityisnotpossible,fastrefreshfromamaterializedviewlogmaynotbepossiblewhentheupdateoperationsareperformedonmultipletables.

REFRESH_FAST_AFTER_ANY_DML Ifthiscapabilityispossible,fastrefreshfromamaterializedviewlogispossibleregardlessofthetypeofupdateoperationorthenumberoftablesupdated.Ifthiscapabilityisnotpossible,fastrefreshfromamaterializedviewlogmaynotbepossiblewhentheupdateoperations(otherthanINSERT )affectmultipletables.

REFRESH_FAST_PCT Ifthiscapabilityispossible,fastrefreshusingPCTispossible.Generally,thismeansthatrefreshispossibleafterpartitionmaintenanceoperationsonthosedetailtableswherePCTisindicatedaspossible.

REWRITE_FULL_TEXT_MATCH Ifthiscapabilityispossible,fulltextmatchqueryrewriteispossible.Ifthiscapabilityisnotpossible,fulltextmatchqueryrewriteisnotpossible.

REWRITE_PARTIAL_TEXT_MATCH Ifthiscapabilityispossible,atleastfullandpartialtextmatchqueryrewritearepossible.Ifthiscapabilityisnotpossible,atleastpartialtextmatchqueryrewriteandgeneralqueryrewritearenotpossible.

REWRITE_GENERAL Ifthiscapabilityispossible,allqueryrewritecapabilitiesarepossible,includinggeneralqueryrewriteandfullandpartialtextmatchqueryrewrite.Ifthiscapabilityisnotpossible,atleastgeneralqueryrewriteisnotpossible.

REWRITE_PCT Ifthiscapabilityispossible,queryrewritecanuseapartiallystalematerializedvieweveninQUERY_REWRITE_INTEGRITY =ENFORCED orTRUSTED modes.Whenthiscapabilityisnotpossible,queryrewritecanuseapartiallystalematerializedviewonlyinQUERY_REWRITE_INTEGRITY =STALE_TOLERATED mode.

()

MV_CAPABILITIES_TABLEColumnDetails

Table88liststhesemanticsforRELATED_TEXT andRELATED_NUM columns.

()()Table88MV_CAPABILITIES_TABLEColumnDetails

MSGNO MSGTXT RELATED_NUM RELATED_TEXT

NULL NULL ForPCTcapabilityonly:[ owner .] name ofthetableuponwhichPCTisenabled

2066 ThisstatementresultedinanOracleerror

Oracleerrornumberthatoccurred

2067 NopartitionkeyorPMARKER orjoindependentexpressioninSELECT list

[ owner .] name ofrelationforwhichPCTisnotsupported

2068 Relationisnotpartitioned

[ owner .] name ofrelationforwhichPCTisnotsupported

2069 PCTnotsupportedwithmulticolumnpartitionkey

[ owner .] name ofrelationforwhichPCTisnotsupported

2070 PCTnotsupportedwiththistypeofpartitioning

[ owner .] name ofrelationforwhichPCTisnotsupported

2071 Internalerror:undefinedPCTfailurecode

TheunrecognizednumericPCTfailurecode

[ owner .] name ofrelationforwhichPCTisnotsupported

2072 Requirementsnotsatisfiedforfastrefreshofnestedmaterializedview

2077 Materializedviewlogisnewerthanlastfullrefresh

[ owner .] table_nameoftableuponwhichthematerializedviewlogisneeded

2078 Materializedviewlogmusthavenewvalues

[ owner .] table_nameoftableuponwhichthematerializedviewlogisneeded

2079 MaterializedviewlogmusthaveROWID

[ owner .] table_nameoftableuponwhichthematerializedviewlogisneeded

2080 Materializedviewlogmusthaveprimarykey

[ owner .] table_nameoftableuponwhichthematerializedviewlogisneeded

2081 Materializedviewlogdoesnothaveallnecessarycolumns

[ owner .] table_nameoftableuponwhichthematerializedviewlogisneeded

2082 Problemwithmaterializedviewlog

[ owner .] table_nameoftableuponwhichthematerializedviewlogisneeded

2099 MaterializedviewreferencesaremotetableorviewintheFROM list

OffsetfromtheSELECTkeywordtothetableorviewinquestion

[ owner .] name ofthetableorviewinquestion

2126 Multiplemastersites

Nameofthefirstdifferentnode,orNULL ifthefirstdifferentnodeislocal

2129 Joinorfiltercondition(s)arecomplex

[ owner .] name ofthetableinvolvedwiththejoinorfiltercondition(orNULL whennotavailable)

2130 Expressionnotsupportedforfastrefresh

OffsetfromtheSELECTkeywordtotheexpressioninquestion

ThealiasnameintheSELECT listoftheexpressioninquestion

2150 SELECT listsmustbeidenticalacrosstheUNIONoperator

OffsetfromtheSELECTkeywordtothefirstdifferentselectitemintheSELECT list

ThealiasnameofthefirstdifferentselectitemintheSELECT list

2182 PCTisenabledthroughajoindependency

[ owner .] name ofrelationforwhichPCT_TABLE_REWRITE isnotenabled

2183 ExpressiontoenablePCTnotin

TheunrecognizednumericPCT

[ owner .] name ofrelationforwhichPCTisnotenabled

PARTITIONBY ofanalyticfunctionormodel

failurecode

2184 ExpressiontoenablePCTcannotberolledup

[ owner .] name ofrelationforwhichPCTisnotenabled

2185 NopartitionkeyorPMARKER intheSELECTlist

[ owner .] name ofrelationforwhichPCT_TABLE_REWRITE isnotenabled

2186 GROUPOUTER JOINispresent

2187 Materializedviewonexternaltable

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