ORACLE教程DBA_les05_undo

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1、ManagingUndoDataObjectivesAftercompletingthislesson,youshouldbeabletodothefollowing:DescribethepurposeofundodataImplementAutomaticUndoManagementCreateandconfigureundosegmentsObtainundosegmentinformationfromthedatadictionaryManagingUndoDataOverviewTwomethodsformanag

2、ingundodataexistsAutomaticUndoManagementManualUndoManagementThislessondiscussesAutomaticUndoManagementThetermundoreplaceswhatwasknowninpreviousversionsofOracleasrollbackUndoSegmentUpdatetransactionOldimageNewimageUndosegmentTableUndoSegments:PurposeTransactionrollb

3、ackTransactionrecoveryUndosegmentReadconsistencyReadConsistencyImageatstartofstatementNewimageTableSELECT*FROMtableTypesofUndoSegmentsSYSTEM:UsedforobjectsintheSYSTEMtablespaceNon-SYSTEM:Usedforobjectsinothertablespaces:AutoMode:RequiresanUNDOtablespaceManualMode:

4、Private:AcquiredbyasingleinstancePublic:AcquiredbyanyinstanceDeferred:Usedwhentablespacesaretakenofflineimmediate,temporary,orforrecoveryAutomaticUndoManagement:ConceptsUndodataismanagedusinganUNDOtablespaceYouallocateoneUNDOtablespaceperinstancewithenoughspacefor

5、theworkloadoftheinstance.TheOracleserverautomaticallymaintainsundodatawithintheUNDOtablespace.AutomaticUndoManagement:ConfigurationConfiguretwoparametersintheinitializationfile:UNDO_MANAGEMENTUNDO_TABLESPACECreateatleastoneUNDOtablespace.Initializationfileundo1db0

6、1.dbfUNDOTablespaceAutomaticUndoManagement:InitializationParametersUNDO_MANAGEMENT:SpecifieswhetherthesystemshoulduseAUTOorMANUALmode.UNDO_TABLESPACE:SpecifiesaparticularUNDOtablespacetobeused.UNDO_MANAGEMENT=AUTOUNDO_TABLESPACE=UNDOTBSAutomaticUndoManagement:UNDO

7、TablespaceYoumaycreatetheUNDOtablespacewiththedatabasebyaddingaclauseintheCREATEDATABASEcommandOrcreateitlaterbyusingtheCREATEUNDOTABLESPACEcommandCREATEDATABASEdb01...UNDOTABLESPACEundo1DATAFILE'undo1db01.dbf'SIZE20MAUTOEXTENDONCREATEUNDOTABLESPACEundo1DATAFILE'un

8、do1db01.dbf'SIZE20M;AutomaticUndoManagement:AlteringanUNDOTablespaceTheALTERTABLESPACEcommandcanmakechangestoUNDOtablespaces.Thefollowingexample