数据库系统概念第六版课后习题部分答案25s

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1、CHAPTER25AdvancedDataTypesandNewApplicationsPracticeExercises25.1Whatarethetwotypesoftime,andhowaretheydifferent?Whydoesitmakesensetohavebothtypesoftimeassociatedwithatuple?Answer:Atemporaldatabasemodelsthechangingstatesofsomeaspectsoftherealworld.Thetimeintervalsr

2、elatedtothedatastoredinatemporaldatabasemaybeoftwotypes-validtimeandtransactiontime.Thevalidtimeforafactisthesetofintervalsduringwhichthefactistrueintherealworld.Thetransactiontimeforadataobjectisthesetoftimeintervalsduringwhichthisobjectispartofthephysicaldatabase

3、.Onlythetransactiontimeissystemdependentandisgeneratedbythedatabasesystem.Supposeweconsideroursamplebankdatabasetobebitemporal.Onlytheconceptofvalidtimeallowsthesystemtoanswerqueriessuchas-“WhatwasSmith’sbalancetwodaysago?”.Ontheotherhand,queriessuchas-“Whatdidwere

4、cordasSmith’sbalancetwodaysago?”canbeansweredbasedonthetransactiontime.Thedifferencebetweenthetwotimesisimportant.Forexample,suppose,threedaysagothetellermadeamistakeinenteringSmith’sbalanceandcorrectedtheerroronlyyesterday.Thiserrormeansthatthereisadifferencebetwe

5、entheresultsofthetwoqueries(ifbothofthemareexecutedtoday).25.2Supposeyouhavearelationcontainingthex,ycoordinatesandnamesofrestaurants.Supposealsothattheonlyqueriesthatwillbeaskedareofthefollowingform:Thequeryspecifiesapoint,andasksifthereisarestaurantexactlyatthatpo

6、int.Whichtypeofindexwouldbepreferable,R-treeorB-tree?Why?Answer:Thegivenqueryisnotarangequery,sinceitrequiresonlysearchingforapoint.ThisquerycanbeefficientlyansweredbyaB-treeindexonthepairofattributes(x,y).12Chapter25AdvancedDataTypesandNewApplications25.3Supposeyou

7、haveaspatialdatabasethatsupportsregionqueries(withcircularregions)butnotnearest-neighborqueries.Describeanalgo-rithmtofindthenearestneighborbymakinguseofmultipleregionqueries.Answer:SupposethatwewanttosearchforthenearestneighborofapointPinadatabaseofpointsintheplane

8、.TheideaistoissuemultipleregionqueriescenteredatP.Eachregionquerycoversalargerareaofpointsthanthepreviousquery.Theprocedurestopswhentheresultofar