软件工程专业外文文献翻译物理数据库设计.docx

软件工程专业外文文献翻译物理数据库设计.docx

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软件工程专业外文文献翻译物理数据库设计

外文原文：

AHISTORICALPERSPECTIVE

Fromtheearliestdaysofcomputers,storingandmanipulatingdatahavebeenamajorapplicationfocus.Thefirstgeneral-purposeDBMSwasdesignedbyCharlesBachmanatGeneralElectricintheearly1960sandwascalledtheIntegratedDataStore.Itformedthebasisforthenetworkdatamodel,whichwasstandardizedbytheConferenceonDataSystemsLanguages（CODASYL）andstronglyinfluenceddatabasesystemsthroughthe1960s.BachmanwasthefirstrecipientofACM’sTuringAward（thecomputerscienceequivalentofaNobelprize）forworkinthedatabasearea;hereceivedtheawardin1973.Inthelate1960s,IBMdevelopedtheInformationManagementSystem（IMS）DBMS,usedeventodayinmanymajorinstallations.IMSformedthebasisforanalternativedatarepresentationframeworkcalledthehierarchicaldatamodel.TheSABREsystemformakingairlinereservationswasjointlydevelopedbyAmericanAirlinesandIBMaroundthesametime,anditallowedseveralpeopletoaccessthesamedatathroughcomputernetwork.Interestingly,todaythesameSABREsystemisusedtopowerpopularWeb-basedtravelservicessuchasTravelocity!

In1970,EdgarCodd,atIBM’sSanJoseResearchLaboratory,proposedanewdatarepresentationframeworkcalledtherelationaldatamodel.Thisprovedtobeawatershedinthedevelopmentofdatabasesystems:

itsparkedrapiddevelopmentofseveralDBMSsbasedontherelationalmodel,alongwitharichbodyoftheoreticalresultsthatplacedthefieldonafirmfoundation.Coddwonthe1981TuringAwardforhisseminalwork.Databasesystemsmaturedasanacademicdiscipline,andthepopularityofrelationalDBMSschangedthecommerciallandscape.Theirbenefitswerewidelyrecognized,andtheuseofDBMSsformanagingcorporatedatabecamestandardpractice.

Inthe1980s,therelationalmodelconsolidateditspositionasthedominantDBMSparadigm,anddatabasesystemscontinuedtogainwidespreaduse.TheSQLquerylanguageforrelationaldatabases,developedaspartofIBM’sSystemRproject,isnowthestandardquerylanguage.SQLwasstandardizedinthelate1980s,andthecurrentstandard,SQL-92,wasadoptedbytheAmericanNationalStandardsInstitute（ANSI）andInternationalStandardsOrganization（ISO）.Arguably,themostwidelyusedformofconcurrentprogrammingistheconcurrentexecutionofdatabaseprograms（calledtransactions）.Userswriteprogramsasiftheyaretoberunbythemselves,andtheresponsibilityforrunningthemconcurrentlyisgiventotheDBMS.JamesGraywonthe1999TuringawardforhiscontributionstothefieldoftransactionmanagementinaDBMS.

Inthelate1980sandthe1990s,advanceshavebeenmadeinmanyareasofdatabasesystems.Considerableresearchhasbeencarriedoutintomorepowerfulquerylanguagesandricherdatamodels,andtherehasbeenabigemphasisonsupportingcomplexanalysisofdatafromallpartsofanenterprise.Severalvendors（e.g.,IBM’sDB2,Oracle8,InformixUDS）haveextendedtheirsystemswiththeabilitytostorenewdatatypessuchasimagesandtext,andwiththeabilitytoaskmorecomplexqueries.Specializedsystemshavebeendevelopedbynumerousvendorsforcreatingdatawarehouses,consolidatingdatafromseveraldatabases,andforcarryingoutspecializedanalysis.

Aninterestingphenomenonistheemergenceofseveralenterpriseresourceplanning（ERP）andmanagementresourceplanning（MRP）packages,whichaddasubstantiallayerofapplication-orientedfeaturesontopofaDBMS.WidelyusedpackagesincludesystemsfromBaan,Oracle,PeopleSoft,SAP,andSiebel.Thesepackagesidentifyasetofcommontasks（e.g.,inventorymanagement,humanresourcesplanning,financialanalysis）encounteredbyalargenumberoforganizationsandprovideageneralapplicationlayertocarryoutthesetasks.ThedataisstoredinarelationalDBMS,andtheapplicationlayercanbecustomizedtodifferentcompanies,leadingtolowerIntroductiontoDatabaseSystemsoverallcostsforthecompanies,comparedtothecostofbuildingtheapplicationlayerfromscratch.Mostsignificantly,perhaps,DBMSshaveenteredtheInternetAge.WhilethefirstgenerationofWebsitesstoredtheirdataexclusivelyinoperatingsystemsfiles,theuseofaDBMStostoredatathatisaccessedthroughaWebbrowserisbecomingwidespread.QueriesaregeneratedthroughWeb-accessibleformsandanswersareformattedusingamarkuplanguagesuchasHTML,inordertobeeasilydisplayedinabrowser.AllthedatabasevendorsareaddingfeaturestotheirDBMSaimedatmakingitmoresuitablefordeploymentovertheInternet.Databasemanagementcontinuestogainimportanceasmoreandmoredataisbroughton-line,andmadeevermoreaccessiblethroughcomputernetworking.Todaythefieldisbeingdrivenbyexcitingvisionssuchasmultimediadatabases,interactivevideo,digitallibraries,ahostofscientificprojectssuchasthehumangenomemappingeffortandNASA’sEarthObservationSystemproject,andthedesireofcompaniestoconsolidatetheirdecision-makingprocessesandminetheirdatarepositoriesforusefulinformationabouttheirbusinesses.Commercially,databasemanage-mentsystemsrepresentoneofthelargestandmostvigorousmarketsegments.Thusthes-tudyofdatabasesystemscouldprovetoberichlyrewardinginmorewaysthanone!

INTRODUCTIONTOPHYSICALDATABASEDESIGN

Likeallotheraspectsofdatabasedesign,physicaldesignmustbeguidedbythenatureofthedataanditsintendeduse.Inparticular,itisimportanttounderstandthetypicalworkloadthatthedatabasemustsupport;theworkloadconsistsofamixofqueriesandupdates.Usersalsohavecertainrequirementsabouthowfastcertainqueriesorupdatesmustrunorhowmanytransactionsmustbeprocessedpersecond.Theworkloaddescriptionandusers’performancerequirementsarethebasisonwhichanumberofdecisionshavetobemadeduringphysicaldatabasedesign.

Tocreateagoodphysicaldatabasedesignandtotunethesystemforperformanceinresponsetoevolvinguserrequirements,thedesignerneedstounderstandtheworkingsofaDBMS,especiallytheindexingandqueryprocessingtechniquessupportedbytheDBMS.Ifthedatabaseisexpectedtobeaccessedconcurrentlybymanyusers,orisadistributeddatabase,thetaskbecomesmorecomplicated,andotherfeaturesofaDBMScomeintoplay.

DATABASEWORKLOADS

Thekeytogoodphysicaldesignisarrivingatanaccuratedescriptionoftheexpectedworkload.Aworkloaddescriptionincludesthefollowingelements:

1.Alistofqueriesandtheirfrequencies,asafractionofallqueriesandupdates.

2.Alistofupdatesandtheirfrequencies.

3.Performancegoalsforeachtypeofqueryandupdate.

Foreachqueryintheworkload,wemustidentify:

Whichrelationsareaccessed.

Whichattributesareretained（intheSELECTclause）.

Whichattributeshaveselectionorjoinconditionsexpressedonthem（intheWHEREclause）andhowselectivetheseconditionsarelikelytobe.Similarly,foreachupdateintheworkload,wemustidentify:

Whichattributeshaveselectionorjoinconditionsexpressedonthem（intheWHEREclause）andhowselectivetheseconditionsarelikelytobe.

Thetypeofupdate（INSERT,DELETE,orUPDATE）andtheupdatedrelation.

ForUPDATEcommands,thefieldsthataremodifiedbytheupdate.

Rememberthatqueriesandupdatestypicallyhaveparameters,forexample,adebitorcreditoperationinvolvesaparticularaccountnumber.Thevaluesoftheseparametersdetermineselectivityofselectionandjoinconditions.

Updateshaveaquerycomponentthatisusedtofindthetargettuples.Thiscomponentcanbenefitfromagoodphysicaldesignandthepresenceofindexes.Ontheotherhand,updatestypicallyrequireadditionalworktomaintainindexesontheattributesthattheymodify.Thus,whilequeriescanonlybenefitfromthepresenceofanindex,anindexmayeitherspeeduporslowdownagivenupdate.Designersshouldkeepthistrade-offerinmindwhencreatingindexes.

NEEDFORDATABASETUNING

Accurate,detailedworkloadinformationmaybehardtocomebywhiledoingtheinitialdesignofthesystem.Consequently,tuningadatabaseafterithasbeendesignedanddeployedisimportant—wemustrefinetheinitialdesigninthelightofactualusagepatternstoobtainthebestpossibleperformance.

Thedistinctionbetweendatabasedesignanddatabasetuningissomewhatarbitrary.

Wecouldconsiderthedesignprocesstobeoveronceaninitialconceptualschemaisdesignedandasetofindexingandclusteringdecisionsismade.Anysubsequentchangestotheconceptualschemaortheindexes,say,wouldthenberegardedasatuningactivity.Alternatively,wecouldconsidersomerefinementoftheconceptualschema（andphysicaldesigndecisionsaffectedbythisrefinement）tobepartofthephysicaldesignprocess.

Wherewedrawthelinebetweendesignandtuningisnotveryimportant.

OVERVIEWOFDATABASETUNING

Aftertheinitialphaseofdatabasedesign,actualuseofthedatabaseprovidesavaluablesourceofdetailedinformationthatcanbeusedtorefinetheinitialdesign.Manyoftheoriginalassumptionsabouttheexpectedworkloadcanbereplacedbyobservedusagepatterns;ingeneral,someoftheinitialworkloadspecificationwillbevalidated,andsomeofitwillturnouttobewrong.Initialguessesaboutthesizeofdatacanbereplacedwithactualstatisticsfromthesystemcatalogs（althoughthisinformationwillkeepchangingasthesystemevolves）.Carefulmonitoringofqueriescanrevealunexpectedproblems;forexample,theoptimizermaynotbeusingsomeindexesasintendedtoproducegoodplans.

Continueddatabasetuningisimportanttogetthebestpossibleperformance.

TUNINGTHE