数据库系统概念全套配套课件PPT ch12.ppt

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1、Chapter12:QueryProcessingChapter12:QueryProcessingOverviewMeasuresofQueryCostSelectionOperationSortingJoinOperationOtherOperationsEvaluationofExpressionsBasicStepsinQueryProcessing1.Parsingandtranslation2.Optimization3.EvaluationBasicStepsinQueryProcessing(Cont.)Parsingandtransl

2、ationtranslatethequeryintoitsinternalform.Thisisthentranslatedintorelationalalgebra.Parsercheckssyntax,verifiesrelationsEvaluationThequery-executionenginetakesaquery-evaluationplan,executesthatplan,andreturnstheanswerstothequery.BasicStepsinQueryProcessing:OptimizationArelationa

3、lalgebraexpressionmayhavemanyequivalentexpressionsE.g.,salary75000(salary(instructor))isequivalenttosalary(salary75000(instructor))EachrelationalalgebraoperationcanbeevaluatedusingoneofseveraldifferentalgorithmsCorrespondingly,arelational-algebraexpressioncanbeevaluatedinm

4、anyways.Annotatedexpressionspecifyingdetailedevaluationstrategyiscalledanevaluation-plan.E.g.,canuseanindexonsalarytofindinstructorswithsalary<75000,orcanperformcompleterelationscananddiscardinstructorswithsalary75000BasicSteps:Optimization(Cont.)QueryOptimization:Amongstallequ

5、ivalentevaluationplanschoosetheonewithlowestcost.Costisestimatedusingstatisticalinformationfromthedatabasecataloge.g.numberoftuplesineachrelation,sizeoftuples,etc.InthischapterwestudyHowtomeasurequerycostsAlgorithmsforevaluatingrelationalalgebraoperationsHowtocombinealgorithmsf

6、orindividualoperationsinordertoevaluateacompleteexpressionInChapter14Westudyhowtooptimizequeries,thatis,howtofindanevaluationplanwithlowestestimatedcostMeasuresofQueryCostCostisgenerallymeasuredastotalelapsedtimeforansweringqueryManyfactorscontributetotimecostdiskaccesses,CPU,or

7、evennetworkcommunicationTypicallydiskaccessisthepredominantcost,andisalsorelativelyeasytoestimate.MeasuredbytakingintoaccountNumberofseeks*average-seek-costNumberofblocksread*average-block-read-costNumberofblockswritten*average-block-write-costCosttowriteablockisgreaterthancostt

8、oreadablockdataisreadbackafterbeingwrittentoens