1AsimilarquerywasusedtoillustratetheParallaxsystemin5 ID: 267474
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Figure1:ExampleofsemanticannotationNotethatintermsofthisdenition,theannotationofadocumentwithconventionalfacetsissimplyasubsetofVNVL;thatis,documentscannotbeusedtoannotateotherdocuments.EXAMPLE2.AssumethatVDincludes(i)documentsPaboutpoliticians;(ii)documentsCaboutchildrenofpoliticians,includ-ingadocumentdtrjaboutTheodoreRooseveltJr.anddkraboutKer-mitRoosevelt;(iii)asetof(documentsabout)countries,includingdusaaboutUSA;and(iv)asetof(documentsabout)universities,in-cludingdhforHarvardUniversityanddsforStanfordUniversity.Moreover,assumethatVLincludesthestringpresident.Asillus-tratedinFigure1,thedocumentsofParetype-annotatedwithval-uesfromVL(oneofwhichispresident),has_child-annotatedwithdocumentfromC,andis_citizen_of-annotatedwithdocumentsabo-utcountries.Finally,thedocumentsofCareis_graduated_from-annotatedwithuniversitydocuments.FacetedQueries.Aconventionalfacetedquerycanbeseenasalteroverasetofdocuments:itspecieswhatfacetnamesshouldbeusedtoannotateatargetdocumentandwhatfacetedval-uesshouldbeusedforeachofthesenames.Theanswerforsuchaqueryisthesubsetofthegivendocumentssatisfyingthelter.Inthesemanticfacetedsearchapproach,wegofurther:asemanticfacetedquerycanrelatesetsS1,:::,Snofdocumentswithannota-tionsbydescribingrelationsbetweenthesesetsandsettingseparateltersovereachoneofthem.Analogouslytoconventionalfacetedqueries,theeffectoftheseltersistoselectsubsetsS01S1,:::,S0nSn.Incontrasttoaconventionalfacetedquery,wecannownavigatebetweendifferenttypesofdocumentsandselectwhatdocumentsS0iSiaretobereturnedasqueryoutput.Inwhatfollows,weformalisethenotionoffacetedquery,start-ingwiththenotionofbasicfacetedquery.DEFINITION3.Abasicfacetedquery(BFQ)overVisapairF=(X;),whereX2VNandVL[VD.Intuitively,adocumentsatisesaBFQ(X;)ifitisX-annotatedwithsomevaluein.WeillustrateBFQsonanexample.EXAMPLE4.ThefollowingareBFQsaboutpoliticians:Q1P=(type;fpresidentg);Q3P=(has_child;fg);Q2P=(is_citizen_of;fdusag):ThequeryQ1Pasksfor(allthedocumentsabout)presidents,Q2PforpoliticianswithAmericancitizenship,Q3Pforpoliticianswithchildren.TheemptyconditionfgisusedinQ2Psinceweimposenorestrictionsonthechildren.Moreover,theBFQQC=(is_graduated_from;fdh;dsg);overthechildrenofpoliticiansinCasksfordocumentsaboutchil-drenofpoliticiansgraduatedfromHarvard(dh)orStanford(ds).WenowdenehowtocombineBFQsincomplexqueries.Facet-edsearchistypicallyinitiatedbyakeywordsearchthatreturnstheinitialsetofdocumentsoverwhichausercansetlters.Tomodelkeyword-basedsearch,weintroduceaspecialsetKWdisjointfromV,andassumethateveryelementS2KWstandsforasetofdocumentsretrievedbyaspecickeyword-basedsearch.DEFINITION5.A(faceted)queryexpressionexpr::=startjstart=restoverVisdenedbythefollowinggrammar:start::=Sj?(S),rest::=pathj(pathpath),step::=Fj?(F),path::=stepjstep=rest,where2f^;_g,S2KWandFisaBFQoverV.A(semantic)facetedquery(FQ)overVisaqueryexpressionoverVwherethesymbol?occursexactlyonce.Thestartingpoint(start)ofaqueryexpressionexprcapturesthecontentoftheinitialsearchusedintypicalinterfaces(priortofacetednavigation),whichisgivenbyakeywordsearch.Ineachstep(step)oftheremainderofthequeryexpression(rest),aBFQisappliedtonarrowdownthesearchresults.Thesestepsareputtogetherinpaths(path),whereeachpathisasequenceofBFQsthathavetobeappliedaccordingtothesequenceorder.Abranch-ing(path1^path2)canbeappliedinaqueryexpressionindicatingthateachofthesearchresultshastosatisfythesequenceofcondi-tionsspeciedbybothpath1andpath2.Finally,thesymbol?inaqueryexpressionindicateswhichdocumentsmustbereturned.Inafacetedquerythissymbolismentionedexactlyonce,astheresultofsuchqueryisonesetofdocuments.WeillustratethesyntaxandsemanticsofFQsinthefollowingexample.EXAMPLE6.ConsiderthefollowinginformationrequestaboutthescenariointroducedinExamples2and4:FindchildrenofUSpresidentswhograduatedfromHarvardUniversityorStanfordUniversity.1ItcorrespondstothefollowingFQ:Spolitician=(Q1P^Q2P^(?(Q3P)=QC));(1)Intuitively,thequeryrstretrievesalldocumentsofSpolitician2KW,i.e.,alldocumentsreturnedbyakeywordsearchwithpoliticians.ThentheseresultsarelteredwithQ1PandQ2P,obtainingthesetofdocumentsaboutAmericanpresidents.Finally,thissetislteredbyusing?(Q3P)=QC,returningthesetofdocumentsaboutchildrenofAmericanpresidentswhograduatedfromHarvardorStanford.SemanticFacetedSearchusingRDFandSPARQL.Se-manticannotationscanberealisedusingdifferenttechnologies.OurdemoimplementationexploitsRDF,whichweseeasthemostnaturalchoice:URIscanbeseenasdocumentsofVD,objectanddatapropertiesasfacet-namesofVN,literalsaslabelsofVL.More-over,annotationsareencodedinRDFastriples:eachtriple(s;p;o)correspondstoap-annotationofadocumentswithafacet-valueo.AswewillseeinSections3and4,RDFgivesmorethanthat:bycombiningRDFwithRDFSandOWL2,onecanusereasoningtoaddressnaturallimitationsofcurrentfacetedsearch.WenowdiscusshowtoembedourfacetedquerylanguageinSPARQL1.1,thestandardlanguageforqueryingRDF.Onecanprovidesemanticstoourfacetedquerylanguagebytranslationintorst-orderlogicandshowthatthatanyFQcorrespondstoalter-freeSPARQL1.1queryQthat:(i)ispositive(neitherOPTIONALnorNOTEXISTSnorMINUSconstructoccursinQ);(ii)istree-shaped(thedependencygraphofQ'svariables,where?xand?yareconnectedifthereisatriple?xZ?yinQ,istree-shaped);and 1AsimilarquerywasusedtoillustratetheParallaxsystemin[5].