TY - JOUR
T1 - Schema evolution and the relational algebra
AU - McKenzie, Edwin
AU - Snodgrass, Richard
N1 - Funding Information:
ation names a schemav ersion to which it is to be ture specificationsin the commandsd efine-relation applied, and an update to one version of an object and modify-relation be a relation’sc urrents ignature could affectt he data storedi n other versions.I n our or a constant.T his extensionw ould supports ignature model, only the most recentv ersionc an be updated, changes dependento n both the current and past and updates do not affect other versions. In the signatureso f relationsi n the database. ENCORE object-orientedD BMS [54,5 51,v ersions A third extensionw ould removet he requirements are identified by transaction numbers, as in our of a relation’ss chemab eing constanto ver the trans-model, but updatesc an be made to any version, as action interval specifiedi n the rollback operation. in the other object-orientedm odels. However, in The major problem is in calculatingt he schemaf or ENCORE, an update applied to one version does the resultingr elation.A generalb ut simplea pproach not affect the other versions. At the risk of over-has not yet been found. simplifying,o bject-orientedsc hemav ersionsr esemble Finally, the effecto f schemae volution on appli-traditional relational views [56] and do not involve cations programsa ccessingt he databases hould be transactiont ime as definedi n this paper. considered [52]. Maintaining consistency between Severalr esearcherhsa veu sedd enotationals eman-such programs and the databases chemab ecomes tics to define formally the semanticso f databases, mored ifficult. Similarly, queryp re-compilations, uch DBMS’s and query languages.S ubieta proposesa n as performedi n SystemR [60],m ay or may not be approachf or definingq ueryl anguagesfo rmally using effectived, ependingo n whethert he time-stampsp ro-denotdtiondl semantics[ 57]. This approach allows vided to the rollback operatorsa re constantso r are powerful query languagesw ith precises emanticst o values supplied by the application program. Howbe definedf or most databasem odels.R ishe proposes ever, it appears that techniquess imilar to those that denotational semanticsb e used to provide a employedb y the WAND system,t hosea ppearingi n uniform treatmento f databases emanticsa t different ORION, and those proposed [46] could serve to information levels based on hierarchieso f domains amortizet he cost of schemac hanges. of mappingsf rom “less semantic” representationosf information into “more semantic” representations Acknowledgements-We wish to thank Bharat Jayaraman [58].N either Subietan or Rishe, however,i nclude in and Peter Mills for suggesting many corrections and im- their approachesa ny facilitiesf or dealingw ith trans-significant improvements to the formalism used here. This provements to this paper and the referees for suggesting action time or an evolving schema.L ee proposesa research was supported by NSF Grants DCR-8402339 and denotationals emanticsf or administratived atabases, IRI-8902707, and ONR grant N00014-86-K-0680. Research where databasesa re regarded as a collection of by the first author also was supported by the United States logical assertions[ 59]. Here. the denotation of an Air Force. Research by the second author was supported in expressionin a first-orderp redicatec alculusi s based, part by an IBM Faculty Development Award. in part. on its evaluation in a time dimension, analogous to valid time, in a possible world, analogoust o a cross-sectiono f a databases tatea t a transaction. An obvious next step would be to implementa n evolving schema that fully supports the rollback operator.O ne approachw e are consideringc onverts the system relations in our prototype [32] to be rollback relations,r ather than snapshotr elationsa s they are now. Changes to the semantic analysis portions of the query analysisw ould be required,b ut it appearst hat changest o the backendo f the DBMS would be minimal. Another step would be to investigatee xtensions to the language.A straightforwarde xtensiono f the languagew ould introduce algebraic operatorst hat map betweent he domain of snapshots tatesa nd the domain of historicals tatesd irectly.T he introduction of such operatorsi nto the snapshota nd historical algebrasw ould rendert he algebrasm ultisorted.B e- causet he two algebras,w ithout theseo perators,a re umsorteda nd becausew e wish to retain this property for the algebras,w e have electedn ot to introduce such conversiono peratorsi nto our language. A seconde xtensionw ould introducea n algebrao f signaturesa, nalogoust o the algebraso f snapshota nd historicals tates,t o removet he restrictiont hat signa-
PY - 1990
Y1 - 1990
N2 - In this paper we discuss extensions to the conventional relational algebra to support both aspects of transaction time, evolution of a database's contents and evolution of a database's schema. We define a relation's schema to be the relation's temporal signature, a function mapping the relation's attribute names onto their value domains and class, indicating the extent of support for time. We also introduce commands to change a relation, now defined as a triple consisting of a sequence of classes, a sequence of signatures, and a sequence of states. A semantic type of system is required to identify semantically incorrect expressions and to enforce consistency constraints among a relation's class, signature and state following update. We show that these extensions are applicable, without change, to historical algebras that support valid time, yielding an algebraic language for the query and update of temporal databases. The additions preserve the useful properties of the conventional algebra.
AB - In this paper we discuss extensions to the conventional relational algebra to support both aspects of transaction time, evolution of a database's contents and evolution of a database's schema. We define a relation's schema to be the relation's temporal signature, a function mapping the relation's attribute names onto their value domains and class, indicating the extent of support for time. We also introduce commands to change a relation, now defined as a triple consisting of a sequence of classes, a sequence of signatures, and a sequence of states. A semantic type of system is required to identify semantically incorrect expressions and to enforce consistency constraints among a relation's class, signature and state following update. We show that these extensions are applicable, without change, to historical algebras that support valid time, yielding an algebraic language for the query and update of temporal databases. The additions preserve the useful properties of the conventional algebra.
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U2 - 10.1016/0306-4379(90)90036-O
DO - 10.1016/0306-4379(90)90036-O
M3 - Article
AN - SCOPUS:0025628863
SN - 0306-4379
VL - 15
SP - 207
EP - 232
JO - Information Systems
JF - Information Systems
IS - 2
ER -