Global-as-View

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Global-as-View (GaV, Global-als-Sicht) is a technical term from computer science that refers to the type of processing of data.

Global-as-View describes a pattern for merging schemas in the context of information integration . The core idea is that individual relations of the global schema are expressed as views of the local schemas of the data sources. In contrast to the reverse approach Local-as-View , which is based on the global schema, Global-as-View is mainly used in materialized integration - especially in data warehouse systems.

Constraints of the global scheme can be modeled in contrast to constraints of the local sources. Since only one relation of the global schema is ever modeled as a view, there are no associations across several relations of the global schema.

A typical feature of the generated views is a union (UNION) of several sources. The resulting duplicates and incomplete data records must be merged by means of duplicate detection and data fusion ( data cleansing ).

With global-local-as-view or both-as-view, there is a method that combines the properties of global-as-view and local-as-view by mapping views to views between global and local schema.

example

A global scheme with two relations is given:

  • Person: Ausweisnummer, Name, Jahrgang
  • Adresse: Ausweisnummer, Ort

These should be displayed as a view of the following local schemes:

  • Q1: Ausweisnummer, Name, Ort
  • Q2: Name, Ausweisnummer, Jahrgang
  • Q3: Ausweisnummer, Jahrgang, Beruf

Only source can be Q1used for the addresses (in SQL syntax): 

  CREATE VIEW Adresse AS 
   SELECT Ausweisnummer, Ort 
   FROM Q1

With the exception of the profession, personal data are contained in the source Q2and in combination in Q1and Q3: 

 CREATE VIEW Person AS
   SELECT Ausweisnummer, Name, Jahrgang 
   FROM Q2
   UNION
   SELECT Q1.Ausweisnummer, Q1.Name, Q3.Jahrgang 
   FROM Q1, Q3 
   WHERE Q1.Ausweisnummer = Q3.Ausweisnummer

Constraints of the global scheme can be adopted. For example, if only people from Munich are to be taken into account, the address view is: 

  CREATE VIEW Adresse AS 
   SELECT Ausweisnummer, Ort 
   FROM Q1 
   WHERE Ort="München"

Inquiry processing

The processing of requests to the global schema is very easy with Global-as-View with nested requests. Relations of the global schema are replaced directly by their views of the local sources. The result is a large nested query that can be optimized by reformulating it.

Example: Request for the above view: 

 SELECT * 
 FROM Person

would then be translated into: 

 SELECT * 
 FROM (
    SELECT Ausweisnummer, Name, Jahrgang 
    FROM Q2
    UNION
    SELECT Q1.Ausweisnummer, Q1.Name, Q3.Jahrgang 
    FROM Q1, Q3 
    WHERE Q1.Ausweisnummer = Q3.Ausweisnummer
  ) AS Person

literature