HELP! * BLUE=global GREY=local Global HTML version of Foils prepared July 23,1995

Foil 51 What are Two-phase Database Schema?

From Full Relational Database HPDC-4 Presentation HPDC-4 Tutorial/CPS616 Spring95 -- August 1, Pentagon City Virginia. by Gang Cheng Marek Podgorny (Geoffrey Fox) * See also color IMAGE

For a RDBMS there are two levels of abstractions of how data being stored and represented in the system:
1) Logical database consists of the conceptual view of a DB system which is described by an ER (entity-relationship) model, defined and access by SQL and represented by tables,columns,views and other abstractions of data-object;
2) The physical database consists of the actual phyical view of the DB which is represented by files,blocks,indexes,clusters,partitions,etc.
End-user and developers only need to deal with the logical level,
While the DBMS and a DBA (database administrator) define and perform the mapping between the two levels.
This is the reason why SQL achieves portability (oompared to f77), from a viewpoint of a data independent model.


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