1.8- 3 Schema Architecture of DBMS | Three Schema Architecture Of DBMS| Gate Net DBMS Tutorials
1.8- 3 Schema Architecture of DBMS | Three Schema Architecture Of DBMS| Gate Net DBMS Tutorials
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Database approach had the following characteristics
(1) self-describing nature of database
(2) insulation of programs and data
(3) support of multiple user views.
To help achieve and visualize these characteristics Three schema architecture was proposed. To separate the user applications from the physical database, schemas can be defined at the following three levels:
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1. The internal level has an internal schema, which describes the physical storage structure of the database. The internal schema uses a physical data model and describes the complete details of data storage and access paths for the database.
2. The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of users. The conceptual schema hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations, and constraints. Usually, a representational data model is used to describe the conceptual schema when a database system is implemented.
3. The external or view level includes a number of external schemas or user
views. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group.
In the three-schema architecture, each user group refers to its own external
schema. Hence, the DBMS must transform a request specified on an external schema into a request against the conceptual schema, and then into a request on the internal schema for processing over the stored database. If the request is a database retrieval, the data extracted from the stored database must be reformatted to match the user’s external view. The processes of transforming requests and results between levels are called mappings.
Data Independence It is defined as the capacity to change the schema at one level of a database system
without having to change the schema at the next higher level.
There are two types of data independence:
a) Logical data independence is the capacity to change the conceptual
schema without having to change external schemas or application
programs. We may change the conceptual schema to expand the database
(by adding a record type or data item), to change constraints, or to reduce
the database (by removing a record type or data item. External schemas
that refer only to the remaining data should not be affected. Only the view
definition and the mappings need to be changed in a DBMS that supports
logical data independence.
b) Physical data independence is the capacity to change the internal schema without having to change the conceptual schema. Hence, the external
schemas need not be changed as well. Changes to the internal schema may be needed because some physical files were reorganized— by creating
additional access structures—to improve the performance of retrieval or
update.
Logical Data independence Physical Data independence is the capacity to change the conceptual schema without having to change external schemas or application programs. is the capacity to change the internal schema without having to change the conceptual schema. it is difficult to achieve logical data independence. it is easier to achieve logical data independence.
We may change the conceptual schema to expand the database (by adding a record type or data item), to change constraints, or to reduce the database (by removing a record type or data item.
Changes to the internal schema may be needed because some physical files were reorganized— by creating additional access structures—to
improve the performance of retrieval or update. Notes Link:
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