LEC-2: DBMS Architecture
1. View of Data (Three Schema Architecture)
a. The major purpose of DBMS is to provide users with an abstract view of the data. That is, the
system hides certain details of how the data is stored and maintained.
b. To simplify user interaction with the system, abstraction is applied through several levels of
abstraction.
c. The main objective of three level architecture is to enable multiple users to access the same data
with a personalized view while storing the underlying data only once
d. Physical level / Internal level
i. The lowest level of abstraction describes how the data are stored.
ii. Low-level data structures used.
iii. It has Physical schema which describes physical storage structure of DB.
iv. Talks about: Storage allocation (N-ary tree etc), Data compression & encryption etc.
v. Goal: We must define algorithms that allow efficient access to data.
e. Logical level / Conceptual level:
i. The conceptual schema describes the design of a database at the conceptual level,
describes what data are stored in DB, and what relationships exist among those data.
ii. User at logical level does not need to be aware about physical-level structures.
iii. DBA, who must decide what information to keep in the DB use the logical level of
abstraction.
iv.Goal: ease to use.
f. View level / External level:
i. Highest level of abstraction aims to simplify users’ interaction with the system by
providing different view to different end-user.
ii. Each view schema describes the database part that a particular user group is interested
and hides the remaining database from that user group.
iii. At the external level, a database contains several schemas that sometimes called as
subschema. The subschema is used to describe the different view of the database.
iv. At views also provide a security mechanism to prevent users from accessing certain parts
of DB.
g.
2. Instances and Schemas
a. The collection of information stored in the DB at a particular moment is called an instance of DB.
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