Data files Indices and Data Dictionary

The data is stored in the data files. The indices are stored in the index files. Indices
provide fast access to data items. For example, a book database may be organised in
the order of Accession number, yet may be indexed on Author name and Book titles.

Data Dictionary:

A Data Dictionary stores information about the structure of the
database. It is used heavily. Hence a good data dictionary should have a good design
and efficient implementation. It is seen that when a program becomes somewhat large
in size, keeping track of all the available names that are used and the purpose for
which they were used becomes more and more difficult. After a significant time if the
same or another programmer has to modify the program, it becomes extremely
difficult.
The problem becomes even more difficult when the number of data types that an
organisation has in its database increases. The data of an organisation is a valuable
corporate resource and therefore some kind of inventory and catalog of it must be
maintained so as to assist in both the utilisation and management of the resource.
It is for this purpose that a data dictionary or dictionary/directory is emerging as a
major tool. A dictionary provides definitions of things. A directory tells you where to
find them. A data dictionary/directory contains information (or data) about the data.
A comprehensive data dictionary would provide the definition of data items, how they
fit into the data structure and how they relate to other entities in the database. In
DBMS, the data dictionary stores the information concerning the external, conceptual
and internal levels of the databases. It would combine the source of each data field
value, that is from where the authenticate value is obtained. The frequency of its use
and audit trail regarding the updates including user identification with the time of each
update is also recorded in Data dictionary.
The Database administrator (DBA) uses the data dictionary in every phase of a
database life cycle, starting from the data gathering phase to the design,
implementation and maintenance phases. Documentation provided by a data
dictionary is as valuable to end users and managers, as it is essential to the

programmers. Users can plan their applications with the database only if they know
exactly what is stored in it. For example, the description of a data item in a data
dictionary may include its origin and other text description in plain English, in
addition to its data format. Thus, users and managers will be able to see exactly what
is available in the database. A data dictionary is a road map which guides users to
access information within a large database.
An ideal data dictionary should include everything a DBA wants to know about the
database.
1. External, conceptual and internal database descriptions.
2. Descriptions of entities (record types), attributes (fields), as well as crossreferences,
origin and meaning of data elements.
3. Synonyms, authorisation and security codes.
4. Which external schemas are used by which programs, who the users are, and
what their authorisations are.
5. Statistics about database and its usage including number of records, etc.
A data dictionary is implemented as a database so that users can query its contents.
The cost effectiveness of a data dictionary increases as the complexity of an
information system increases. A data dictionary can be a great asset not only to the
DBA for database design, implementation and maintenance, but also to managers or
end users in their project planning.

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