What is a data dictionary's role in RDBMS?

A data dictionary in RDBMS is a centralised repository of information about data such as meaning, relationships, origin, usage and format.

In a Relational Database Management System (RDBMS), a data dictionary plays a crucial role in maintaining the integrity, organisation and security of data. It is essentially a catalogue that contains metadata, which is data about data. This metadata provides a detailed description of all the objects within the database, including tables, indexes, columns, keys, constraints, views, procedures, and more.

The data dictionary is automatically updated by the RDBMS whenever changes are made to the database structure or schema. For instance, if a new table is created, the data dictionary will be updated with information about the table's name, the names and data types of its columns, any constraints applied to it, and so on. This automatic updating ensures that the data dictionary always provides an accurate and up-to-date reflection of the database's structure.

One of the key roles of a data dictionary is to enforce data integrity. It does this by storing the rules that define the relationships between tables and the constraints on the data they contain. These rules are then enforced by the RDBMS whenever data is added, updated or deleted. This helps to prevent inconsistencies and errors in the data.

The data dictionary also plays a vital role in database security. It stores information about user privileges and roles, which the RDBMS uses to control access to the data. This ensures that only authorised users can view or modify the data, protecting it from unauthorised access or manipulation.

Furthermore, the data dictionary aids in database performance tuning. It contains statistics about the data, such as the number of rows in each table and the distribution of values in the columns. These statistics can be used by the RDBMS's query optimiser to choose the most efficient way to execute queries, improving the performance of the database.

In summary, the data dictionary in an RDBMS is a vital tool for managing and maintaining the database. It provides a centralised source of information about the data, enforces data integrity, controls access to the data, and aids in performance tuning.