Järjestelmäkatalogi

Järjestelmäkatalogi, known in English as a system catalog or data dictionary, is a collection of metadata about the data stored in a database system. It acts as a central repository of information that describes the structure, contents, and constraints of the database. This metadata includes details about tables, columns, data types, indexes, views, stored procedures, users, permissions, and other database objects.

The primary purpose of a system catalog is to provide the database management system (DBMS) with the necessary information to manage and process queries efficiently. When a user or application issues a query, the DBMS consults the system catalog to understand the structure of the data being accessed, locate the relevant data, and ensure that the query is valid according to the database's schema and constraints.

Beyond its role in query processing, the system catalog is also a valuable resource for database administrators and developers. It allows them to understand the database design, track changes, monitor performance, and enforce data integrity. Access to the system catalog is typically restricted to authorized users and is often managed through specific views or tables provided by the DBMS. Different database systems, such as PostgreSQL, MySQL, and Oracle, have their own implementations and naming conventions for their system catalogs, but the fundamental concept of storing metadata remains the same.