DB2 Catalog

Enhanced Definition

The DB2 Catalog is a fundamental set of system tables within a DB2 for z/OS subsystem that stores all metadata about the DB2 environment. It contains definitions, attributes, relationships, and statistics for every DB2 object, including tables, indexes, views, stored procedures, plans, packages, users, and privileges. It is essential for DB2's operation, allowing the database manager to understand and manage its own structure and data. The DB2 Catalog is a specialized set of system tables within a DB2 for z/OS subsystem that stores all metadata about the database objects, the DB2 environment, and its configuration. It acts as the central data dictionary, containing definitions for tables, indexes, views, plans, packages, users, privileges, and more. DB2 relies on the Catalog for all operational aspects, from query optimization to security enforcement.

Key Characteristics

- System-Managed: The DB2 Catalog is automatically updated by DB2 whenever Data Definition Language (DDL) statements (e.g., CREATE, ALTER, DROP) are executed or internal system events occur.
- Read-Only for Users: While users and applications can query the catalog using standard SQL, direct modification of catalog tables is strictly prohibited and can lead to database corruption.
- Critical for DB2 Operation: Every DB2 operation, from parsing SQL queries to optimizing access paths and enforcing security, relies on information stored in the catalog.
- Standard SQL Interface: Catalog information is exposed through a set of system tables and views (e.g., SYSTABLES, SYSCOLUMNS, SYSINDEXES, SYSVIEWS, SYSPLAN) that can be queried like any other DB2 table.
- Stores Optimizer Statistics: It contains crucial statistics about data distribution, table sizes, and index characteristics, which the DB2 Optimizer uses to determine the most efficient access paths for SQL queries.
- Schema and Object Definitions: Provides a comprehensive dictionary of all database objects, their properties, and their relationships, forming the backbone of the DB2 data dictionary.

Use Cases

- Application Development: Developers query the catalog to understand existing table structures, column data types, constraints, and relationships when designing or modifying applications.
- Database Administration (DBA): DBAs use the catalog for monitoring database health, auditing security permissions, managing storage, and performing impact analysis before making schema changes.
- SQL Optimization: The DB2 Optimizer extensively uses the statistics stored in the catalog to generate efficient query execution plans, making RUNSTATS a critical utility.
- Impact Analysis: Before dropping a table or altering a column, DBAs can query the catalog to identify dependent objects (e.g., views, stored procedures, packages) to assess potential impacts.
- Security Auditing: Reviewing catalog tables like SYSGRANTS allows administrators to audit user privileges and object ownership across the DB2 subsystem.

Related Concepts

The DB2 Catalog is inextricably linked to the DB2 Subsystem itself, serving as its self-describing metadata repository. It is constantly updated by DDL (Data Definition Language) statements, which define and modify database objects. The DB2 Optimizer relies heavily on the statistics within the catalog to formulate optimal Access Paths for SQL queries. While the catalog stores object definitions, the DB2 Directory (another set of system tables) stores internal control structures, such as information about active threads, logs, and internal database pages, complementing the catalog for complete subsystem management.

Best Practices:

Regularly Update Statistics: Use the RUNSTATS utility frequently to ensure the catalog's statistics are current, especially after significant data changes, to enable the DB2 Optimizer to generate efficient access paths.
Avoid Direct Updates: Never attempt to update DB2 Catalog tables directly using SQL INSERT, UPDATE, or DELETE statements, as this can corrupt the DB2 subsystem and lead to data loss or instability.
Monitor Catalog Growth: Periodically monitor the size and activity of key catalog tablespaces (e.g., DSNDB06.DSNDB06) to proactively manage storage and performance, especially in environments with frequent DDL activity.
Include in Backup and Recovery: Ensure that the DB2 Catalog tablespaces are always included in the regular backup and recovery strategy for the DB2 subsystem, as their loss would render the entire database unusable.
Grant Appropriate Access: Limit direct SELECT access to sensitive catalog tables and views to authorized personnel, as they contain critical information about the database structure and security.