IBM Db2 System Catalog Overview | Generated by AI
Introduction to IBM Db2 System Tables
The system catalog in IBM Db2 (also known as the Db2 catalog) is a built-in metadata repository that stores detailed information about every object and element in your database. This includes tables, views, indexes, columns, users, privileges, routines, and more. It’s essentially the “phone book” of your database—automatically maintained by Db2 whenever you create, alter, or drop objects. The catalog helps database administrators (DBAs), developers, and tools query the structure and status of the database without needing to parse application code or external files.
Unlike regular user tables, the system catalog is read-only for most users and optimized for querying metadata rather than high-volume data operations. It’s created automatically when you create a new database and resides in special table spaces (like SYSCATSPACE in Db2 LUW).
Key Components and Structure
The system catalog consists of:
- Base Tables: These are the underlying, normalized tables where raw metadata is stored. They are in the SYSIBM schema and are not directly queryable by end users to prevent accidental modifications or performance issues. Examples include SYSIBM.SYSTABLES (basic table info) and SYSIBM.SYSCOLUMNS (column details).
- Catalog Views: User-friendly, denormalized views built on top of the base tables. These are easier to query and provide a standardized interface compliant with SQL standards (like ISO). They are grouped into schemas:
- SYSCAT: Core metadata about database objects (e.g., tables, indexes, triggers).
- SYSCOLUMNS: Detailed column-level information.
- SYSSTAT: Statistical data used by the query optimizer (e.g., row counts, cardinalities).
- SYSPROC and others: For procedures, functions, and XML-related info.
In Db2 for z/OS, the catalog is in database DSNDB06, but the concepts are similar across platforms (LUW, z/OS, i).
Purpose
- Discovery: Find out what objects exist, their properties, and relationships.
- Administration: Monitor privileges, dependencies, and performance stats.
- Development: Generate DDL scripts, validate schemas, or integrate with tools like Db2 Data Studio.
- Optimization: The query optimizer uses SYSSTAT views to choose execution plans.
How to Access and Query
- Connect to the Database: Use
db2 connect to <dbname>. - Permissions: By default, PUBLIC has SELECT on catalog views. No special grants needed for basic queries, but SYSIBM base tables require SYSADM or higher.
- Querying: Use standard SQL SELECT statements. Views are schema-qualified (e.g.,
SELECT * FROM SYSCAT.TABLES).
Example Queries:
- List all tables in the current schema:
SELECT TABSCHEMA, TABNAME, TYPE FROM SYSCAT.TABLES WHERE TABSCHEMA = CURRENT SCHEMA;- Output: Shows schema, table name, and type (e.g., ‘T’ for table, ‘V’ for view).
- Get column details for a table:
SELECT COLNAME, TYPENAME, LENGTH, NULLS FROM SYSCOLUMNS WHERE TABSCHEMA = 'YOUR_SCHEMA' AND TABNAME = 'YOUR_TABLE';- Output: Column names, data types, lengths, and nullability.
- Check table statistics:
SELECT CARD, NPAGES FROM SYSSTAT.TABLES WHERE TABNAME = 'YOUR_TABLE';- CARD: Estimated row count; NPAGES: Number of pages.
For full lists, run SELECT * FROM SYSCAT.VIEWS WHERE VIEWNAME LIKE 'SYSCAT%' to discover all available views.
Important Notes
- Performance: Catalog queries are fast but can be resource-intensive on large databases; use filters (e.g., WHERE clauses) to limit results.
- Updates: Db2 updates the catalog automatically—don’t try to INSERT/UPDATE it manually.
- Version Differences: In Db2 11.5+, views are more standardized. For z/OS, some views have platform-specific columns.
- Tools Integration: Use Db2’s
db2lookcommand to generate DDL from catalog data:db2look -d <dbname> -t <table>. - Avoid direct base table access unless you’re a sysadmin, as it can lead to inconsistencies.
For deeper dives, explore the official docs or experiment in a test database.