A Brief Introduction to the DB2 Catalog

The system catalog, or the DB2 Catalog, offers a wealth of information about DB2. If the DB2 optimizer is the heart and soul of DB2, the DB2 Catalog is its memory. The knowledge base of every object known to DB2 is stored in the DB2 Catalog, along with the DB2 Directory and the BSDS (Bootstrap Data Set).

The tables in the DB2 Catalog collectively describe the objects and resources available to DB2. You can think of the DB2 Catalog as a metadata repository for your DB2 databases. As of Version 10, the DB2 Catalog is composed of 90 table spaces and 137 tables all in a single database named DSNDB06. These numbers have grown considerably since the early days of DB2. The DB2 Catalog consisted of 25 tables in 11 table spaces for the first version of DB2 and as recently as DB2 V8, there were only 21 table spaces and 87 tables. The following table runs down the history:

Over the course of the past couple releases, the DB2 Catalog has undergone many significant changes. For most of its life, the DB2 Catalog contained many multi-table table spaces. As of DB2 10 for z/OS, IBM made an effort to clean that up, and now only a few table spaces are in the DB2 Catalog with more than one table defined. As of V10, most of the table spaces in the DB2 Catalog are now universal table spaces. In addition, the DB2 Catalog now must be SMS-managed.

Even as many new tables have been added to the DB2 Catalog to support new features such as trusted context, XML, and access path management, some tables have been removed. The SYSPROCEDURES table, which was used to register stored procedures in earlier version of DB2, was removed as of DB2 V9. And the SYSLINKS table was removed for  DB2 V10.

The SYSLINKS table was used to record the links (or pointers) that existed in several of the older DB2 Catalog table spaces (SYSDBASE, SYSPLAN, SYSDBAUT, SYSVIEW, SYSGROUP), as well as in the DB2 Directory (DBD01). Links were used to tie tables together hierarchically—not unlike an IMS database—using a special type of relationship. However, links are obsolete in DB2 as of V10.

Each DB2 Catalog table maintains data about an aspect of the DB2 environment. In that respect, the DB2 Catalog functions as a data dictionary for DB2, supporting and maintaining data about the DB2 environment. The DB2 Catalog records all the information required by DB2 for the following functional areas:

• Database Objects: Storage groups, databases, table spaces, partitions, tables, auxiliary tables, columns, user-defined distinct types, views, synonyms, aliases, sequences, indexes, index keys, foreign keys, relationships, schemas, user-defined functions, stored procedures, triggers, and so on.
• Programs: Plans, packages, DBRMs, and Java/JAR information
• XML: XML Schema Repository tables
• Security: Database privileges, plan privileges, schema privileges, system privileges, table privileges, view privileges, use privileges, trusted contexts, roles, and audit ­policies
• Utility: Image copy data sets, REORG executions, LOAD executions, and object organization efficiency information
• Communication: How DB2 subsystems are connected for communication, data distribution, and DRDA usage
• Performance: Statistics, profiles, queries, and auto alerts
• Environmental: Control and administrative information (such as details on image copies and the dummy tables)

How does the DB2 Catalog support data about these areas? For the most part, the tables of the DB2 Catalog cannot be modified using standard SQL data manipulation language statements. You do not use INSERT statements, DELETE statements, or UPDATE statements (with a few exceptions) to modify these tables. Instead, the DB2 Catalog operates as a semi-active, integrated, and non-subvertible data dictionary. The definitions of these three adjectives follow.

First, the DB2 Catalog is semi-active. An active dictionary is built, maintained, and used as the result of the creation of the objects defined to the dictionary. In other words, as the user is utilizing the intrinsic functions of the DBMS, metadata is being accumulated and populated in the active data dictionary.

The DB2 Catalog, therefore, is active in the sense that when standard DB2 SQL is issued, the DB2 Catalog is either updated or accessed. All the information in the DB2 Catalog, however, is not completely up-to-date, and some of the tables must be proactively populated (such as SYSIBM.IPNAMES and SYSIBM.IPLIST). But, for the most part, the DB2 Catalog operates as an active data dictionary, particularly with regard to SQL. Remember that the three types of SQL are DDL, DCL, and DML. When DDL is issued to create DB2 objects such as databases, table spaces, and tables, the pertinent descriptive information is automatically stored in the DB2 Catalog.

When a CREATE, DROP, or ALTER statement is issued, information is recorded or updated in the DB2 Catalog. For example, upon successfully issuing a CREATE TABLE statement, DB2 populates the metadata for the table into SYSTABLES and SYSCOLUMNS, as well as possibly into SYSSEQUENCES, SYSFIELDS, SYSCHECKS, and SYSCHECKDEP depending upon the exact DDL that was issued.

The same is true for security SQL data control language statements. The GRANT and REVOKE statements cause information to be added or removed from DB2 Catalog tables. For example, if you issue GRANT TABLE, DB2 potentially adds metadata to SYSTABAUTH and SYSCOLAUTH.

Data manipulation language SQL (SELECT, INSERT, UPDATE, MERGE, DELETE) statements use the DB2 Catalog to ensure that the statements accurately reference the DB2 objects being manipulated (such as column names and data types).

Why then is the DB2 Catalog classified as only semi-active rather than completely active? The DB2 Catalog houses important information about the physical organization of DB2 objects. For example, the following information is maintained in the DB2 Catalog:

• The number of rows in a given DB2 table or a given DB2 table space

• The number of distinct values in a given DB2 index

• The physical order of the rows in the table for a set of keys

This information is populated by means of the DB2 RUNSTATS utility. A truly active data dictionary would update this information as data is populated in the application table spaces, tables, and indexes. Some of these statistics are now actively populated in the Real Time Statistics table in the DB2 Catalog, making them active. But because some of the information in the DB2 Catalog is not always completely up-to-date, it is only a semi-active system catalog.

I also decsribed the DB2 Catalog as being integrated. The DB2 Catalog and the DB2 DBMS are inherently bound together, neither having purpose or function without the other. The DB2 Catalog without DB2 defines nothing; DB2 without the DB2 Catalog has nothing defined that it can operate on.

The final adjective used to classify the DB2 Catalog is non-subvertible. This simply means that the DB2 Catalog is continually updated as DB2 is being used; the most important metadata in the DB2 Catalog cannot be updated behind DB2’s back. Suppose that you created a table with 20 columns. You cannot subsequently update the DB2 Catalog to indicate that the table has 15 columns instead of 20 without using standard DB2 data definition language SQL statements to drop and re-create the table.

An Exception to the Rule  

As with most things in life, there are exceptions to the basic rule that the SQL data manipulation language cannot be used to modify DB2 Catalog tables. You can modify columns (used by the DB2 optimizer) that pertain to the physical organization of table data. 

Querying the DB2 Catalog

Because the DB2 Catalog consists of DB2 tables, you can write SQL queries to easily retrieve the metadata information about your DB2 environment. You can write queries to discover all sorts of interesting and useful information about DB2 across the following broad categories:

• Navigational queries, which help you to maneuver through the sea of DB2 objects in your DB2 subsystems
  
• Physical analysis queries, which depict the physical state of your application table spaces and indexes
  
• Queries that aid programmers (and other analysts) in identifying the components of DB2 packages and plans
  
• Application efficiency queries, which combine DB2 Catalog statistics with the PLAN_TABLE output from EXPLAIN to identify problem queries quickly
  
• Authorization queries, which identify the authority implemented for each type of DB2 security
  
• Historical queries, which use the DB2 Catalog HIST tables to identify and monitor changing data patterns
  
• Partition statistics queries, which aid the analysis of partitioned table spaces 

In addition to aiding development, DB2 Catalog queries can also aid performance tuning and administration of your production environment. An effective strategy for monitoring DB2 objects using catalog queries can help to catch and forestall problems before they affect performance. By monitoring DB2 objects using DB2 Catalog queries, you can more effectively forecast disk needs and other resource usage, making it easier to plan for future capacity needs.

Summary

The DB2 Catalog is a rich source of information about your DB2 subsystem and applications. Be sure to use it to simplify your DB2 development and administrative efforts. 

Note: This blog post was adapted from material in the sixth and latest edition of Craig's book, DB2 Developer's Guide.