DB2 Locking, Part 2: Table Space and Table Locks

Today's post is the second in our DB2 locking series and it covers the topic of table space and table locks.

Table Space Locks

A table space lock is acquired when a DB2 table or index is accessed. Note that I said accessed, not updated. The table space is locked even when simple read-only access is ­occurring.

Refer to to Table 1 below for a listing of the types of table space locks that can be acquired during the execution of an SQL statement. Every table space lock implies two types of access: the access acquired by the lock requester and the access allowed to other subsequent, concurrent processes.

Table 1. Table Space Locks

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When an SQL statement is issued and first accesses data, it takes an intent lock on the table space. Later in the process, actual S-, U-, or X-locks are taken. The intent locks (IS, IX, and SIX) enable programs to wait for the required S-, U-, or X-lock that needs to be taken until other processes have released competing locks.

The type of table space lock used by DB2 during processing is contingent on several factors, including the table space LOCKSIZE specified in the DDL, the bind parameters chosen for the plan being run, and the type of processing requested. Table 2 provides a synopsis of the initial table space locks acquired under certain conditions.

Table 2. How Table Space Locks Are Acquired

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A table space U-lock indicates intent to update, but an update has not occurred. This is caused by using a cursor with the FOR UPDATE OF clause. A U-lock is non-exclusive because it can be taken while tasks have S-locks on the same table space. More information on table space lock compatibility follows in Table 3.

An additional consideration is that table space locks are usually taken in combination with table and page locks, but they can be used on their own. When you specify the ­LOCKSIZE TABLESPACE DDL parameter, table space locks alone are used as the locking mechanism for the data in that table space. This way, concurrent access is limited and concurrent update processing is eliminated.

Similar in function to the LOCKSIZE DDL parameter is the LOCK TABLE statement. The LOCK TABLE statement requests an immediate lock on the specified table. The LOCK TABLE statement has two forms—one to request a share lock and one to request an exclusive lock.

     LOCK TABLE table_name IN SHARE MODE; 
     LOCK TABLE table_name IN EXCLUSIVE MODE;

You also can issue LOCK TABLE against a specific partition, for example:

     LOCK TABLE table-name 
          PARTITION integer IN SHARE MODE;

A locking scheme is not effective unless multiple processes can secure different types of locks on the same resource concurrently. With DB2 locking, some types of table space locks can be acquired concurrently by discrete processes. Two locks that can be acquired concurrently on the same resource are said to be compatible with one another.

Refer to Table 3 for a breakdown of DB2 table space lock compatibility. A Yes in the matrix indicates that the two locks are compatible and can be acquired by distinct processes on the same table space concurrently. A No indicates that the two locks are incompatible. In general, two locks cannot be taken concurrently if they allow concurrent processes to negatively affect the integrity of data in the table space.


Table 3. Table Space Lock Compatability

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Table Locks

When segmented table spaces are involved, DB2 can use table locks . Table locks are always associated with a corresponding table space lock.

The same types of locks are used for table locks as are used for table space locks. S, U, X, IS, IX, and SIX table locks can be acquired by DB2 processes when data in segmented table spaces is accessed. Table 1 describes the options available to DB2 for table locking. The compatibility chart in Table 3 applies to table locks as well as table space locks.

For a table lock to be acquired, an IS-lock must first be acquired on the segmented table space in which the table exists. The type of table lock to be taken depends on the LOCKSIZE specified in the DDL, the bind parameters chosen for the plan being run, and the type of processing requested. Table 4 (below) is a modified version of Table 2 (shown earlier), depicting the initial types of table spaces and table locks acquired given a certain set of conditions. Table locks are not acquired when the LOCKSIZE TABLESPACE parameter is used.


Table 4. How Table Locks Are Acquired

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Summary

And so concludes Part 2 of our multi-part series on DB2 locking. Be sure to keep watching this blog for additional postings in this series!

DB2 Locking, Part 1: An Overview

DB2 automatically guarantees the integrity of data by enforcing several locking strategies. These strategies permit multiple users from multiple environments to access and modify data concurrently. Basically, DB2 combines the following strategies to implement an overall locking strategy:

  • Table and table space locking

  • IRLM page and row locking

  • Internal page and row latching

  • Claims and drains to achieve partition independence

  • Checking commit log sequence numbers (CLSN) and PUNC bits to achieve lock avoidance

  • Global locking through the coupling facility in a data sharing environment

But what exactly is locking? How does DB2 utilize these strategies to lock pages and guarantee data integrity? Why does DB2 have to lock data before it can process it? What is the difference between a lock and a latch? How can DB2 provide data integrity while operating on separate partitions concurrently? Finally, how can DB2 avoid locks and still guarantee data integrity?

Today's blog post will offer a high level introduction to DB2 locking, why it is needed, as well as the benefits and drawbacks that it introduces to using DB2 databases.

Anyone accustomed to application programming when access to a database is required understands the potential for concurrency problems. When one application program tries to read data that is in the process of being changed by another, the DBMS must forbid access until the modification is complete to ensure data integrity. Most DBMS products, DB2 included, use a locking mechanism for all data items being changed. Therefore, when one task is updating data on a page, another task cannot access data (read or update) on that same page until the data modification is complete and committed.

When multiple users can access and update the same data at the same time, a locking mechanism is required. This mechanism must be capable of differentiating between stable data and uncertain data. Stable data has been successfully committed and is not involved in an update in a current unit of work. Uncertain data is currently involved in an operation that could modify its contents. 

Consider two DB2 application programs. If program #1 updates a piece of data on page 1, you must ensure that program #2 cannot access the data until program #1 commits the unit of work. Otherwise, a loss of integrity could result. Without a locking mechanism, the following sequence of events would be possible:

1. Program #1 retrieves a row from DSN81010.EMP for EMPNO ‘000010’.
2. Program #1 issues an update statement to change that employee’s salary to 55000.
3. Program #2 retrieves the DSN81010.EMP row for EMPNO ‘000010’. Because the change was not committed, the old value for the salary, 52750, is retrieved.
4. Program #1 commits the change, causing the salary to be 55000.
5. Program #2 changes a value in a different column and commits the change.
6. The value for salary is now back to 52750, negating the change made by program #1.

The DBMS avoids this situation by using a locking mechanism. DB2 supports locking at four levels, or granularities: table space-, table-, page-, and row-level locking. DB2 also provides LOB locking for large objects (BLOBs, CLOBs, and DBCLOBs).

Locks can be taken at any level in the locking hierarchy without taking a lock at the lower level. However, locks cannot be taken at the lower levels without a compatible higher-level lock also being taken. For example, you can take a table space lock without taking any other lock, but you cannot take a page lock without first securing a table space-level lock (and a table lock as well if the page is part of a table in a segmented table space).

Additionally, a page lock does not have to be taken before a row lock is taken. Your locking strategy requires an “either/or” type of choice by table space: either row locking or page locking. An in-depth discussion on the merits of both are beyond the scope of today's blog post, but will be covered in future posts. 

Both page locks and row locks can escalate to a table level and then to a table space level for segmented tables or straight to a table space level for partitioned table spaces. A table or table space cannot have both page locks and row locks held against it at the same time.

Many modes of locking are supported by DB2, but they can be divided into two types:

  • Locks to enable the reading of data                                

  • Locks to enable the updating of data

But remember, we are still talking at a very high, and somewhat simplistic level. DB2 uses varieties of these two types of locks to indicate the type of locking required. 

Locks Versus Latches

A true lock is handled by DB2 using the IRLM. However, whenever doing so is practical, DB2 tries to lock resources without going to the IRLM. This type of lock is called a latch. Whereas true locks are always set in the IRLM, latches are set internally by DB2, without going to the IRLM.

When a latch is taken instead of a lock, it is handled in the Buffer Manager by internal DB2 code; so the cross-memory service calls to the IRLM are eliminated. Latches are usually held only briefly—for a shorter duration than locks. Also, a latch requires about one-third the number of instructions as a lock. Therefore, latches are more efficient than locks because they avoid the overhead associated with calling an external address space. Latches are used when a resource serialization situation is required for a short time. Both latches and locks guarantee data integrity. In subsequent sections, any usage of the term lock generically, refers to both locks and latches.

Lock Duration

Before you learn about the various types of locks that can be acquired by DB2, you should understand lock duration, which refers to the length of time that a lock is maintained. The duration of a lock is based on the BIND options chosen for the program requesting locks. Locks can be acquired either immediately when the program is requested to be run or iteratively as needed during the execution of the program. Locks can be released when the plan is terminated or when they are no longer required for a unit of work.

The BIND parameters affecting DB2 locking are summarized below:

The BIND parameters that impact table space locks are the ACQUIRE and RELEASE parameters. 

• ACQUIRE(ALLOCATE | USE): The ALLOCATE option specifies that locks will be acquired when the plan is allocated, which normally occurs when the first SQL statement is issued. The USE option indicates that locks will be acquired only as they are required, SQL statement by SQL statement. As of DB2 10, ACQUIRE(ALLOCATE) is no longer supported when binding or rebinding plans.
  
• RELEASE(DEALLOCATE | COMMIT): When you specify DEALLOCATE for a program, locks are not released until the plan is terminated. When you specify COMMIT, table space locks are released when a COMMIT is issued.

The BIND parameter affecting page and row locks is the ISOLATION parameter. There are four choices for isolation level:

• ISOLATION(CS), or Cursor Stability, acquires and releases page locks as pages are read and processed. CS provides the greatest level of concurrency at the expense of potentially different data being returned by the same cursor if it is processed twice during the same unit of work.
• ISOLATION(RR), or Repeatable Read, holds page and row locks until a COMMIT point; no other program can modify the data. If data is accessed twice during the unit of work, the same exact data will be returned.
• ISOLATION(RS), or Read Stability, holds page and row locks until a COMMIT point, but other programs can INSERT new data. If data is accessed twice during the unit of work, new rows may be returned, but old rows will not have changed.
• ISOLATION(UR), or Uncommitted Read, is also known as dirty read processing. UR avoids locking altogether, so data can be read that never actually exists in the ­database.

Regardless of the ISOLATION level chosen, all page locks are released when a COMMIT is encountered.

I think that is enough for this first installment on DB2 locking... come back soon and we will expand on table space locks in the next installment of this blog series on DB2 locking...

DB2 Table Editors

In today's blog post I want to briefly discuss one of the more useful, yet often ignored, DB2 tools available on the market -- Table Editors...

Typically, the only method of updating DB2 data (indeed, any data stored in a relational database) is with  SQL data manipulation language statements DELETE, INSERT, and UPDATE (or with a database load). Because these SQL statements operate on data a set at a time, multiple rows -- or even all of the rows -- can be affected by a single SQL statement. Coding SQL statements for every data modification required during the application development and testing phase can be time-consuming and error-prone.

A table editing tool can reduce the time needed to make simple data modifications by providing full-screen edit capability for database tables. The user specifies the table to edit and is placed into an edit session. The data is presented to the user as a series of rows, with the columns separated by spaces. A header line indicates the column names. The data can be scrolled up and down as well as left and right. To change data, the user simply types over the current data.

This type of tool is ideal for supporting the application development process. A programmer can make quick changes without coding SQL. Also, if properly implemented, a table editor can reduce the number of erroneous data modifications made by beginning SQL users.

When a table editor is used, all columns are available for update. Thus, if a table editor is used to change production data, a simple mis-keying can cause unwanted updates. Native SQL should be used if you must ensure that only certain columns are updated.

One final note: Tested SQL statements and application programs are characterized by their planned nature. These modification requests are well thought out and tested. This is not true for changes implemented through a table editor, so always exercise caution when using a table editor.

Examples of DB2 Table Editors include:

• BMC Catalog Manager (which includes an integrated table editor)
• CA RC/Update (which includes an integrated table editor)
• IBM DB2 Table Editor 
• RazorSQL DB2 Table Editor

The Importance of Database Design

It is impossible to over-estimate the importance of database design on the effectiveness and efficiency of application systems. The first step, of course, is to create a logical data model of the business information that must be stored in, and accessed through, the database. This is a non-trivial task, but it is not the focus of today’s blog post, which is implementing optimal and well-designed database schemata.

Oftentimes the database design does not get the attention it deserves. This can occur for numerous reasons such as:

• Insufficient specifications and/or poor logical data modeling
• Not enough time in the development schedule
• Too  many changes occurring throughout the development cycle
• Database design assigned to, or performed by novices

And even when the database design is being performed by experienced professionals with sufficient time and a solid logical model, it is very easy for design flaws to creep into the database. This is especially the case for larger and more complex databases required to support mission critical applications.

Of course, the first step in constructing a physical database should be transforming the logical design using best practices. The transformation consists of the following:

• Transforming entities into tables
• Transforming attributes into columns
• Transforming domains into data types and constraints
• Transforming relationships into primary and foreign keys

But a simple transformation will not result in a complete and correct physical database design – it is merely the first step.  And design flaws can be introduced even during such a transformation.

The process of normalizing your data should be conducted during the logical design phase, but sometimes mistakes are made during the logical modeling process. In simple terms, normalization is the process of identifying the one best place where each fact belongs. A normalized design minimizes data redundancy and optimizes data structures by systematically and properly placing data elements into the appropriate groupings.

• First normal form (1NF) eliminates repeating groups and non-atomic data from an entity.
• Second normal form (2NF) ensures that all the attributes of each entity are dependent on the primary key.
• Third normal form (3NF) ensures that no relationships between attributes exist within an entity.

Although normalization is a logical process and does not necessarily dictate physical database design it is good practice to implement normalized physical databases – especially with today’s powerful hardware and database systems. A normalized data model will ensure that each entity is well formed and that each attribute is assigned to the proper entity. And denormalized data structures should always be documented.

A database that is not normalized can be difficult to modify (because facts may be strewn about within the database) and prone to data quality issues (again because one fact may be in more than one place). Failing to achieve appropriate normalization is a significant design flaw.

Design flaws can make it difficult to access a database in an efficient manner. Something as simple as creating columns with inconsistent data type and length across tables can introduce inefficiency. For example, perhaps JOB_CODE is defined as CHAR(10) in one table and CHAR(15) in another. If these columns need to be compared or joined, additional work is required by the DBMS to make the columns comparable.  It is easy to see how a database design issue can make applications inefficient no matter how adept the developers are. And this is just one type of design flaw.

There are numerous other types of design flaws that can negatively impact the usability and efficiency of a database implementation (and the applications that use it). Improperly defined constraints (referential, check, uniqueness) can cause data quality problems. Improper indexing (to support constraints and other physical structures) can cause a database to operate inefficiently or even cease operating altogether. In DB2 for z/OS, failing to explicitly name a clustering index causes DB2 to default cluster sequence to the oldest index. Changing index structures, which sometimes require dropping and recreating the index, can cause the data to be ordered in a completely different physical sequence, thereby impacting performance.

Some database design flaws are more subtle. What happens if you create two incompatible check constraints? For example, consider the following:

     CHECK (empno < 100)

     CHECK (empno >= 101)

No data can be added to the table because no number is less than 100 and greater than or equal to 101. Of course, this is an extreme example to highlight the problem. Another situation can occur if the default value is not one of the values permitted by the check constraint, for example:

    emp_type  CHAR(8)  DEFAULT 'new'

       CHECK (emp_type IN ('temp',
                           'fulltime',
                           'contract')), ...

Cycles are another potential problem that can be created in a physical database schema. A cycle is a referential path that connects a table to itself. This can occur when multiple tables are related back to each other and it looks like a loop when diagrammed. DB2 forbids a table from being delete-connected to itself in a cycle. A table is delete-connected to another table if it is a dependent of a table specified with a CASCADE delete rule.

Nested triggers can also create problems. A trigger is fired by an INSERT, UPDATE, or DELETE operation. However, a trigger can also contain INSERT, UPDATE, or DELETE logic within itself. Therefore, a trigger is fired by a data modification, but can also cause another data modification, thereby firing yet another trigger. When a trigger contains INSERT, UPDATE, and/or DELETE logic (including MERGE statements), the trigger is said to be a nested trigger. If referential integrity is combined with triggers, additional cascading updates and/or deletes can occur.

DB2 restricts the number of nested triggers that can be executed within a single firing event. If this were not done, it could be quite possible to have triggers firing triggers ad infinitum until all of the data was removed from an entire database. DB2 limits this cascading effect to 16 levels to prevent endless looping. If more than 16 levels of nesting occur, the transaction is aborted. Such nesting may restrict certain types of data modification from happening at all because the number of nested calls will always be exceeded. So nesting levels need to be controlled and managed in the database schema to avoid problems.

And this is by no means a comprehensive list of database design flaws that can cause real problems for application developers and end users. Getting the database design correct is imperative if you wish to have any hope of assuring application performance.

So how can you go about examining the correctness of your database structures? The best approach is an automated one such as that provided by Database Examiner, a product offered by DBE Software that performs a comprehensive validation of a database schema. The in-depth technology incorporated into Database Examiner allows you to review your database schema, identify problems, and remediate the issues to ensure a quality database implementation offering peak performance.

Simply provide Database Examiner with the data model, DDL, or a link to the active database and it will perform a series of comprehensive diagnostics. Database Examiner applies the rules of relational technology to detect flaws, inconsistencies and lack of integrity. The product applies more than 50 diagnostics that can be organized by category or severity levels. And you can customize the diagnostics by selecting those to be executed and assigning each one a severity level.

But Database Examiner doesn’t just highlight the flaws, it also helps to fix them be providing recommendations and corrections for each issue it detects, including the generation of SQL DDL scripts to remedy the flaws. Even better, Database Examiner provides multi-platform DBMS support, including DB2 for z/OS, DB2 LUW, Oracle, Microsoft SQL Server and Sybase.

And DBE Software is currently offering folks the ability to download and use a scaled down “Performance Edition” of Database Examiner, free ofcharge for a limited time. Be sure to enter the promo code MULL on the download request to get the best offer available.

Hopefully this blog entry has convinced you that database design is important… important enough to take some time to evaluate the quality of your existing database structures. And to take a look at automating the process using Database Examiner (using the promo code MULL ).

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.