Wednesday, August 29, 2012

Managing DB2 for z/OS Application Performance

Applications that access databases are only as good as the performance they achieve. And every user wants their software to run as fast as possible. As such, performance tuning and management is one of the biggest demands on the DBA’s time. When asked what is the single most important or stressful aspect of their job, DBAs typically respond “assuring optimal performance.” Indeed, a recent Forrester Research survey indicates that “performance and troubleshooting” tops the list of most challenging DBA tasks.
But when you are dealing with data in a database management system there are multiple interacting components that must be managed and tuned to achieve optimal performance. That is, every database application, at its core, requires three components to operate:
  • the system (that is, the DBMS itself, the network, and the O/S),
  • the database (that is, the DDL and database schema), and
  • the application (that is, the SQL and program logic).
Each of these components requires care and attention, but today I want to focus on the high-level aspects of performance management from the perspective of the application. Furthermore, I will discuss this in terms of DB2 for z/OS.

So where do we begin? For DB2, a logical starting point is with BIND Parameters. There are many parameters and values that must be chosen from and specified when you bind a DB2 application program. The vast array of options at our disposal can render the whole process extremely confusing – especially if you don’t bind on a daily basis. And even if you do, some of the options still might be confusing if you rarely have to change them. You know what I’m talking about, parameters like ACQUIRE, RELEASE, VALIDATE, and DEGREE.

I will not delve into the myriad bind options as there are many articles and books, as well as the IBM DB2 manuals that you can use to guide you along that path. Suffice it to say, that there are some standard parameters and values that should be chosen “most of the time” in certain situations. As such, a wise DBA group will set up canned routines for the programmers to use for compiling and binding their applications. Choices such as: “CICS transaction”, “DB2 batch”, or “analytical query” can be presented to the developer and then, based on which of the various types of programs and environments that are available, the canned script can choose the proper bind options. Doing so can greatly diminish the problems that can be encountered when the “wrong” parameters or values are chosen at bind time.

This same process can be put in place for production binding to ensure that the appropriate parameters and values are chosen. This is especially useful when the binds are not done by a DBA, but are automated in production or done by a less-experienced change control clerk.

Of course, there should always be a method for over-riding the “standard” values for special situations, although these overrides should not be available to anyone other than a well-trained individual (DBA or otherwise).

I want to make one small exception here regarding advice on bind parameters, and that is the EXPLAIN parameter. In production, always bind your plans and packages specifying EXPLAIN YES. Failing to do so means that access paths will be generated, but you will not know what they are. This is akin to blinding yourself to what DB2 is doing and is not advisable.

Access Path Management

Bind and Rebind are important components to achieve optimal DB2 application performance. Bind/Rebind determine the access paths to the data that is accessed by your program. As such, it is vital that you develop an appropriate strategy for when and how to Rebind your programs.

There are several common approaches taken by DB2 users. By far, the best approach is to Rebind your applications over time as the data changes. This approach involves some form of regular maintenance that keeps DB2 statistics up to date and formulates new access paths as data volumes and patterns change. More on this in a moment.

Other approaches include Rebinding only when a new version of DB2 is installed, or perhaps more ambitious, whenever new PTFs are applied to DB2. Another approach is to rebind automatically after a regular period of time, whether it is days, weeks, months, or whatever period of time you deem significant. This approach can work if the period of time is wisely chosen based on the application data – but it still can pose significant administrative issues.

The final approach is from the “if it ain’t broke don’t fix it” school of thought. This approach is the worst of the several approaches discussed here. This approach penalizes every program for fear that a single program (or two) might experience a degraded access path. Yet, the possibility of degraded performance is real. That is why this approach has been adopted at some sites. The problem is being able to find which statements may be worse. The ideal situation would be to be able to review the access path changes before hand to determine if they are better or worse. But DB2 itself does not provide any systematic method of administering access paths that way. There are third party tools that can help you achieve this though.
Anyway, let’s go back to the best approach again, and that is to Rebind regularly as your data changes. This involves what is known as the three Rs: REORG, RUNSTATS, and Rebind. At any rate, your goal should be to keep your access paths up-to-date with the current state of your data. Failing to do this means that DB2 is accessing data based upon false assumptions.

By Rebinding you will generally improve the overall performance of your applications because the access paths will be better designed based on an accurate view of the data. And as you apply changes to DB2 (new releases/PTFs) optimizer improvements and new access techniques can be used. If you never Rebind, not only are you forgoing better access paths due to data changes but you are also forgoing better access paths due to changes to DB2 itself.

To adopt the Three R’s you need to determine when to REORG. This means looking at either RUNSTATS or Real-Time Statistics (RTS). So, perhaps we need at least 4 R’s:

  1. Real Time Statistics (or RUNSTATS)
  2. REORG

But is this enough? Probably not because we need to review the access paths after rebinding to make sure that there are no rogue access paths. So, let’s add another “R” – to Review the access paths generated by the REBIND. As we mentioned, the optimizer can make mistakes. And, of course, so can you. Users don't call you when performance is better (or the same). But if performance gets worse, you can bet on getting a call from irate users.

So we need to put in place best practices whereby we test Rebind results to compare the before and after impact of the optimizer’s choices. Only then can we assure that we are achieving optimal DB2 application performance.

Tuning The Code Itself

Of course, everything we’ve discussed so far assumes that the code is written efficiently to begin with – and that is a big assumption. We also need to make sure that we are implementing efficient application code. The application code consists of two parts: the SQL code and the host language code in which the SQL is embedded.

SQL is simple to learn and easy to start using. But SQL tuning and optimization is an art that takes years to master. Some general rules of thumb for creating efficient SQL statements include:
  • Let SQL do the work instead of the application program. For example, code an SQL join instead of two cursors and a programmatic join.
  • Simpler is generally better, but complex SQL can be very efficient.
  • Retrieve only the columns required, never more.
  • Retrieve the absolute minimum number of rows by specifying every WHERE clause that is appropriate.
  • When joining tables, always provide join predicates. In other words, avoid Cartesian products.
  • Favor using Stage 1 and Indexable predicates.
  • Avoid sorting if possible by creating indexes for ORDER BY, GROUP BY, and DISTINCT operations.
  • Avoid black boxes – that is, avoid I/O routines that are called by programs instead of using embedded SQL.
  •  Avoid deadlocks by updating tables in the same sequence in every program.
  • Issue data modification statements (INSERT, UPDATE, DELETE) as close as possible to the COMMIT statement as possible.
  • Be sure to build a COMMIT strategy into every batch program that changes data. Failing to COMMIT can cause locking problems.
To tune SQL the DBA must be able to interpret the output of the access paths produced by EXPLAIN. This information is encoded in the PLAN_TABLEs. IBM and other vendors offer tools to simplify this process, such as IBM's Data Studio.

Finally, some attention must be paid to the host language code. Host language code refers to the application programs written in C, COBOL, Java, Visual Basic or the programming language du jour. SQL statements are usually embedded into host language code and it is quite possible to have finely tuned SQL inside of inefficient host language code. And, of course, that would cause a performance problem.

The Bottom Line

Although DBAs must understand all three aspects of database performance management concentrating on the application aspects of performance will most likely provide the most bang-for-the-buck. Of course, we have only touched the tip of the DB2 application performance iceberg today. But even this high-level view into application performance can serve as a nice starting place for tuning your DB2 applications.

Good luck with DB2 for z/OS and happy performance tuning! 

Wednesday, August 15, 2012

Don’t Forget About DB2 Session Variables

Session variables, set by DB2 or by the user, offer another way to provide additional information to applications. Session variables are set by DB2, and are accessible to application programs and end users. By accessing session variables, an application program can determine many aspects of its environment and the DB2 features in effect.

There are a plethora of session variables available to the DB2 programmer as outlined in the accompanying table. 

Table 1. DB2 Session Variables.
 Session Variable
The application encoding scheme specified for the subsystem. 
The string delimiter. Value will be DEFAULT, , or .
Name of the data sharing group.
The date format in use. Value will be ISO, JIS, USA, EUR, or LOCAL.
The LOCAL DATE LENGTH install parameter. Value is 10-254, or 0 for no exit.
The DECIMAL ARITHMETIC install parameter. Value is DEC15, DEC31, 15, or 31.
The DECIMAL POINT install parameter. Value is ‘.’ or ‘,’.
The DECFLOAT ROUNDING MODE install parameter.
The value of GROUP ATTACH field on the DSNTIPK installation panel or the SUBSYSTEM NAME field on the DSNTIPM installation panel. 
The LANGUAGE DEFAULT install parameter. Value is ASM, C, CPP, IBMCOB, FORTRAN, or PL/I.

The value of LOCALE LC_CTYPE install parameter.
The fully qualified data set name of the data set from which the DSNHDECP or a user-specified application defaults module was loaded. 

The USE FOR DYNAMICRULES install parameter. Value is YES or NO.
The DEF ENCODING SCHEME install parameter. Value is EBCDIC, ASCII, or UNICODE.
The MIXED DATA install parameter. Value is YES or NO.
The INSTALL TYPE parameter. Value is INSTALL, UPDATE, MIGRATE, or ENFM; reflects the setting of the DSNHDECP variable NEWFUN.

Name of the package currently in use.
Version of the current package.
Schema name of the current package.
 The PAD NUL-TERMINATED install parameter. Value is YES or NO.
Name of the plan currently being run.
The user’s security label (if any); null if not defined.
The SQL STRING DELIMITER install parameter. Value is DEFAULT, or 
DB2 subsystem identifier.
The STD SQL LANGUAGE install parameter. Value is YES or NO.
Name of the system, as defined in field SUBSYSTEM NAME on installation panel DSNTIPM.
A comma-delimited string of the ASCII CCSIDs in use on this system.
A comma-delimited string of the EBCDIC CCSIDs in use on this system.
A comma-delimited string of the UNICODE CCSIDs in use on this system.

The TIME FORMAT install parameter. Value is ISO, JIS, USA, EUR, or LOCAL.
The LOCAL TIME LENGTH install parameter. Value is 8-254, or 0 for no exit.
Version of the DB2 subsystem. This value is a string, formatted as pppvvrrm where:

  • ppp is a product string set to the value ‘DSN’.
  • vv is a two-digit version identifier such as ‘09’.
  • rr is a two-digit release identifier such as ‘01’.
  • m is a one-digit maintenance level identifier.

Each session variable must be qualified by SYSIBM. A built-in function named GETVARIABLE can retrieve session variable values. So, you could create a view based on a security label, for example:

  SELECT column-list
  FROM   table-name

The GETVARIABLE built-in function can be used in views, triggers, stored procedures, and constraints to enforce a security policy.

Users can add up to ten session variables by setting the name and value in their connection or sign-on exits. User-created session variables are qualified by SESSION. For example, the customer might have a connection or sign-on exit that examines the SQL user’s IP address, and maps the IP address to the user’s site within the company. This is recorded in a session variable, named say, USER_SITE. This session variable is then accessible using the built-in function, for example:


Using session variables much more information is available to application programs as they execute, and more control and security is provided, as well. Additionally, session variables can be trusted. They are set by DB2 and an application cannot modify them.