Wednesday, November 08, 2006

DB2 Access Paths and Change Management Procedures

No one working as a DB2 DBA or performance analyst would deny that one of the most important components of assuring efficient DB2 performance is making sure that DB2 access paths are appropriate for your DB2 programs. Binding programs with EXPLAIN(YES) specified is important to ensure that we know what access paths DB2 has chosen for each SQL statement. Without the information that EXPLAIN puts in the PLAN_TABLE we would be "flying blind."

Anyway, programs need to be rebound periodically to ensure that DB2 has forumlated access paths based on up-to-date statistics and to ensure that you are taking advantage of all the latest and greatest DB2 optimizer features. Whether you are implementing changes into your DB2 applications, upgrading to a new version of DB2, or simply trying to achieve optimum performance for existing application plan and packages, an exhaustive and thorough REBIND management process is a necessity.

But not every organization does what is necessary to keep access paths up-to-date with the current state of their data. Oh, there are always reasons given as to why the acknowledged “best practice” of REORG/RUNSTATS/REBIND is not followed religiously. But these reasons are not always reasonable - especially when there are approaches to overcome them.

But let's approach this subject from another perspective: that is, from a change management procedures perspective. On the mainframe, change has traditionally been strictly controlled. But one exception has been DB2 access paths. In a mainframe shop everything we do is tightly controlled. If we make even a minor change to an application program, that program is thoroughly tested before it ever reaches a production environment. The program progresses through unit testing, QA testing, volume testing, and so on. As developers, we do a good job of testing a change to minimize the risk that the change might have unintended consequences.

We do the same type of due diligence with most other changes in the mainframe world. Database changes are planned and thoroughly tested. Many shops use change manager software to ensure that database changes are implemented appropriately and in a controlled manner. System software (e.g. CICS, WebSphere, etc.), including subsystem and DB2 changes, are all subject to strict change control procedures. This is done to minimize disruption to the production work being conducted by our business folks.

But, if you think about it, there is one exception to this tight change control environment: Binds and Rebinds are typically done in the production environments without the benefit of oversight or prior testing. This lack of change control results in unpredictable performance impacts. In most shops, programs are moved to production and bound there. Indeed, we are at the mercy of the DB2 optimizer, which generates access paths on the fly when we Bind or Rebind our programs. Any issues with inefficient access paths are then dealt with in a reactive mode. That is, problems are addressed after the fact.

One of the biggest reasons for not implementing strict change control processes for access paths is the lack of built-in methods for ensuring access path change control discipline. Let’s face it, manually evaluating thousands of packages and tens of thousands of SQL statements can be quite impractical. But there are things that can be done to help alleviate this problem.

Rebinding does not always produce DB2 performance improvements—and in some cases rebinding can cause DB2 performance to degrade. Typically, 75% to 90% of all rebinds are unnecessary. Bind ImpactExpert (from NEON Enterprise Software) manages the bind and rebind processes to assure optimal application performance by checking what access path changes will be before your rebind, and then only rebinding those programs where performance would improve. How does it do this?

Well, Bind ImpactExpert helps to eliminate the unpredictable results of rebinds that DBAs experience daily. By filtering out the rebinds that are likely to have a negative impact on DB2 performance, Bind ImpactExpert guarantees improved or consistent performing DB2 applications.

And one of the most popular features of Bind ImpactExpert is its ability to eliminate the unpredictability of rebinds when moving between DB2 releases. The EarlyPrecheck feature assists you in moving to new DB2 releases. The EarlyPrecheck feature allows you to evaluate and correct access path problems for both dynamic and static SQL prior to an actual DB2 version upgrade. You can perform access path evaluation weeks or months prior to the migration date to allow for in-depth analysis and pre-emptive correction of access path issues.

Bind ImpactExpert can also be stop unnecessary binds. It does this by determining whether a DBRM contains changed SQL and skipping the bind step for those that do not. This process eliminates unnecessary binds, optimizes and accelerates the compile procedure, reduces CPU usage, and reduces locks to the DB2 catalog.

All forward-thinking organizations should adopt a liberal Rebind process to ensure optimal access paths based on up-to-date statistics. Keeping abreast of data changes and making sure that your programs are optimized for the current state of the data is the best approach. This means regular executions of RUNSTATS, REORG, and Rebind.

So, if you are not rebinding your programs on a regular basis because you are afraid of degrading a few access paths, it is time to take a look at Bind ImpactExpert. 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. I mean, why would you want to penalize every DB2 program in your subsystem for fear that a program or two may have a few degraded access paths? Especially when NEON Enterprise Software offers an automated solution to the problem...

The bottom line is this: failing to keep your access paths aligned with your data is a sure recipe for declining DB2 application performance.

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