Friday, August 21, 2009

Approaches to Access Path Management

BIND and REBIND are important components in assuring efficient DB2 applications. Because the
BIND/REBIND process determines exactly how your DB2 data is accessed, it is important that you develop an appropriate strategy for when and how to REBIND your programs.

There are several common REBIND approaches taken by DB2 users. By far, the best approach is to REBIND your applications over time as your data and systems change. 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.

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 (days, weeks, months, etc.). This approach can work if the period of time is wisely chosen based on the application data – but it still can pose administrative issues.

The final approach can be summarized as “if it ain’t broke don’t fix it!” This is the worst of the several approaches discussed here. The biggest problem with this approach is that you are penalizing every program in your subsystem for fear that a program or two may have a degraded access path. This results in potentially many programs having sub-optimal performance because the optimizer never gets a chance to create better access paths as the data changes.

Of course, the possibility of degraded performance is real – and that is why this approach has been adopted at some sites. The problem is being able to find which statements have degraded. In an ideal world we would be to be able to review the access path changes beforehand 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 on a regular basis as your data changes. This approach has become known as the three Rs. To implement this approach you:
  1. Regularly reorganize the data to ensure that it is optimally structured.
  2. Follow that with RUNSTATS to be sure that the reorganized state of the data is reflected in the DB2 Catalog.
  3. And follow that with a REBIND for all the application programs that access the data structures impacted by the REORG and RUNSTATS.
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. DB2 is unlikely to make the same access path choice as your data grows – and as patterns within the data change.

By REBINDing you can generally improve the overall performance of your applications because the access paths will be better designed based on an accurate view of the data. Additionally, as DB2 changes are introduced (PTFs, new version/release) optimizer improvements and new access techniques can be incorporated into the access paths. That is, 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.

Adopting the Three R’s approach can pose additional questions. For example, when should you reorganize? In order to properly determine when a REORG is needed you’ll have to look at statistics. This means looking at either RUNSTATS or Real-Time Statistics (RTS). So, we need to add another R – in other words:
  1. RUNSTATS or better yet, RTS
  2. REORG
Now it is true that some folks don’t rely on statistics to schedule a REORG. Instead, they just build the JCL to REORG their database objects when they create the object. So they create a table space then build the REORG job and schedule it to run monthly, or quarterly, or on some regular basis. This is better than no REORG at all, but it is probably not the best approach because you are most likely either reorganizing too soon (in which case you waste the CPU cycles to do the REORG) or you are reorganizing too late (in which case performance is suffering for a period of time before the REORG runs). Better to base your REORGs off of statistics and thresholds using either RUNSTATS or RTS.

Without accurate statistics there is little hope that the optimizer will formulate the best access path to retrieve your data. If the optimizer doesn’t have accurate information on the size, organization, and particulars of your data then it will be creating access paths based on either default or inaccurate statistics. Incorrect statistics will cause bad choices to be made – such as choosing a merge-scan join when a nested loop join would be better, or failure to invoke sequential prefetch, or using the wrong index – or no index at all. And the problem of inaccurate statistics is pervasive.

There are shops out there that never, or rarely, run RUNSTATS to gather up-to-date statistics. Make sure yours is not one of those shops!

When should you run RUNSTATS? One answer is “As frequently as possible based on how often your data changes.” To do this you will need to know a thing or two about your data growth patterns: what is its make-up, how is it used, how fast does it grow, and how often does it change? These patterns will differ for every table space in your system.

Next we need to decide when to REBIND? The best answer for this is when statistics have changed significantly enough to change access paths. When we know that data has significantly changed it makes sense to REBIND after the RUNSTATS completes. But the trick is determining exactly when we have a “significant” change in our data. Without an automated method of comparing and contrasting statistics (or even better yet, access paths) coming up with an answer in a manual way can be time-consuming and error-prone – especially if we have thousands of DB2 programs to manage.

As we REBIND, we always must be on alert for rogue access paths. A rogue access path is created when the optimizer formulates a new access path that performs worse than the previous access path. This can happen for a variety of reasons. Of course, number one is that the optimizer, though good, is not perfect. So mistakes can happen. Other factors can cause degraded access paths, too. The access paths for volatile tables depend on when you run the RUNSTATS. Volatile tables are those that start out empty, get rows added to them during processing, and are emptied out at the end of the day. And, of course, if the catalog or statistics are not accurate we can get problems, too.

So adopting the Four R’s approach implies that you will have to develop a methodology for reviewing your access paths and taking care of any “potential” problem access paths. Indeed, the Four R’s becomes the Five R’s as we add a step to review the access paths after REBINDing to make sure that there are no rogue access paths:

  1. Start RTS (or a RUNSTATS) to determine when to REORG.
  2. Reorganize the table spaces (and/or indexes)
  3. After reorganizing, run RUNSTATS again,
  4. Follow that with the REBINDs.
  5. Then we need that fifth R – which is to Review the access paths generated by the REBIND.
The review is of utmost importance because the optimizer can make mistakes. And, of course, so can you. But your users will not call you when performance is better (or the same). They only ring you up when performance gets worse. As such, proactive shops will put best practices in place to test REBIND results comparing the before and after impact of the optimizer’s choices.