Before going into the various methods that can be used to alter access paths or influence access path selection, let's first take a moment to laud the DB2 optimizer and the fine minds that built it. The DB2 optimizer is one of the most intricate pieces of software on the market. It does an admirable job of optimizing SQL requests. To achieve this level of success, the optimizer contains a great deal of performance-specific expertise. For example, the optimizer estimates both elapsed times and CPU times when choosing an access path. When a SQL statement is rebound, the optimizer might choose a new access path that increases CPU time but decreases elapsed time.
Now many shops may choose to enhance elapsed time at the expense of additional CPU use because elapsed time has a measurable effect on user productivity. In other words, it can be good to trade off CPU cycles for user satisfaction, and the DB2 optimizer attempts to accomplish this. Of course, if both CPU and elapsed time can be reduced, the optimizer tries to do that, too.
But the DB2 optimizer is not infallible. Sometimes the application analyst or DBA understands the nature of the data better than DB2. And that is the reason that there are ways to influence the optimizer into choosing an access path that you know is a better one but the optimizer thinks is a worse one.
As the functionality and complexity of the optimizer is enhanced from release to release of DB2, the need to trick the optimizer in this way invariably diminishes. But until the day when software is perfect, the need to tweak it will remain (and I'm pretty sure that will be the case throughout my lifetime).
There are five ways to influence the optimizer’s access path decisions:
- Updating DB2 Catalog statistics
- Standard, DB2-based methods
- Tweaking SQL statements
- Specifying the OPTIMIZE FOR n ROWS clause
- Using OPTHINT to indicate that an access path in the PLAN_TABLE should be chosen
Over the course of the ensuing weeks, we will examine each of these methods, and more. So stay tuned as we delve into the techniques at your disposal to influence the choices made by the DB2 optimizer.