Understanding object-level tuning

Performance tuning boils down to the user experience. Bad performance derives from the very subjective notion that a given application is somehow different from the acceptable norms. The common complaint heard from the user is, "It was fast yesterday but it's slow today". It is the job of the DBA to take these subjective ideas and discover objective facts that point to the cause for this unacceptable performance. In the end, database performance tuning is an analysis of why SQL statements are executing at a suboptimal rate. Even if the root cause is CPU starvation, incorrect configuration parameters, or just a poorly written DELETE statement, the key is first finding out what is slow and then what can be done to rectify the situation. Thus, it is wise to look first at how SQL statements are executed against database objects and what conditions can affect how they are executed.

Understanding the Oracle optimizer

Let's take the following simple SQL statement...