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- It is important that statistics are up to date so that the simulation is as accurate as possible.
- What-If is only as good as the data it has to work on. It is dependent on statements being loaded into the PMDB and explained by the “Explain Statements” PMDB process.
- Until all functions within an application are exercised, Precise for DB2 cannot give a true picture of all affected statements.
- The index recommendation feature in the SQL tab only examines a single statement. It may be the case that the What-If tab suggests that this recommendation should not be implemented because of its impact on other statements. It is always a good idea to use both tabs together.
See “Getting Getting current application information” on page 30, “Examining information, Examining performance over time” on page 39time, and “Analyzing Statement plans” on page 73 Analyzing statement plans.
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evaluate changes
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- “Adding an index” on page 85
Adding an index
Simulating an index change is a two-step process. First you need a list of proposed index changes. Typically this is done automatically for you by following the link on the Recommendation tab in the SQL tab. Alternatively, you can manually propose the addition, modification, or deletion of indexes.
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The DB2 Optimizer is used to compare the cost of the original execution plan with the cost of the new execution plan. To simulate removing an index, you still need to add a proposed index change to the table in the Main area. This is either done for you on the Advice tab in the SQL tab, or you can click Add to open the What if... dialog box.