SQL optimization is a four-phase process:
Tip: The DB2 LUW Cost is only an estimate of the resources it takes to execute a SQL statement. It is essential to run Batch Execute on the alternatives to determine which statement actually performs the best.
Click to open a SQL Optimizer session.
On the SQL tab, enter the SQL statement you want to optimize.
Click . This step launches the SQL Rewrite process that automatically transforms the syntax of the SQL statement.
After the rewrite, the Rewrite Details dialog shows the total number of semantically equivalent SQL statements, the number of alternative statements with access plans different from your original statement, and a warning message if the number of SQL transformations reaches any of the optimization quotas set in the Options.
The SQL Optimizer window shows several tabs that provide information about the original SQL statement and its SQL alternatives. The SQL tab displays the SQL text and the access plan of the currently selected SQL alternative (or the original SQL statement). At the bottom of the tab is the Run Time pane. This pane lists the original SQL statement, the SQL alternatives, and the run-time statistics for all of these after they are executed. At this point, since you have not yet run Batch Execute, the pane shows displays only DB2 LUW cost value for the original SQL statement and each SQL alternative.
In the Run Time pane, select an SQL alternative.
Go to the Access Plan tab to view the alternative's access plan and statistics for the objects accessed by the SQL statement.
To see how the syntax of an alternative SQL statement differs from that of your original SQL, do the following:
Go to the Compare tab.
Your original SQL statement is displayed in one pane of the page and the alternative statement in the other pane. Blue highlighted shows differences in the SQL syntax.
Click , , or to customize what is displayed on the page.
Go to the Plan Statistics tab to compare the cost estimates between the original SQL access plan and the plan for each alternative.
In the SQL Optimizer window, click to generate virtual index sets. These index sets include those that the SQL Optimizer's Index Expert component recommends and those that DB2 recommends.
Note: If you are connected toDB2 LUW 8 or later, Index Expert generates its own virtual index sets and optionally includes indexes that DB2 recommends. If you are using DB2 LUW 7, only DB2-recommended index sets are retrieved.
The resulting virtual index-sets display as alternatives in the in the Run Time pane. Index-set alternatives recommended by Index Expert are labeled Setx; those recommended by DB2 are labeled DB2 LUW.
To prepare to execute the original SQL, SQL alternatives, and index-set alternatives, click .
In the Batch Run Criteria window, select the Selected SQL/Index Set tab.
Select which alternatives to execute. The blue checkmark in the left column indicates that the alternative is selected. By default, all alternatives are selected.
To unselect a statement, right-click the alternative and select the appropriate option.
Select the defaults on the remaining tabs in the Batch Criteria window, and click OK.
Note: If you need to make edits to any criteria, see Retrieve the Run Time for a Group of Alternatives for more information.
The Batch Run window opens enabling you to view the results as each statement executes.
When all the selected SQL statements have finished executing, the Batch Run Details window appears. Click OK.
In the Run Time pane on the SQL tab, review the columns that contain various types of execution time.
Once you have identified the most-efficient alternative SQL statement, you can any of the following:
Copy and paste it back in your application.
Save the alternative SQL statement in a text file either individually or with multiple SQL statements in the Optimized SQL report.
Save your SQL optimization results for later review. Select SQL | Saved Optimized SQL.