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SQL Optimizer for DB2 LUW 4.3.1 - Installation Guide

SQL Optimizer Tutorial

SQL optimization is a four-phase process:

  • The SQL Rewrite step creates virtual alternative SQL statements, retrieves their respective access plans from DB2 LUW, and indicates each statement’s DB2 LUW cost. These alternatives produce the same results as your original SQL statement, but each has a different access plan.
  • The Generate Indexes step creates virtual index sets and adds these as alternatives.
  • The next phase test-runs your original SQL statement and any or all of the alternatives against your database to obtain run times.
  • The last phase is determining the SQL or index-set alternative that performs best.

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.

Rewrite the SQL statement

  1. Click to open a SQL Optimizer session.

  2. On the SQL tab, enter the SQL statement you want to optimize.

  3. Click image\B_Optimize.gif. This step launches the SQL Rewrite process that automatically transforms the syntax of the SQL statement.


    • The use of SQL Options and other optimization options such as temp table generation, ANSI JOIN syntax are optional and configurable in the Options.
    • The degree of the SQL transformation process is controlled by the Intelligence Level in the Options. The Intelligence Levels control how many options are applied to transformed SQL and how many SQL alternatives are created.
    • If your SQL statement uses a temporary table, see the section User-Defined Temp Table for the steps to create a temporary table in the User-Defined Temp Table module.

    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.

  4. Click Close.

    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.

  5. In the Run Time pane, select an SQL alternative.

  6. Go to the Access Plan tab to view the alternative's access plan and statistics for the objects accessed by the SQL statement.

  7. To see how the syntax of an alternative SQL statement differs from that of your original SQL, do the following:

    1. 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.

    2. Click , , or to customize what is displayed on the page.

  8. Go to the Plan Statistics tab to compare the cost estimates between the original SQL access plan and the plan for each alternative.

  9. Go back to the SQL tab.

Create index-set candidates

  1. In the SQL Optimizer window, click image\B_GenerateIndexes.gif 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.

Batch test SQL alternatives

  1. To prepare to execute the original SQL, SQL alternatives, and index-set alternatives, click image\B_BatchRun.gif.

  2. In the Batch Run Criteria window, select the Selected SQL/Index Set tab.

    1. Select which alternatives to execute. The blue checkmark in the left column indicates that the alternative is selected. By default, all alternatives are selected.

    2. To unselect a statement, right-click the alternative and select the appropriate option.

  3. 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.

  4. When all the selected SQL statements have finished executing, the Batch Run Details window appears. Click OK.

Review test results

  1. In the Run Time pane on the SQL tab, review the columns that contain various types of execution time.

  2. 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.


Related Topics

Generate Index-Set Alternatives

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