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This section includes the following topics:
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The SQL tab lets you analyze execution plans and explain results so that you can tune statements and achieve optimal results.
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Table 7-1 Launch in-context to following entities
Tab Entities
Dashboard Statement
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| Tab | Entities |
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| Dashboard | Statement |
| Current | Statement, Active or Current Session that is currently executing a statement. |
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| Activity | Statement |
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| If | Statement that is affected by one or more index evaluations. |
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If the SQL tab is opened with no statement in-context, the following error message is displayed:
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In the Current tab, you can either launch the SQL tab by clicking the SQL tab button when the selected entity in the Main area is a statement, or by selecting a statement in the Association area and clicking the Tune icon that appears before the SQL text.
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There can be statements with many execution plans. See “About the Dashboard tab” on page 23.See , “About the Current tab” on page 30. See , “About the Activity tab” on page 39. See , and “About the Objects tab” on page 67.
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The following sections explain how to view information and navigate through the SQL tab:
- About viewing the execution plan of a statement
- About interpreting text highlighting on the Highlights tab
- About viewing which objects are referenced by the execution plan
- About viewing information on all operators and predicates in the Execution plan
- About viewing the statement text
- About viewing recommended indexes for a statement
- About viewing additional execution plan details
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The Plan Tree area displays the execution plan of a statement and related information, such as statistics, referenced objects, and operations performed in the execution plan. It lets you assess and tune statements based on real information.
Two types of execution plans are available when launching to the SQL tab:
Virtual execution
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plan. The plan which was collected from DB2 V$ tables. Additional real-time statistics are available for these plans. For example, you can view a breakdown of In DB2 time for a plan as compared to other plans, or a breakdown of its I/O when accessing DB2 objects.
When launching to the SQL tab with a specific SQL statement in context, Precise for DB2 presents actual execution plan information, including actual plan steps, and information and statistics for multiple plans. For example, if a statement has several different execution plans, all are displayed.
Estimated execution
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plan. Depending upon the version of DB2 running on your system, you may be only able to generate and run an estimated execution plan. At times, real execution plan information is lost (as when they are removed from the system before Precise for DB2 could access them). If this happens, all real-time statistics (such as, I/O, and In DB2) are considered to belong to other plans.
The Plan view is divided into three panes. The Plan Tree area and the Text View area are displayed in the left pane. The Details area is displayed in the right pane. The information displayed in the Details area is controlled by the information tabs (Highlights, Objects, Operators, Text, and Recommendations) that are located at the top of this area.
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Table 7-2 Information tabs
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| Tabs | Description |
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| Highlights | The default tab, it displays the statement’s Optimized text. When opening a new plan tree or when clicking on the root of the tree, the original text is displayed (Text tab). When an operator is clicked, the text will be switched to the optimized text of the statement. |
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| Objects | Displays information on all referenced objects in the execution plan, including their tables, indexes, and columns. Statistical and general details are displayed for each object. |
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| Operators | Displays information on all operators in the Execution plan tree. When you sort a table, you can locate operators by their total cost or by any other column in the table. |
| Text |
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| Displays the Original text of the statement. |
| Recommendations |
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Displays new index recommendations for the statement. These are additional indexes that can improve the statement.
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About the Execution Plan tree
The Execution Plan tree displays both the Virtual execution plan and the Estimated execution plan, of the specified statement.
The execution plan's text appears both in the Text View area (Text tab) at the bottom of the execution plan tree and at the top of the Details area (also a Text tab). The text that relates to the selected step in the Plan tree is highlighted. This lets you view the text of the statement, the execution plan, and additional information, such as the objects referenced by the statement, all at once. In the Text View area you can also view two additional tabs displaying Optimized Text and Remote Query Text.
Actions that can be performed on the tree
You can perform the following actions on the execution plan tree:
- The Playback controls, located at the top of the tree, enable you to freely move within the execution plan of an explained statement (step by step).
- By moving the pointer over the execution plan steps, you can view a ToolTip that contains statistical information, such as Estimated Cost, Estimated Rows, and Estimated I/O Cost, and a description of the specified step.
- Selecting a specific step (operator) highlights nested steps and affects the information displayed in the Detail tabs: the relevant Optimized text is shown on the Highlights tab; the referenced tables, indexes, and columns are shown on the Objects tab; the operators and predicates are shown on the Operators tab; and the Original text is shown on the Text tab. The Recommendations tab, only displayed when you generate recommendations, shows index details and the DDL for creating the indexes.
- Clicking the hatchet mark ( ), located at the bottom of the tree, displays the Tree View area with three tabs allowing you to select which statement text to view:
- Optimized Text
- Original Text
- Remote Query Text
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When launching to the SQL tab, the Highlights tab in the Details area and the Text tab in the Text View area are selected by default. |
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The Highlights tab displays the statement’s Optimized text as generated by the DB2 optimizer. Depending upon the operator selected on the Tree, the text will have different colors highlighting relevant tables, columns, and predicates.
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Note: Not all operators include color fonts. Color fonts are used only when relevant for performance tuning.
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The Objects tab displays three tables (Tables used in Plan, Indexes on Table, Columns in Table) that list all referenced objects in the execution plan, including their indexes and columns. Statistical details and general details are displayed for each object and its sub-entities.
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Key Number The position within the chosen index.
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The Operators tab displays information on all operators in the Execution plan tree. When you sort a table, you can locate operators by their total cost or by any other column in the table.
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Sub Select Indicates when the subquery used in this predicate is evaluated.
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The Text tab displays the statement’s original text.
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The Recommendations tab displays the output of the Recommend algorithm. The Recommendations tab is divided into three areas:
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The Details area displays statistical information of the recommended index. The DDL area displays the create index statement.
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You can view additional execution plan details by hovering over a specific execution plan row with the mouse. A ToolTip is displayed showing some of the following details, depending upon which row you are hovering over:
- Operator Type
- Description
- Estimated Total Cost
- First row cost
- Estimated I/O cost
- Estimated CPU cost
- Estimated communication cost
- Contribution Ratio
- Estimated total cost
- Operator Arguments such as:
- MAXPAGES=ALL
- PREFETCH=NONE
- ROWLOCK=EXCLUSIVE
- SCANDIR=FORWARD
- TABLOCK=INTENT EXCLUSIVE
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Use the Actions menu to perform the following operations:
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