120 lines
8.5 KiB
HTML
120 lines
8.5 KiB
HTML
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<meta name="DC.Title" content="Overview of information available from Visual Explain" />
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<meta name="abstract" content="You can use Visual Explain to view many types of information." />
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<meta name="DC.subject" content="query optimizer index advisor, in Visual Explain, Visual Explain, index advisor" />
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<meta name="DC.Relation" scheme="URI" content="idxadvisor.htm" />
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<title>Overview of information available from Visual Explain</title>
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<body id="rzajqovrve"><a name="rzajqovrve"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Overview of information available from Visual Explain</h1>
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<div><p>You can use Visual Explain to view many types of information.</p>
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<div class="section"><p>The information includes:</p>
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<ul><li>Information about each operation (icon) in the query graph</li>
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<li>Highlight expensive icons</li>
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<li>The statistics and index advisor</li>
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<li>The predicate implementation of the query</li>
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<li>Basic and detailed information in the graph</li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">Information about each operation (icon) in the query graph</h4><p>As
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stated before, the icons in the graph represent operations that occur during
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the implementation of the query. The order of operations is shown by the arrows
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connecting the icons. If parallelism was used to process an operation, the
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arrows are doubled. Occasionally, the optimizer "shares" hash tables with
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different operations in a query, causing the lines of the query to cross.</p>
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</div>
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<div class="section"><p>You can view information about an operation by selecting the icon.
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Information is displayed in the <span class="uicontrol">Attributes</span> table in
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the right pane. To view information about the environment, click an icon and
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then select <span class="uicontrol">Display query environment</span> from the <span class="uicontrol">Action</span> menu.
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Finally, you can view more information about the icon by right-clicking the
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icon and selecting <span class="uicontrol">Help</span>.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Highlight expensive icons</h4><p>You can highlight problem
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areas (expensive icons) in your query using Visual Explain. Visual Explain
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offers you two types of expensive icons to highlight: by processing time or
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number of rows. You can highlight icons by selecting <span class="uicontrol">Highlight
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expensive icons</span> from the <span class="uicontrol">View</span> menu.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">The statistics and index advisor</h4><p>During the implementation
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of a query, the optimizer can determine if statistics need to be created or
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refreshed, or if an index might make the query run faster. You can view these
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recommendations using the Statistics and Index Advisor from Visual Explain.
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Start the advisor by selecting <span class="uicontrol">Advisor</span> from the <span class="uicontrol">Action</span> menu.
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Additionally, you can begin collecting statistics or create an index directly
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from the advisor.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">The predicate implementation of the query</h4><p>Visual
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explain allows you to view the implementation of query predicates. Predicate
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implementation is represented by a blue plus sign next to an icon. You can
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expand this view by right-clicking the icon and selecting <span class="uicontrol">Expand</span>.
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or open it into another window. Click an icon to view attributes about the
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operation. To collapse the view, right-click anywhere in the window and select <span class="uicontrol">Collapse</span>.
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This function is only available on V5R3 or later systems.</p>
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</div>
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<div class="section"><p>The optimizer can also use the Look Ahead Predicate Generation
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to minimize the random the I/O costs of a join. To highlight predicates that
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used this method, select <span class="uicontrol">Highlight LPG</span> from the <span class="uicontrol">View</span> menu.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Basic and full information in the graph</h4><p>Visual Explain
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also presents information in two different views: basic and full. The basic
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view only shows those icons that are necessary to understand the implementation
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of the SQL statement, thus excluding some preliminary or intermediate operations
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that are not essential for understanding the main flow of query implementation.
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The full view may show more icons that further depict the flow of the execution
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tree. You can change the graph detail by select <span class="uicontrol">Graph Detail</span> from
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the <span class="uicontrol">Options</span> menu and selecting either <span class="uicontrol">Basic</span> or <span class="uicontrol">Full</span>.
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The default view is <span class="uicontrol">Basic</span>. Note that in order to see
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all of the detail for a <span class="uicontrol">Full</span> view, you will need to
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change the Graph Detail to <span class="uicontrol">Full</span>, close out Visual Explain,
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and run the query again. The setting for Graph Detail will persist.</p>
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</div>
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<div class="section"><p>For more information about Visual Explain and the different options
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that are available, see the Visual Explain online help.</p>
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</div>
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<div class="section"><img src="./delta.gif" alt="Start of change" /><h4 class="sectiontitle">Refresh the Visual Explain diagram</h4><p>For
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long running queries, you can refresh the visual explain graph with runtime
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statistical information before the query is complete. Refresh also updates
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the appropriate information in the attributes section of the icon shown on
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the right of the screen. In order to use the <span class="uicontrol">Refresh</span> option,
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you need to select <span class="uicontrol">Explain while Running</span> from the Run
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SQL Scripts window.</p>
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<p>To refresh the diagram, select <span class="uicontrol">Refresh</span> from
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the <span class="uicontrol">View</span> menu. Or click the <span class="uicontrol">Refresh</span> button
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in the toolbar.</p>
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<img src="./deltaend.gif" alt="End of change" /></div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="visexpl.htm" title="You can use the Visual Explain tool with iSeries Navigator to create a query graph that graphically displays the implementation of an SQL statement. You can use this tool to see information about both static and dynamic SQL statements. Visual Explain supports the following types of SQL statements: SELECT, INSERT, UPDATE, and DELETE.">View the implementation of your queries with Visual Explain</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="idxadvisor.htm" title="The query optimizer analyzes the row selection in the query and determines, based on default values, if creation of a permanent index improves performance. If the optimizer determines that a permanent index might be beneficial, it returns the key columns necessary to create the suggested index.">Query optimizer index advisor</a></div>
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</div>
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</div>
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</body>
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