133 lines
8.2 KiB
HTML
133 lines
8.2 KiB
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<meta name="DC.Title" content="Temporary index probe" />
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<meta name="abstract" content="A temporary index probe operation is identical to the index probe operation that is performed upon the permanent radix index. Its main function is to provide a form of quick access against the index keys of the temporary index; however it can still used to retrieve the rows from a table in a keyed sequence." />
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<title>Temporary index probe</title>
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<body id="rzajqtempinprobe"><a name="rzajqtempinprobe"><!-- --></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">Temporary index probe</h1>
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<div><p>A temporary index probe operation is identical to the index probe
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operation that is performed upon the permanent radix index. Its main function
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is to provide a form of quick access against the index keys of the temporary
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index; however it can still used to retrieve the rows from a table in a keyed
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sequence.</p>
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<div class="section"><p>The temporary index is used by the optimizer to satisfy the join
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portion of the query request. </p>
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</div>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. Temporary index probe attributes</caption><thead align="left"><tr><th valign="top" width="31.979695431472084%" id="d0e42">Data access method</th>
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<th valign="top" width="68.02030456852792%" id="d0e44">Temporary index probe</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Description</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 ">The index is quickly probed based upon the selection
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criteria that were rewritten into a series of ranges. Only those keys that
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satisfy the selection will be used to generate a table row number. </td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Advantages</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 "><ul><li>Only those index entries that match any selection continue to be processed.
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Provides very quick access to the selected rows </li>
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<li>Potential to extract all of the data from the index keys' values, thus
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eliminating the need for a Table Probe</li>
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<li>Returns the rows back in a sequence based upon the keys of the index</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Considerations</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 ">Generally requires a Table Probe to be performed to
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extract any remaining columns required to satisfy the query. Can perform poorly
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when a large number of rows are selected because of the random I/O associated
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with the Table Probe. </td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Likely to be used</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 "><ul><li>When the ability to probe the rows required for the query (for example,
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joins) exists</li>
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<li>When the selection columns cannot be matched against the leading key columns
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of the index</li>
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<li>When the overhead cost associated with the creation of the temporary index
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can be justified against other alternative methods to implement this query</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Example SQL statement</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 "><pre>SELET * FROM Employee XXX, Department YYY
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WHERE XXX.WorkDept = YYY.DeptNo
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OPTIMIZE FOR ALL ROWS </pre>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Messages indicating use</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 ">There are multiple ways in which a temporary index probe
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can be indicated through the messages. The messages in this example illustrate
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one example of how the Classic Query Engine will indicate a temporary index
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probe was used. <ul><li>Optimizer Debug: <pre>CPI4321 -- Access path built for file DEPARTMENT.
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CPI4327 -- File EMPLOYEE processed in join
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position 1.
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CPI4326 -- File DEPARTMENT processed in join
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position 2.</pre>
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</li>
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<li>PRTSQLINF: <pre>SQL4007 -- Query implementation for join
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position 1 table 1.
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SQL4010 -- Table scan access for table 1.
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SQL4007 -- Query implementation for join
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position 2 table 2.
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SQL4009 -- Index created for table 2.</pre>
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</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>SMP parallel enabled</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 ">Yes</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Also referred to as</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 ">Index Probe <p>Index Probe, Preload</p>
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<p>Index Probe,
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Distinct</p>
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<p>Index Probe Distinct, Preload</p>
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<p>Index Probe, Key Selection</p>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e42 "><strong>Visual Explain icon</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e44 "><br /><img src="rzajq531.gif" alt="Temporary index probe" /><br /></td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="section"><p>Using the example above, the optimizer chose to create a temporary
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index over the DeptNo column to help satisfy the join requirement against
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the DEPARTMENT table. A temporary index probe was then performed against the
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temporary index to process the join criteria between the two tables. In this
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particular case, there was no additional selection that might be applied against
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the DEPARTMENT table while the temporary index was being created. </p>
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</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="rzajqtempindex.htm" title="A temporary index is a temporary object that allows the optimizer to create and use a radix index for a specific query. The temporary index has all of the same attributes and benefits as a radix index that is created by a user through the CREATE INDEX SQL statement or Create Logical File (CRTLF) CL command.">Temporary index</a></div>
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</div>
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</div>
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</body>
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</html>
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