125 lines
7.9 KiB
HTML
125 lines
7.9 KiB
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<meta name="DC.Title" content="Temporary index scan" />
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<meta name="abstract" content="A temporary index scan operation is identical to the index scan operation that is performed upon the permanent radix index. It is still used to retrieve the rows from a table in a keyed sequence; however, the temporary index object must first be created. All of the rows in the index will be sequentially processed, but the resulting row numbers will be sequenced based upon the key columns." />
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<meta name="description" content="A temporary index scan operation is identical to the index scan operation that is performed upon the permanent radix index. It is still used to retrieve the rows from a table in a keyed sequence; however, the temporary index object must first be created. All of the rows in the index will be sequentially processed, but the resulting row numbers will be sequenced based upon the key columns." />
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<meta name="DC.subject" content="definitions, temporary index scan access method, temporary index, scan access method, index, temporary, access method, temporary index scan" />
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<title>Temporary index scan</title>
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<body id="rzajqtempinscan"><a name="rzajqtempinscan"><!-- --></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 scan</h1>
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<div><p>A temporary index scan operation is identical to the index scan
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operation that is performed upon the permanent radix index. It is still used
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to retrieve the rows from a table in a keyed sequence; however, the temporary
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index object must first be created. All of the rows in the index will be sequentially
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processed, but the resulting row numbers will be sequenced based upon the
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key columns.</p>
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<div class="section"><p>The sequenced rows can be used by the optimizer to satisfy a portion
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of the query request (such as ordering or grouping).</p>
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</div>
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<div class="section">
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. Temporary index scan attributes</caption><thead align="left"><tr><th valign="top" width="31.979695431472084%" id="d0e43">Data access method</th>
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<th valign="top" width="68.02030456852792%" id="d0e45">Temporary index scan</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="31.979695431472084%" headers="d0e43 "><strong>Description</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 ">Sequentially scan and process all of the keys associated
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with the temporary index. </td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e43 "><strong>Advantages</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 "><ul><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="d0e43 "><strong>Considerations</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 ">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="d0e43 "><strong>Likely to be used</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 "><ul><li>When sequencing the rows is required for the query (for example, ordering
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or grouping) </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="d0e43 "><strong>Example SQL statement</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 "><pre>SELECT * FROM Employee
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WHERE WorkDept BETWEEN 'A01' AND 'E01'
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ORDER BY LastName
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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="d0e43 "><strong>Messages indicating use</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 "><ul><li>Optimizer Debug: <pre>CPI4321 -- Access path built for file EMPLOYEE.</pre>
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</li>
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<li>PRTSQLINF: <pre>SQL4009 -- Index created for table 1.</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="d0e43 "><strong>SMP parallel enabled</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 ">Yes</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e43 "><strong>Also referred to as</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 ">Index Scan <p>Index Scan, Preload</p>
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<p>Index Scan,
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Distinct</p>
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<p>Index Scan Distinct, Preload</p>
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<p>Index Scan, 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="d0e43 "><strong>Visual Explain icon</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e45 "><br /><img src="rzajq530.gif" alt="Temporary radix index scan" /><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>
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<div class="section"><p>Using the example above, the optimizer chose to create a temporary
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index to sequence the rows based upon the LastName column. A temporary index
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scan might then be performed to satisfy the ORDER BY clause in this query.</p>
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</div>
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<div class="section"><p>The optimizer will determine where the selection against the WorkDept
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column best belongs. It can be performed as the temporary index itself is
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being created or it can be performed as a part of the temporary index scan.
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Adding the selection to the temporary index creation has the possibility of
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making the open data path (ODP) for this query non-reusable. This ODP reuse
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is taken into consideration when determining how selection will be performed. </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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