124 lines
7.4 KiB
HTML
124 lines
7.4 KiB
HTML
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<meta name="DC.Title" content="Radix index scan" />
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<meta name="abstract" content="A radix index scan operation is used to retrieve the rows from a table in a keyed sequence. Like a Table Scan, 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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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<title>Radix index scan</title>
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</head>
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<body id="rzajqindscan"><a name="rzajqindscan"><!-- --></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">Radix index scan</h1>
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<div><p>A radix index scan operation is used to retrieve the rows from
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a table in a keyed sequence. Like a Table Scan, all of the rows in the index
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will be sequentially processed, but the resulting row numbers will be sequenced
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based upon the 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). They can be also used
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to provide faster throughput by performing selection against the index keys
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rather than all the rows in the table. Since the I/Os associated with the
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index will only contain the index keys, typically more rows can be paged into
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memory in one I/O against the index than the rows from a table with a large
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number of columns.</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. Radix index scan attributes</caption><thead align="left"><tr><th valign="top" width="31.979695431472084%" id="d0e44">Data access method</th>
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<th valign="top" width="68.02030456852792%" id="d0e46">Radix 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="d0e44 "><strong>Description</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 ">Sequentially scan and process all of the keys associated
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with the index. Any selection is applied to every key value of the index before
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a table row </td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e44 "><strong>Advantages</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 "><ul><li>Only those index entries that match any selection continue to be processed</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="d0e44 "><strong>Considerations</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 ">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="d0e44 "><strong>Likely to be used</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 "><ul><li>When asking for or expecting only a few rows to be returned from the index </li>
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<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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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e44 "><strong>Example SQL statement</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 "><pre>CREATE INDEX X1 ON Employee (LastName, WorkDept)
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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 30 ROWS</pre>
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</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e44 "><strong>Messages indicating use</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 "><ul><li>Optimizer Debug: <pre> CPI4328 -- Access path of file X1 was used by query.</pre>
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</li>
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<li>PRTSQLINF: <pre> SQL4008 -- Index X1 used 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="d0e44 "><strong>SMP parallel enabled</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 ">Yes</td>
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</tr>
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<tr><td valign="top" width="31.979695431472084%" headers="d0e44 "><strong>Also referred to as</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 ">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="d0e44 "><strong>Visual Explain icon</strong></td>
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<td valign="top" width="68.02030456852792%" headers="d0e46 "><br /><img src="rzajq507.gif" alt="Radix index scan icon" /><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>
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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="perf21b.htm" title="An SQL index (or keyed sequence access path) is a permanent object that is created over a table and used by the optimizer to provide a sequenced view of the data for a scan or probe operation.">Radix index</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="ipuc.htm" title="Some complex queries can perform better by using a sort or hashing method to evaluate the query instead of using or creating an index.">Effects of the ALWCPYDTA parameter on database performance</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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