98 lines
6.6 KiB
HTML
98 lines
6.6 KiB
HTML
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="DC.Title" content="Recommendations for EVI use" />
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<meta name="abstract" content="Encoded vector indexes are a powerful tool for providing fast data access in decision support and query reporting environments, but to ensure the effective use of EVIs, you should implement EVIs with the following guidelines:" />
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<title>Recommendations for EVI use</title>
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</head>
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<body id="recommendationsforevi"><a name="recommendationsforevi"><!-- --></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">Recommendations for EVI use</h1>
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<div><p>Encoded vector indexes are a powerful tool for providing fast data
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access in decision support and query reporting environments, but to ensure
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the effective use of EVIs, you should implement EVIs with the following guidelines: </p>
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<div class="section"><h4 class="sectiontitle">Create EVIs on</h4><ul><li>Read-only tables or tables with a minimum of INSERT, UPDATE, DELETE activity.</li>
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<li>Key columns that are used in the WHERE clause - local selection predicates
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of SQL requests.</li>
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<li>Single key columns that have a relatively small set of distinct values.</li>
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<li>Multiple key columns that result in a relatively small set of distinct
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values. </li>
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<li>Key columns that have a static or relatively static set of distinct values. </li>
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<li>Non-unique key columns, with many duplicates.</li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">Create EVIs with the maximum byte code size expected</h4><ul><li>Use the "WITH n DISTINCT VALUES" clause on the CREATE ENCODED VECTOR INDEX
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statement. </li>
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<li>If unsure, use a number greater than 65,535 to create a 4 byte code, thus
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avoiding the EVI maintenance overhead of switching byte code sizes.</li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">When loading data</h4><ul><li>Drop EVIs, load data, create EVIs.</li>
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<li>EVI byte code size will be assigned automatically based on the number
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of actual distinct key values found in the table. </li>
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<li>Symbol table will contain all key values, in order, no keys in overflow
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area. </li>
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</ul>
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</div>
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<div class="section"><h4 class="sectiontitle">Consider SMP and parallel index creation and maintenance</h4><p>Symmetrical
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Multiprocessing (SMP) is a valuable tool for building and maintaining indexes
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in parallel. The results of using the optional SMP feature of <span class="keyword">i5/OS™</span> are
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faster index build times, and faster I/O velocities while maintaining indexes
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in parallel. Using an SMP degree value of either *OPTIMIZE or *MAX, additional
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multiple tasks and additional server resources are used to build or maintain
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the indexes. With a degree value of *MAX, expect linear scalability on index
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creation. For example, creating indexes on a 4 processor server
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can be 4 times as fast as a 1 processor server. </p>
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</div>
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<div class="section"><h4 class="sectiontitle">Checking values in the overflow area</h4><p>You can also
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use the <span class="cmdname">Display File Description (DSPFD)</span> command (or iSeries™ Navigator
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- Database) to check how many values are in the overflow area. Once the <span class="cmdname">DSPFD</span> command
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is issued, check the overflow area parameter for details on the initial and
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actual number of distinct key values in the overflow area. </p>
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</div>
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<div class="section"><h4 class="sectiontitle">Using CHGLF to rebuild an index's access path</h4><p>Use
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the <span class="cmdname">Change Logical File (CHGLF)</span> command with the attribute
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Force Rebuild Access Path set to <em>YES</em> (FRCRBDAP(*YES)). This command
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accomplishes the same thing as dropping and recreating the index, but it does
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not require that you know about how the index was built. This command is especially
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effective for applications where the original index definitions are not available,
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or for refreshing the access path.</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="whatareevi.htm" title="An encoded vector index (EVI) is an index object that is used by the query optimizer and database engine to provide fast data access in decision support and query reporting environments.">Encoded vector indexes</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../db2/rbafzmstxcindx.htm">SQL Create Index statement</a></div>
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<div><a href="../cl/chglf.htm">Change Logical File (CHGLF) command</a></div>
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<div><a href="../cl/dspfd.htm">Display File Description (DSPFD) command</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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