79 lines
4.1 KiB
HTML
79 lines
4.1 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE 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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<html lang="en-us" xml:lang="en-us">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Use indexes" />
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<meta name="abstract" content="The use of indexes can improve significantly the performance of your applications." />
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<meta name="description" content="The use of indexes can improve significantly the performance of your applications." />
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<meta name="DC.Relation" scheme="URI" content="rzaikdbdesign.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1999, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1999, 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="indexes" />
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<meta name="DC.Language" content="en-us" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Use indexes</title>
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</head>
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<body id="indexes"><a name="indexes"><!-- --></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">Use indexes</h1>
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<div><p>The use of indexes can improve significantly the performance of
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your applications. </p>
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<div class="section"><p>This is because the <a href="rzaikoptimizer.htm#optimizer">Optimizer</a> uses
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them for performance optimization. Indexes are created in five different ways:
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</p>
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<ul><li>CREATE INDEX (in <span class="keyword">SQL</span>)</li>
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<li>CRTPF, with key</li>
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<li>CRTLF, with key</li>
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<li>CRTLF, as join logical file</li>
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<li>CRTLF, with select/omit specifications, without a key, and without dynamic
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selection (DYNSLT).</li>
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</ul>
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</div>
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<div class="section"><p>Indexes are used to enable row selection by means of index-versus-table
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scanning, which is usually slower. Table scanning sequentially processes all
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rows in a table. If a permanent index is available, building a temporary index
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can be avoided. Indexes are required for: </p>
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<ul><li>Join tables</li>
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<li>ORDER BY</li>
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<li>GROUP BY</li>
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</ul>
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</div>
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<div class="section"><p>Indexes will be created, if no permanent index exists.</p>
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</div>
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<div class="section"><p>Manage the number of indexes to minimize the extra server cost
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of maintaining the indexes during update operations. Below are general rules
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for particular types of tables: </p>
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<dl><dt class="dlterm">Primarily read-only tables:</dt>
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<dd>Create indexes over columns as needed. Consider creating an index only
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if a table is greater than approximately 1,000 rows or is going to be used
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with ORDER BY, GROUP BY, or join processing. Index maintenance could be costlier
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than occasionally scanning the entire table.</dd>
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<dt class="dlterm">Primarily read-only table, with low update rate:</dt>
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<dd>Create indexes over columns as needed. Avoid building indexes over columns
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that are updated frequently. INSERT, UPDATE, and DELETE will cause maintenance
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to all indexes related to the table.</dd>
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<dt class="dlterm">High update-rate tables:</dt>
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<dd>Avoid creating many indexes. An example of a table that has a high update
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rate is a logging or a history table.</dd>
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</dl>
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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="rzaikdbdesign.htm" title="Use the following topics to determine what tables you require in your database and to understand the relationship between those tables.">Database design</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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