61 lines
3.6 KiB
HTML
61 lines
3.6 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="When to create EVIs" />
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<meta name="abstract" content="There are several instances when you should consider creating EVIs." />
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<meta name="description" content="There are several instances when you should consider creating EVIs." />
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<meta name="DC.subject" content="encoded vector index, creating" />
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<meta name="keywords" content="encoded vector index, creating" />
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<meta name="DC.Relation" scheme="URI" content="whatareevi.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="whentouseevis" />
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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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<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>When to create EVIs</title>
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</head>
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<body id="whentouseevis"><a name="whentouseevis"><!-- --></a>
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<img src="./delta.gif" alt="Start of change" /><!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">When to create EVIs</h1>
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<div><p>There are several instances when you should consider creating EVIs.</p>
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<div class="section"><p>Encoded vector indexes should be considered when any one of the
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following is true:</p>
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<ul><li>You want to gather 'live' statistics</li>
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<li>Full table scan is currently being selected for the query</li>
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<li>Selectivity of the query is 20%-70% and using skip sequential access with
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dynamic bitmaps will speed up the scan</li>
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<li>When a star schema join is expected to be used for star schema join queries. </li>
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<li>When grouping or distinct queries are specified against a column, the
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columns has a small number of distinct values and (if a column function is
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specified at all) only the COUNT column function is used. </li>
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</ul>
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</div>
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<div class="section"><p>Encoded vector indexes should be created with: </p>
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<ul><li>Single key columns with a low number of distinct values expected</li>
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<li>Keys columns with a low volatililty (they don't change often)</li>
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<li>Maximum number of distinct values expected using the WITH n DISTINCT VALUES
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clause</li>
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<li> Single key over foreign key columns for a star schema model</li>
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</ul>
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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>
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<img src="./deltaend.gif" alt="End of change" /></body>
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</html>
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