59 lines
4.3 KiB
HTML
59 lines
4.3 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 name="security" content="public" />
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<meta name="Robots" content="index,follow" />
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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="Optimize the number of columns that are selected with SELECT statements" />
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<meta name="abstract" content="The number of columns that you specify in the select list of a SELECT statement causes the database manager to retrieve the data from the underlying tables and map the data into host variables in the application programs. By minimizing the number of columns that are specified, processing unit resource usage can be conserved." />
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<meta name="description" content="The number of columns that you specify in the select list of a SELECT statement causes the database manager to retrieve the data from the underlying tables and map the data into host variables in the application programs. By minimizing the number of columns that are specified, processing unit resource usage can be conserved." />
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<meta name="DC.subject" content="improving performance, SELECT statements, using effectively, performance improvement, SELECT statements, using effectively, SELECT statement, using effectively to improve performance" />
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<meta name="keywords" content="improving performance, SELECT statements, using effectively, performance improvement, SELECT statements, using effectively, SELECT statement, using effectively to improve performance" />
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<meta name="DC.Relation" scheme="URI" content="progtech.htm" />
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<meta name="DC.Relation" scheme="URI" content="../db2/rbafzmstintsel.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="selectopt" />
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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>Optimize the number of columns that are selected with SELECT statements</title>
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</head>
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<body id="selectopt"><a name="selectopt"><!-- --></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">Optimize the number of columns that are selected with SELECT statements</h1>
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<div><p>The number of columns that you specify in the select list of a
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SELECT statement causes the database manager to retrieve the data from the
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underlying tables and map the data into host variables in the
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application programs. By minimizing the number of columns that are specified,
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processing unit resource usage can be conserved.</p>
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<div class="section"><p>Even though it is convenient to code SELECT *, it is far better
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to explicitly code the columns that are actually required for the application.
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This is especially important if index-only access is wanted or if all of
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the columns will participate in a sort operation (as happens for SELECT DISTINCT
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and for SELECT UNION).</p>
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</div>
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<div class="section"><p>This is also important when considering index only access, since
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you minimize the number of columns in a query and thereby increase the odds
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that an index can be used to completely satisfy the request for all the data.</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="progtech.htm" title="By changing the coding of your queries, you can improve their performance.">Programming techniques for database performance</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/rbafzmstintsel.htm">select-statement</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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