66 lines
4.0 KiB
HTML
66 lines
4.0 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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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Grouping, joining, and selection: Open Query File (OPNQRYF) command performance" />
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<meta name="abstract" content="When using an existing index, the optimizer ensures that the attributes of the selection, join, and grouping fields match the attributes of the keys in the existing index." />
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<meta name="description" content="When using an existing index, the optimizer ensures that the attributes of the selection, join, and grouping fields match the attributes of the keys in the existing index." />
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<meta name="DC.subject" content="performance, grouping, joining, and selection, grouping, joining, selection" />
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<meta name="keywords" content="performance, grouping, joining, and selection, grouping, joining, selection" />
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<meta name="DC.Relation" scheme="URI" content="rbafooqfsst.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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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Grouping, joining, and selection: Open Query File (OPNQRYF) command
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performance</title>
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</head>
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<body id="rbafooqfpcgrp"><a name="rbafooqfpcgrp"><!-- --></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">Grouping, joining, and selection: Open Query File (OPNQRYF) command
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performance</h1>
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<div><p>When using an existing index, the optimizer ensures that the attributes
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of the selection, join, and grouping fields match the attributes of the keys
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in the existing index.</p>
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<p>Also, the sort sequence table associated with the query must match the
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sequence table (a sort sequence table or an alternate collating sequence table)
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associated with the key field of the existing index. If the sequence tables
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do not match, the existing index cannot be used.</p>
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<div class="p">However, if the sort sequence table associated with the query is a unique-weight
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sequence table (including *HEX), some additional optimization is possible.
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The optimizer acts as though no sort sequence table is specified for any grouping
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fields or any selection or join predicates that use the following operators
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or functions: <ul><li>*EQ</li>
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<li>*NE</li>
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<li>*CT</li>
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<li>%WLDCRD</li>
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<li>%VALUES</li>
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</ul>
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</div>
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<div class="p">The advantage is that the optimizer is free to use any existing access
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path where the keys match the field and the access path either: <ul><li>Does not contain a sequence table.</li>
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<li>Contains a unique-weight sequence table (the table does not have to match
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the unique-weight sort sequence table associated with the query).</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="rbafooqfsst.htm" title="Here are tips and techniques for optimizing the performance of sort sequence tables.">Open Query File (OPNQRYF) command: Performance considerations for sort sequence tables</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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