57 lines
3.9 KiB
HTML
57 lines
3.9 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="Implementation and optimization of the UNION and DISTINCT clauses with DB2 Multisystem" />
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<meta name="abstract" content="If a unioned SELECT statement refers to a distributed file, the statement is processed as a distributed query." />
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<meta name="description" content="If a unioned SELECT statement refers to a distributed file, the statement is processed as a distributed query." />
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<meta name="DC.subject" content="UNION clause, optimization, implementation, DISTINCT clause, DISTINCT clause" />
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<meta name="keywords" content="UNION clause, optimization, implementation, DISTINCT clause, DISTINCT clause" />
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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.Identifier" content="union" />
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<title>Implementation and optimization of the UNION and DISTINCT clauses with DB2 Multisystem</title>
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</head>
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<body id="union"><a name="union"><!-- --></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">Implementation and optimization of the UNION and DISTINCT clauses with DB2<sup>®</sup> Multisystem</h1>
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<div><p>If a unioned SELECT statement refers to a distributed file, the
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statement is processed as a distributed query.</p>
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<p>The processing of the statement can occur in parallel. However, the records
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from each unioned SELECT are brought back to the coordinator node to perform
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the union operation. In this regard, the union operators are processed serially.</p>
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<p>If an ORDER BY clause is specified with a union query, all of the records
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from each node are received on the coordinator node and are sorted before
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any records are returned.</p>
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<p>When the DISTINCT clause is specified for a distributed query, adding an
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ORDER BY clause returns records faster than if no ORDER BY clause was specified.
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DISTINCT with an ORDER BY allows each node to order the records in parallel.
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A final merge on the coordinator node reads the ordered records from each
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node, merges the records in the proper order, and eliminates duplicate records
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without having to do a final sort.</p>
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<p>When the DISTINCT clause is specified without an ORDER BY clause, all of
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the records from each node are sent to the coordinator node where a sort is
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performed. Duplicate records are eliminated as the sorted records are returned.</p>
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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="qqp.htm" title="This topic provides you with some guidelines for designing queries so that they use query resources more efficiently when you run queries that use distributed files.">Query design for performance with DB2 Multisystem</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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