57 lines
3.9 KiB
HTML
57 lines
3.9 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE html
|
|
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
|
<html lang="en-us" xml:lang="en-us">
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<meta name="security" content="public" />
|
|
<meta name="Robots" content="index,follow" />
|
|
<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))' />
|
|
<meta name="DC.Type" content="concept" />
|
|
<meta name="DC.Title" content="Implementation and optimization of the UNION and DISTINCT clauses with DB2 Multisystem" />
|
|
<meta name="abstract" content="If a unioned SELECT statement refers to a distributed file, the statement is processed as a distributed query." />
|
|
<meta name="description" content="If a unioned SELECT statement refers to a distributed file, the statement is processed as a distributed query." />
|
|
<meta name="DC.subject" content="UNION clause, optimization, implementation, DISTINCT clause, DISTINCT clause" />
|
|
<meta name="keywords" content="UNION clause, optimization, implementation, DISTINCT clause, DISTINCT clause" />
|
|
<meta name="DC.Relation" scheme="URI" content="qqp.htm" />
|
|
<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
|
|
<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1998, 2006" />
|
|
<meta name="DC.Format" content="XHTML" />
|
|
<meta name="DC.Identifier" content="union" />
|
|
<meta name="DC.Language" content="en-us" />
|
|
<!-- All rights reserved. Licensed Materials Property of IBM -->
|
|
<!-- US Government Users Restricted Rights -->
|
|
<!-- Use, duplication or disclosure restricted by -->
|
|
<!-- GSA ADP Schedule Contract with IBM Corp. -->
|
|
<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
|
|
<link rel="stylesheet" type="text/css" href="./ic.css" />
|
|
<title>Implementation and optimization of the UNION and DISTINCT clauses with DB2 Multisystem</title>
|
|
</head>
|
|
<body id="union"><a name="union"><!-- --></a>
|
|
<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
|
|
<h1 class="topictitle1">Implementation and optimization of the UNION and DISTINCT clauses with DB2<sup>®</sup> Multisystem</h1>
|
|
<div><p>If a unioned SELECT statement refers to a distributed file, the
|
|
statement is processed as a distributed query.</p>
|
|
<p>The processing of the statement can occur in parallel. However, the records
|
|
from each unioned SELECT are brought back to the coordinator node to perform
|
|
the union operation. In this regard, the union operators are processed serially.</p>
|
|
<p>If an ORDER BY clause is specified with a union query, all of the records
|
|
from each node are received on the coordinator node and are sorted before
|
|
any records are returned.</p>
|
|
<p>When the DISTINCT clause is specified for a distributed query, adding an
|
|
ORDER BY clause returns records faster than if no ORDER BY clause was specified.
|
|
DISTINCT with an ORDER BY allows each node to order the records in parallel.
|
|
A final merge on the coordinator node reads the ordered records from each
|
|
node, merges the records in the proper order, and eliminates duplicate records
|
|
without having to do a final sort.</p>
|
|
<p>When the DISTINCT clause is specified without an ORDER BY clause, all of
|
|
the records from each node are sent to the coordinator node where a sort is
|
|
performed. Duplicate records are eliminated as the sorted records are returned.</p>
|
|
</div>
|
|
<div>
|
|
<div class="familylinks">
|
|
<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>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html> |