57 lines
3.6 KiB
HTML
57 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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<meta name="DC.Title" content="Sources of join query performance problems" />
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<meta name="abstract" content="The optimization algorithms described above benefit most join queries, but the performance of a few queries may be degraded." />
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<meta name="description" content="The optimization algorithms described above benefit most join queries, but the performance of a few queries may be degraded." />
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<meta name="DC.subject" content="problems, join query performance" />
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<meta name="keywords" content="problems, join query performance" />
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<meta name="DC.Relation" scheme="URI" content="perf24.htm" />
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<meta name="copyright" 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>Sources of join query performance problems</title>
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</head>
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<body id="rzajqjoinproblems"><a name="rzajqjoinproblems"><!-- --></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">Sources of join query performance problems</h1>
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<div><p>The optimization algorithms described above benefit most join queries,
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but the performance of a few queries may be degraded.</p>
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<div class="section"><p>This occurs when:</p>
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<ul><li>An index is not available which provides average number of duplicate values
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statistics for the potential join columns. </li>
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<li>The query optimizer uses default filter factors to estimate the number
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of rows being selected when applying local selection to the table because
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indexes or column statistics do not exist over the selection columns. <p>Creating
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indexes over the selection columns allow—s the query optimizer to make a more
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accurate filtering estimate by using key range estimates.</p>
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</li>
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<li>The particular values selected for the join columns yield a significantly
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greater number of matching rows than the average number of duplicate values
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for all values of the join columns in the table (for example, the data is
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not uniformly distributed). </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="perf24.htm" title="A join operation is a complex function that requires special attention in order to achieve good performance. This section describes how DB2 Universal Database for iSeries implements join queries and how optimization choices are made by the query optimizer. It also describes design tips and techniques which help avoid or solve performance problems.">Join optimization</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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