81 lines
5.1 KiB
HTML
81 lines
5.1 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="Join optimization with DB2 Multisystem" />
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<meta name="abstract" content="The distributed query optimizer generates a plan to join distributed files." />
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<meta name="description" content="The distributed query optimizer generates a plan to join distributed files." />
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<meta name="DC.subject" content="join, optimization" />
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<meta name="keywords" content="join, optimization" />
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<meta name="DC.Relation" scheme="URI" content="joins.htm" />
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<meta name="DC.Relation" scheme="URI" content="partkeysjoin.htm" />
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<meta name="DC.Relation" scheme="URI" content="joins.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>Join optimization with DB2 Multisystem</title>
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</head>
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<body id="joinopt"><a name="joinopt"><!-- --></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">Join optimization with DB2<sup>®</sup> Multisystem</h1>
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<div><p>The distributed query optimizer generates a plan to join distributed
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files.</p>
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<p>The distributed query optimizer looks at file sizes, expected number of
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records selected for each file, and the type of distributed joins that are
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possible; and then the optimizer breaks the query into multiple steps. Each
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step creates an intermediate result file that is used as input for the next
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step.</p>
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<p>During optimization, a cost is calculated for each join step based on the
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type of distributed join. The cost reflects, in part, the amount of data movement
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required for that join step. The cost is used to determine the final distributed
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plan.</p>
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<p>As much processing as possible is completed during each step; for example,
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record selection isolated to a given step is performed during that step, and
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as many files as possible are joined for each step. Each join step might involve
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more than one type of distributed join. A collocated join and a directed join
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can be combined into one collocated join by directing the necessary file first.
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A directed join and a re-partitioned join can be combined by directing all
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the files first and then performing the join. Note that directed and re-partitioned
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joins are really just a collocated join, with one or more files being directed
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before the join occurs.</p>
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<p>When joining distributed files with local files, the distributed
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query optimizer calculates a cost, similar to the cost calculated when joining
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distributed files. Based on this cost, the distributed query optimizer can
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choose to perform one of the following actions:</p>
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<ul><li>Broadcast all of the local files to the data nodes of the distributed
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file and perform a collocated join.</li>
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<li>Broadcast all of the local and distributed files to the data nodes of
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the largest distributed file and perform a collocated join.</li>
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<li>Direct the distributed files back to the coordinator node and perform
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the join there.</li>
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</ul>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="partkeysjoin.htm">Partitioning keys over join fields with DB2 Multisystem</a></strong><br />
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From the preceding sections on the types of joins, you can see that data movement is required for all distributed join types except a collocated join.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="joins.htm" title="In addition to the performance considerations for nondistributed join queries, more performance considerations exist for queries involving distributed files.">Implementation and optimization of join operations with DB2 Multisystem</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="joins.htm" title="In addition to the performance considerations for nondistributed join queries, more performance considerations exist for queries involving distributed files.">Implementation and optimization of join operations 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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