74 lines
4.3 KiB
HTML
74 lines
4.3 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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<head>
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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Two-step grouping with DB2 Multisystem" />
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<meta name="abstract" content="If the partitioning key is not included in the grouping fields, then grouping must be done using two-step grouping, because the like values of a field are not located on the same node." />
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<meta name="description" content="If the partitioning key is not included in the grouping fields, then grouping must be done using two-step grouping, because the like values of a field are not located on the same node." />
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<meta name="DC.subject" content="grouping, two-step, optimization, two-step grouping" />
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<meta name="keywords" content="grouping, two-step, optimization, two-step grouping" />
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<meta name="DC.Relation" scheme="URI" content="grouping.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="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Two-step grouping with DB2 Multisystem</title>
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</head>
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<body id="twostep"><a name="twostep"><!-- --></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">Two-step grouping with DB2<sup>®</sup> Multisystem</h1>
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<div><p>If the partitioning key is not included in the grouping fields,
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then grouping must be done using two-step grouping, because the like values
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of a field are not located on the same node.</p>
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<div class="section"><p>The following code is an example of two-step grouping.
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</p>
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<p>SQL statement:</p>
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<pre> SELECT JOB, AVG(SALARY)
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FROM EMPLOYEE
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GROUP BY JOB
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</pre>
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<p>OPNQRYF command:</p>
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<pre> OPNQRYF FILE((EMPLOYEE)) FORMAT(GRPFMT2)
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GRPFLD(JOB)
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MAPFLD((AVGSAL '%AVG(SALARY)'))</pre>
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<p>In this example, note that for the group where JOB is <tt>Clerk</tt>,
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the value <tt>Clerk</tt> is on two different nodes in the EMPLOYEE distributed
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file. Grouping is implemented by first running grouping in parallel on all
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three nodes. This results in an initial grouping which is placed in a temporary
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file at the coordinator node. The query is modified, and the grouping is run
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again at the coordinator node to get the final set of grouping results.</p>
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<p>Whole-file
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grouping (no group by fields) is always implemented using two steps.</p>
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<p>If
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the query contains either a HAVING clause or the group selection expression
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(GRPSLT) parameter on the OPNQRYF command, all groups from the first grouping
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step are returned to the coordinator node. The HAVING clause or the GRPSLT
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parameter is then processed as part of second grouping step.</p>
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<p>If the
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query contains a DISTINCT column (aggregate) function and two-step grouping
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is required, no grouping is done in the first step. Instead, all records are
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returned to the coordinator node, and all grouping is run at the coordinator
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node as part of the second step.</p>
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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="grouping.htm" title="The implementation method for grouping in queries that use distributed files is dependent on whether the partitioning key is included in the grouping criteria.">Implementation and optimization of grouping 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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