103 lines
6.2 KiB
HTML
103 lines
6.2 KiB
HTML
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<meta name="abstract" content="Temporary result writers are system-initiated jobs that are always active." />
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<title>Temporary result writer with DB2 Multisystem</title>
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</head>
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<body id="tempresult"><a name="tempresult"><!-- --></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">Temporary result writer with DB2<sup>®</sup> Multisystem</h1>
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<div><p>Temporary result writers are system-initiated jobs that are always
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active.</p>
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<div class="section"><p>On the system, temporary result writers are paired jobs called
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QQQTEMP1 and QQQTEMP2. Temporary result writers handle requests from jobs
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that are running queries. These requests consist of a query (of the query
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step) to run and the name of a system temporary file to fill from the results
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of the query. The temporary result writer processes the request and fills
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the temporary file. This intermediate temporary file is then used by the requesting
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job to complete the original query.</p>
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<p>The following example shows a query
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that requires a temporary result writer and the steps needed to process the
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query. </p>
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<p>SQL statement:</p>
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<pre>SELECT COUNT(*)
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FROM DEPARTMENT a, EMPLOYEE b
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WHERE a.ADMRDEPT = b.WORKDEPT
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AND b.JOB = 'Manager'
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</pre>
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<p>OPNQRYF command:</p>
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<pre> OPNQRYF FILE((DEPARTMENT) (EMPLOYEE))
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FORMAT(FMTFILE)
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MAPFLD((CNTFLD '%COUNT'))
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JFLD((1/ADMRDEPT 2/WORKDEPT))
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QRYSLT('2/JOB = 'Manager')</pre>
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<div class="p">WORKDEPT
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is the partitioning key for EMPLOYEE, but ADMRDEPT is not the partitioning
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key for DEPARTMENT. Because the query must be processed in two steps, the
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optimizer splits the query into the following steps: <pre> INSERT INTO SYS_TEMP_FILE
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SELECT a.DEPTNAME, a.ADMRDEPT
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FROM DEPARTMENT a</pre>
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</div>
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<div class="p">and <pre> SELECT COUNT(*) FROM SYS_TEMP_FILE x, EMPLOYEE b
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WHERE x.ADMRDEPT = b.WORKDEPT AND b.JOB = 'Manager'</pre>
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</div>
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<p>If a temporary result writer is
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allowed for the job (controlled by the Change Query Attributes (CHGQRYA) options),
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the optimizer:</p>
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<ol><li>Creates the temporary file (SYS_TEMP_FILE) into library QRECOVERY.</li>
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<li>Sends the request that populates SYS_TEMP_FILE to the temporary result
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writer.</li>
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<li>Continues to finish opening the final query (while the temporary result
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writer is filling the temporary file).</li>
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<li>After the final query is opened, waits until the temporary
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result writer has finished filling the temporary file before returning control
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to its caller.</li>
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</ol>
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</div>
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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="tempad.htm">Temporary result writer job: Advantages with DB2 Multisystem</a></strong><br />
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The advantage of using a temporary result writer job in processing a request is that the temporary result writer can process its request at the same time (in parallel) that the main job is processing another step of the query.</li>
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<li class="ulchildlink"><strong><a href="tempdisad.htm">Temporary result writer job: Disadvantages with DB2 Multisystem</a></strong><br />
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The temporary result writer also has disadvantages that must be considered when you determine its usefulness for queries.</li>
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<li class="ulchildlink"><strong><a href="tempcontrol.htm">Control of the temporary result writer with DB2 Multisystem</a></strong><br />
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By default, queries do not use the temporary result writer. Temporary result writer usage, however, can be enabled by using the Change Query Attributes (CHGQRYA) command.</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="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 class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="chgqrya.htm" title="The CHGQRYA command has two parameters that are applicable to distributed queries.">Changes to the change query attributes (CHGQRYA) command 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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