65 lines
3.8 KiB
HTML
65 lines
3.8 KiB
HTML
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<meta name="DC.Title" content="Summary of MQT query recommendations" />
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<meta name="abstract" content="Follow these recommendations when using MQT queries." />
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<title>Summary of MQT query recommendations</title>
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</head>
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<body id="rzajqmqtsummary"><a name="rzajqmqtsummary"><!-- --></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">Summary of MQT query recommendations</h1>
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<div><p>Follow these recommendations when using MQT queries.</p>
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<div class="section"> <ul><li>Do not include local selection or constants in the MQT because that limits
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the number of user specified queries that query optimizer can use the MQT
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in.</li>
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<li>For grouping MQTs, only use the SUM, COUNT, MIN, and MAX grouping functions.
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The query optimizer can recalculate AVG, STDDEV, and VAR_POP in user specified
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queries.</li>
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<li>Specifying FETCH FIRST N ROWS in the MQT limits the number of user specified
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queries that the query optimizer can use the MQT in and is not recommended.</li>
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<li>If the MQT is created with DATA INITIALLY DEFERRED, consider specifying
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the DISABLE QUERY OPTIMIZATION clause to prevent the query optimizer from
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using the MQT until it has been populated. When the MQT has been populated
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and is ready for use, the ALTER TABLE statement with the ENABLE QUERY OPTIMIZATION
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clause can used to enable the MQT for the query optimizer.</li>
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</ul>
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</div>
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<div class="section"><p>MQT tables need to be optimized just like non-MQT tables. Indexes
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should be created over the MQT columns that are used for selection, join and
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grouping as appropriate. Column statistics are collected for MQT tables.</p>
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</div>
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<div class="section"><p>The database monitor shows the list of MQTs considered during
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optimization. This information is in the 3030 record. If MQT usage has been
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enabled through the QAQQINI file and a MQT exists over at least one of the
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tables in the query, there will be a 3030 record for the query. Each MQT
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has a reason code indicating that it was used or if it was not used, why it
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was not used.</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="rzajqmqt.htm" title="Materialized query tables (MQTs) (also referred to as automatic summary tables or materialized views) can provide performance enhancements for queries.">Materialized query table 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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