78 lines
4.2 KiB
HTML
78 lines
4.2 KiB
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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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Distinct optimization" />
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<meta name="abstract" content="Distinct is used to compare a value with another value." />
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<title>Distinct optimization</title>
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</head>
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<body id="rzajqdistinct"><a name="rzajqdistinct"><!-- --></a>
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<img src="./delta.gif" alt="Start of change" /><!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Distinct optimization</h1>
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<div><p>Distinct is used to compare a value with another value.</p>
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<div class="section"><p>There are two methods to write a query that returns distinct values
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in SQL. One method uses the DISTINCT keyword:</p>
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<pre><strong>SELECT DISTINCT</strong> COL1, COL2
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<strong>FROM</strong> TABLE1</pre>
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<p>The second method uses GROUP BY:</p>
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<pre><strong>SELECT</strong> COL1, COL2
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<strong>FROM</strong> TABLE1
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<strong>GROUP BY</strong> COL1, COL2</pre>
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<p>All queries that contain a DISTINCT,
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and are run using SQE, will be rewritten into queries using GROUP BY. This
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rewrite enables queries using DISTINCT to take advantage of the many grouping
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techniques available to the optimizer.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Distinct to Grouping implementation</h4><p>Below is an
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example of a query with a DISTINCT:</p>
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<pre><strong>SELECT</strong> DISTINCT COL1, COL2
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<strong>FROM</strong> T1
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<strong>WHERE</strong> COL2 > 5 AND COL3 = 2</pre>
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<p>The optimizer will rewrite
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it into this query:</p>
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<pre><strong>SELECT</strong> COL1, COL2
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<strong>FROM</strong> T1
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<strong>WHERE</strong> COL2 > 5 AND COL3 = 2
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<strong>GROUP BY</strong> COL1, COL2</pre>
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</div>
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<div class="section"><h4 class="sectiontitle">Distinct removal</h4><p>A query containing a DISTINCT over
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whole-file aggregation (no grouping or selection) allows the DISTINCT to be
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removed. For example, look at this query with DISTINCT:</p>
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<pre><strong>SELECT DISTINCT COUNT</strong>(C1), <strong>SUM</strong>(C1)
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<strong>FROM</strong> TABLE1</pre>
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<p>The optimizer rewrites this query as the
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following:</p>
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<pre><strong>SELECT COUNT</strong>(C1), <strong>SUM</strong>(C1)
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<strong>FROM</strong> TABLE1</pre>
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<p>If the DISTINCT and the GROUP BY fields
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are identical, the DISTINCT can be removed. If the DISTINCT fields are
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a subset of the GROUP BY fields (and there are no aggregates), the DISTINCTs
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can be removed.</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="per0001.htm" title="This overview of the query optimizer provides guidelines for designing queries that will perform and will use server resources more efficiently.">Processing queries: Overview</a></div>
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</div>
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</div>
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<img src="./deltaend.gif" alt="End of change" /></body>
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