85 lines
5.4 KiB
HTML
85 lines
5.4 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.Title" content="Indexes versus column statistics" />
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<meta name="abstract" content="If you are trying to decide whether to use statistics or indexes to provide information to the Statistics Manager, keep the following differences in mind." />
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<meta name="description" content="If you are trying to decide whether to use statistics or indexes to provide information to the Statistics Manager, keep the following differences in mind." />
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<meta name="DC.subject" content="statistics manager, statistics versus indexes, index" />
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<meta name="keywords" content="statistics manager, statistics versus indexes, index" />
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<meta name="copyright" 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>Indexes versus column statistics</title>
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</head>
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<body id="statsvindex"><a name="statsvindex"><!-- --></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">Indexes versus column statistics</h1>
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<div><p>If you are trying to decide whether to use statistics or indexes
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to provide information to the Statistics Manager, keep the following differences
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in mind.</p>
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<div class="section"><p>One major difference between indexes and column statistics is
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that indexes are permanent objects that are updated when changes to the underlying
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table occur, while column statistics are not. If your data is constantly changing,
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the Statistics Manager may need to rely on stale column statistics. However,
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maintaining an index after each change to the table might take up more system
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resources than refreshing the stale column statistics after a group of changes
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to the table have occurred. </p>
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</div>
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<div class="section"><p>Another difference is the effect that the existence of new indexes
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or column statistics has on the Optimizer. When new indexes become available,
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the Optimizer will consider them for implementation. If they are candidates,
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the Optimizer will re-optimize the query and try to find a better implementation.
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However, this is not true for column statistics. When new or refreshed column
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statistics are available, the Statistics Manager will interrogate immediately.
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Reoptimization will occur only if the answers are significantly different
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from the ones that were given before these refreshed statistics. This means
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that it is possible to use statistics that are refreshed without causing a
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reoptimization of an access plan.</p>
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</div>
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<div class="section"><p>When trying to determine the selectivity of predicates, the Statistics
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Manager considers column statistics and indexes as resources for its answers
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in the following order:</p>
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</div>
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<div class="section"> <ol><li>Try to use a multi-column keyed index when ANDed or ORed predicates reference
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multiple columns</li>
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<li>If there is no perfect index that contains all of the columns in the predicates,
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it will try to find a combination of indexes that can be used.</li>
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<li>For single column questions, it will use available column statistics</li>
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<li>If the answer derived from the column statistics shows a selectivity of
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less than 2%, indexes are used to verify this answer</li>
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</ol>
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</div>
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<div class="section"><p>Accessing column statistics to answer questions is faster than
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trying to obtain these answers from indexes.</p>
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</div>
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<div class="section"><p>Column statistics can only be used by SQE. For CQE, all statistics
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are retrieved from indexes.</p>
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</div>
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<div class="section"><p>Finally, column statistics can be used only for query optimization.
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They cannot be used for the actual implementation of a query, whereas indexes
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can be used for both.</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="statsmanager.htm" title="As stated earlier, the collection of statistics is handled by a separate component called the Statistics Manager. Statistical information can be used by the query optimizer to determine the best access plan for a query. Since the query optimizer bases its choice of access plan on the statistical information found in the table, it is important that this information be current.">Collecting statistics with the Statistics Manager</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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