62 lines
4.1 KiB
HTML
62 lines
4.1 KiB
HTML
<?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 http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="security" content="public" />
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<meta name="Robots" content="index,follow" />
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Automatic statistics refresh" />
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<meta name="abstract" content="Column statistics are not maintained when the underlying table data changes. The Statistics Manager determines if columns statistics are still valid or if they no longer represent the column accurately (stale)." />
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<meta name="description" content="Column statistics are not maintained when the underlying table data changes. The Statistics Manager determines if columns statistics are still valid or if they no longer represent the column accurately (stale)." />
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<meta name="DC.subject" content="statistics manager, automatic statistics refresh" />
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<meta name="keywords" content="statistics manager, automatic statistics refresh" />
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<meta name="DC.Relation" scheme="URI" content="statsmanager.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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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="autostatsrefresh" />
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<meta name="DC.Language" content="en-us" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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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>Automatic statistics refresh</title>
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</head>
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<body id="autostatsrefresh"><a name="autostatsrefresh"><!-- --></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">Automatic statistics refresh</h1>
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<div><p>Column statistics are not maintained when the underlying table
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data changes. The Statistics Manager determines if columns statistics are
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still valid or if they no longer represent the column accurately (stale).</p>
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<div class="section"><p>This validation is done each time one of the following occurs: </p>
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<ul><li>A full open occurs for a query where column statistics were used to create
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the access plan</li>
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<li>A new plan is added to the plan cache, either because a completely new
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query was optimized or because an existing plan was re-optimized. </li>
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</ul>
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<p>To validate the statistics, the Statistics Manager checks to see if
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any of the following apply: </p>
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<ul><li>Number of rows in the table has changed by more than 15% of the total
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table row count</li>
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<li>Number of rows changed in the table is more than 15% of the total table
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row count</li>
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</ul>
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</div>
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<div class="section"><p>If the statistics is determined to be stale, the Statistics Manager
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still uses the stale column statistics to answer the questions from the optimizer,
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but it also marks the column statistics as stale in the Plan Cache and generates
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a request to refresh the statistics.</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> |