72 lines
4.4 KiB
HTML
72 lines
4.4 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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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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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="concept" />
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<meta name="DC.Title" content="How the EVI works" />
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<meta name="abstract" content="EVIs work in different ways for costing and implementation." />
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<meta name="description" content="EVIs work in different ways for costing and implementation." />
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<meta name="DC.Relation" scheme="URI" content="whatareevi.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.Identifier" content="howeviworks" />
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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>How the EVI works</title>
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</head>
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<body id="howeviworks"><a name="howeviworks"><!-- --></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">How the EVI works</h1>
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<div><p>EVIs work in different ways for costing and implementation.</p>
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<p>For costing, the optimizer uses the symbol table to collect metadata information
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about the query.</p>
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<p>For implementation, the optimizer may use the EVI in one of the following
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ways:</p>
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<ul><li><strong>Selection (WHERE clause)</strong><p>If the optimizer decides to use an EVI
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to process the query, the database engine uses the vector to build the dynamic
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bitmap (or a list of selected row ids) that contains one bit for each row
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in the table, the bit being turned on for each selected row. Like a bitmap
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index, these intermediate dynamic bitmaps (or lists) can be AND'ed and OR'ed
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together to satisfy an ad hoc query.</p>
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<p>For example, if a user wants to
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see sales data for a certain region during a certain time period, you can
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define an EVI over the region column and the Quarter column of the database.
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When the query runs, the database engine builds dynamic bitmaps using the
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two EVIs and then ANDs the bitmaps together to produce a bitmap that contains
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only the relevant rows for both selection criteria. This AND'ing capability
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drastically reduces the number of rows that the server must read and test.
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The dynamic bitmap(s) exists only as long as the query is executing. Once
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the query is completed, the dynamic bitmap(s) are eliminated.</p>
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</li>
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<li><strong>Grouping or Distinct</strong><p>The symbol table within the EVI contains
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the distinct values for the specified columns in the key definition, along
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with a count of the number of records in the base table that have each distinct
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value. The symbol table in effect contains the grouping results of the columns
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in that key. Therefore, queries involving grouping or distinct on the columns
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in that key are potential candidates for a technique that uses the symbol
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table directly to determine the query result. Note that the symbol table contains
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only the key values and their associated counts. Therefore, queries involving
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column function COUNT are eligible for this technique, but queries involving
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column functions MIN or MAX on other columns are not eligible (since the min
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and max values are not stored in the symbol table).</p>
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</li>
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</ul>
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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="whatareevi.htm" title="An encoded vector index (EVI) is an index object that is used by the query optimizer and database engine to provide fast data access in decision support and query reporting environments.">Encoded vector indexes</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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</html>
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