72 lines
4.4 KiB
HTML
72 lines
4.4 KiB
HTML
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<!DOCTYPE html
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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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<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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