74 lines
5.0 KiB
HTML
74 lines
5.0 KiB
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<!DOCTYPE html
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<meta name="DC.Title" content="Encoded vector index" />
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<meta name="abstract" content="An encoded vector index is a permanent object that provides access to a table by assigning codes to distinct key values and then representing those values in a vector." />
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<meta name="description" content="An encoded vector index is a permanent object that provides access to a table by assigning codes to distinct key values and then representing those values in a vector." />
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<title>Encoded vector index</title>
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</head>
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<body id="perf21c"><a name="perf21c"><!-- --></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">Encoded vector index</h1>
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<div><p>An encoded vector index is a permanent object that provides access
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to a table by assigning codes to distinct key values and then representing
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those values in a vector.</p>
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<div class="section"><p>The size of the vector will match the number of rows in the underlying
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table. Each vector entry represents the table row number in the same position.
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The codes generated to represent the distinct key values can be 1, 2 or 4
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bytes in length, depending upon the number of distinct values that need to
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be represented. Because of their compact size and relative simplicity, the
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EVI can be used to process large amounts of data very efficiently.</p>
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</div>
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<div class="section"><p>Even though an encoded vector index is used to represent the values
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stored in a table, the index itself cannot be used to directly gain access
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to the table. Instead, the encoded vector index can only be used to generate
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either a temporary row number list or a temporary row number bitmap. These
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temporary objects can then be used in conjunction with a Table Probe to specify
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the rows in the table that the query needs to process. The main difference
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with the Table Probe associated with an encoded vector index (versus a radix
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index) is that the paging associated with the table can be asynchronous. The
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I/O can now be scheduled more efficiently to take advantage of groups of selected
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rows. Large portions of the table can be skipped over where no rows are selected.</p>
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</div>
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<div class="section"><p>Visual explain icon:</p>
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</div>
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<div class="section"><p><img src="rzajq509.gif" alt="Encoded vector index icon" /></p>
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</div>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rzajqeviscan.htm">Encoded vector index symbol table scan</a></strong><br />
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An encoded vector index symbol table scan operation is used to retrieve the entries from the symbol table portion of the index.</li>
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<li class="ulchildlink"><strong><a href="rzajqeviprobe.htm">Encoded vector index probe</a></strong><br />
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The encoded vector index (EVI) is quickly probed based upon the selection criteria that were rewritten into a series of ranges. It produces either a temporary row number list or bitmap.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzajqpermobj.htm" title="The database objects and access methods used by the query engine can be broken down into three basic types of operations that are used to manipulate the permanent and temporary objects -- Create, Scan, and Probe.">Permanent objects and access methods</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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