84 lines
5.5 KiB
HTML
84 lines
5.5 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 name="DC.Title" content="Temporary hash table" />
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<meta name="abstract" content="The temporary hash table is a temporary object that allows the optimizer to collate the rows based upon a column or set of columns. The hash table can be either scanned or probed by the optimizer to satisfy different operations of the query." />
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<meta name="description" content="The temporary hash table is a temporary object that allows the optimizer to collate the rows based upon a column or set of columns. The hash table can be either scanned or probed by the optimizer to satisfy different operations of the query." />
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<meta name="DC.subject" content="definitions, temporary hash table, hash table, access method, access method" />
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<meta name="keywords" content="definitions, temporary hash table, hash table, access method, access method" />
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<meta name="DC.Relation" scheme="URI" content="rzajqtempobj.htm" />
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<meta name="DC.Relation" scheme="URI" content="rzajqhtblscan.htm" />
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<meta name="DC.Relation" scheme="URI" content="rzajqhtblprobe.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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<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>Temporary hash table</title>
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</head>
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<body id="rzajqtemphash"><a name="rzajqtemphash"><!-- --></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">Temporary hash table</h1>
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<div><p>The temporary hash table is a temporary object that allows the
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optimizer to collate the rows based upon a column or set of columns. The hash
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table can be either scanned or probed by the optimizer to satisfy different
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operations of the query.</p>
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<div class="section"><p>A temporary hash table is an efficient data structure because
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the rows are organized for quick and easy retrieval after population has occurred.
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This is primarily due to the hash table remaining resident within main memory
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so as to avoid any I/Os associated with either the scan or probe against the
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temporary object. The optimizer will determine the optimal size for the hash
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table based upon the number of unique combinations (for example, cardinality)
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of the columns used as keys for the creation.</p>
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</div>
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<div class="section"><p>Additionally the hash table can be populated with all of the necessary
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columns to satisfy any further processing, avoiding any random I/Os associated
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with a Table Probe operation. However, the optimizer does have the ability
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to selectively include columns in the hash table when the calculated size
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will exceed the memory pool storage available for this query. In those cases,
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a Table Probe operation is required to recollect the missing columns from
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the hash table before the selected rows can be processed.</p>
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</div>
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<div class="section"><p>The optimizer also has the ability to populate the hash table
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with distinct values. If the query contains grouping or distinct processing,
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then all of the rows with the same key value are not required to be stored
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in the temporary object. They are still collated, but the distinct processing
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is performed during the population of the hash table itself. This allows a
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simple scan to be performed on the result in order to complete the grouping
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or distinct operation.</p>
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</div>
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<div class="section"><p>A temporary hash table is an internal data structure and can only
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be created by the database manager</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="rzajq512.gif" alt="Temporary hash table 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="rzajqhtblscan.htm">Hash table scan</a></strong><br />
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During a Hash Table Scan operation, the entire temporary hash table is scanned and all of the entries contained within the hash table will be processed.</li>
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<li class="ulchildlink"><strong><a href="rzajqhtblprobe.htm">Hash table probe</a></strong><br />
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A hash table probe operation is used to retrieve rows from a temporary hash table based upon a probe lookup operation.</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="rzajqtempobj.htm" title="Temporary objects are created by the optimizer in order to process a query. In general, these temporary objects are internal objects and cannot be accessed by a user.">Temporary 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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