119 lines
7.0 KiB
HTML
119 lines
7.0 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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<meta name="DC.Title" content="Hash table scan" />
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<meta name="abstract" content="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." />
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<meta name="description" content="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." />
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<meta name="DC.subject" content="definitions, hash table scan access method, hash table, scan access method, access method, hash table scan" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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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>Hash table scan</title>
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</head>
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<body id="rzajqhtblscan"><a name="rzajqhtblscan"><!-- --></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">Hash table scan</h1>
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<div><p>During a Hash Table Scan operation, the entire temporary hash table
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is scanned and all of the entries contained within the hash table will be
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processed.</p>
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<div class="section"><p>The optimizer considers a hash table scan when the data values
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need to be collated together, but the sequence of the data is not required.
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The use of a hash table scan will allow the optimizer to generate a plan that
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can take advantage of any non-join selection while creating the temporary
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hash table. An additional benefit of using a hash table scan is that the data
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structure of the temporary hash table will typically cause the table data
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within the hash table to remain resident within main memory after creation,
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thus reducing paging on the subsequent hash table scan operation.</p>
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</div>
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<div class="section">
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. Hash table scan attributes</caption><thead align="left"><tr><th valign="top" width="30.96446700507614%" id="d0e37">Data access method</th>
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<th valign="top" width="69.03553299492386%" id="d0e39">Hash table scan</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Description</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 "> Read all of the entries in a temporary hash table.
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The hash table may perform distinct processing to eliminate duplicates or
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takes advantage of the temporary hash table to collate all of the rows with
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the same value together.</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Advantages</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 "><ul><li>Reduces the random I/O to the table generally associated with longer running
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queries that may otherwise use an index to collate the data</li>
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<li>Selection can be performed before generating the hash table to subset
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the number of rows in the temporary object</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Considerations</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 ">Generally used for distinct or group by processing.
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Can perform poorly when the entire hash table does not stay resident in memory
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as it is being processed.</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Likely to be used</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 "><ul><li>When the use of temporary results is allowed by the query environmental
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parameter (ALWCPYDTA)</li>
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<li>When the data is required to be collated based upon a column or columns
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for distinct or grouping</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Example SQL statement</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 "><pre>SELECT COUNT(*), FirstNme FROM Employee
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WHERE WorkDept BETWEEN 'A01' AND 'E01'
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GROUP BY FirstNme</pre>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Messages indicating use</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 ">There are multiple ways in which a hash scan can be
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indicated through the messages. The messages in this example illustrate how
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the SQL Query Engine will indicate a hash scan was used. <ul><li>Optimizer Debug: <pre>CPI4329 -- Arrival sequence was used for file
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EMPLOYEE.</pre>
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</li>
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<li>PRTSQLINF: <pre>SQL4010 -- Table scan access for table 1.
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SQL4029 -- Hashing algorithm used to process
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the grouping.</pre>
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</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>SMP parallel enabled</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 ">Yes</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Also referred to as</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 ">Hash Scan, Preload <p>Hash Table Scan Distinct</p>
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<p>Hash
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Table Scan Distinct, Preload</p>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e37 "><strong>Visual Explain icon</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e39 "><br /><img src="rzajq513.gif" alt="Hash table scan icon" /><br /></td>
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</tr>
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</tbody>
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</table>
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</div>
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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="rzajqtemphash.htm" title="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.">Temporary hash table</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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