113 lines
7.0 KiB
HTML
113 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="View materialization implementation" />
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<meta name="abstract" content="The view materialization implementation runs the query of the view and places the results in a temporary result. The view reference in the user's query is then replaced with the temporary, and the query is run against the temporary result." />
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<meta name="description" content="The view materialization implementation runs the query of the view and places the results in a temporary result. The view reference in the user's query is then replaced with the temporary, and the query is run against the temporary result." />
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<meta name="DC.Relation" scheme="URI" content="viewopt.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="viewmaterialize" />
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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>View materialization implementation</title>
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</head>
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<body id="viewmaterialize"><a name="viewmaterialize"><!-- --></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">View materialization implementation</h1>
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<div><p>The view materialization implementation runs the query of the view
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and places the results in a temporary result. The view reference in the user's
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query is then replaced with the temporary, and the query is run against the
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temporary result.</p>
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<div class="section"><p><img src="./delta.gif" alt="Start of change" />View materialization is done whenever it is not possible
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to create a view composite. Note that for SQE, view materialization is optional.
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The following types of queries require view materialization:<img src="./deltaend.gif" alt="End of change" /></p>
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<img src="./delta.gif" alt="Start of change" /><ul><li>The outermost select of the view contains grouping, the query contains
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grouping, and refers to a column derived from a column function in the view
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in the HAVING or select-list.</li>
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<li>The query is a join and the outermost select of the view contains grouping
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or DISTINCT.</li>
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<li>The outermost select of the view contains DISTINCT, and the query has
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UNION, grouping, or DISTINCT and one of the following: <ul><li>Only the query has a shared weight NLSS table</li>
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<li>Only the view has a shared weight NLSS table</li>
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<li>Both the query and the view have a shared weight NLSS table, but the tables
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are different.</li>
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</ul>
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</li>
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<li>The query contains a column function and the outermost select of the view
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contains a DISTINCT</li>
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<li>The view does not contain an access plan. This can occur when a view references
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a view and a view composite cannot be created because of one of the reasons
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listed above. This does not apply to nested table expressions and common table
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expressions.</li>
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<li>The Common table expression (CTE) is reference more than once in the query's
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FROM clause(s) and the CTE's SELECT clause references a MODIFIES or EXTERNAL
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ACTION UDF.</li>
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</ul><img src="./deltaend.gif" alt="End of change" />
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<p>When a temporary result table is created, access methods that are
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allowed with ALWCPYDTA(*OPTIMIZE) may be used to implement the query. These
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methods include hash grouping, hash join, and bitmaps.</p>
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</div>
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<div class="section"><p>See the following examples:</p>
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<pre><strong>CREATE VIEW</strong> AVGSALVW <strong>AS</strong>
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<strong>SELECT</strong> WORKDEPT, AVG(SALARY) <strong>AS</strong> AVGSAL
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>GROUP BY</strong> WORKDEPT</pre>
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</div>
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<div class="section"><p>SQL example: </p>
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<pre> <strong>SELECT</strong> D.DEPTNAME, A.AVGSAL
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<strong>FROM</strong> CORPDATA.DEPARTMENT D, AVGSALVW A
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<strong>WHERE</strong> D.DEPTNO=A.WORKDEPT</pre>
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</div>
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<div class="section"><p>In this case, a view composite cannot be created since a join
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query references a grouping view. The results of AVGSALVW are placed in a
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temporary result table (*QUERY0001). The view reference AVGSALVW is replaced
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with the temporary result table. The new query is then run. The generated
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query looks like the following: </p>
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<pre><strong>SELECT</strong> D.DEPTNAME, A.AVGSAL
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<strong>FROM</strong> CORPDATA.DEPARTMENT D, *QUERY0001 A
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<strong>WHERE</strong> D.DEPTNO=A.WORKDEPT</pre>
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<div class="note"><span class="notetitle">Note:</span> The new query that the
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query optimizer generates is not visible to users. Only the original query
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against the view will be seen by users and database performance tools.</div>
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</div>
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<div class="section"><p>Whenever possible, isolatable selection from the query, except
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subquery predicates, is added to the view materialization process. This results
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in smaller temporary result tables and allows existing indexes to be used
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when materializing the view. This will not be done if there is more than
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one reference to the same view or common table expression in the query. The
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following is an example where isolatable selection is added to the view materialization: </p>
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<pre><strong>SELECT</strong> D.DEPTNAME,A.AVGSAL
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<strong>FROM</strong> CORPDATA.DEPARTMENT D, AVGSALVW A
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<strong>WHERE</strong> D.DEPTNO=A.WORKDEPT <strong>AND</strong>
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A.WORKDEPT <strong>LIKE</strong> 'D%' <strong>AND</strong> AVGSAL>10000</pre>
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</div>
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<div class="section"><p>The isolatable selection from the query is added to the view resulting
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in a new query to generate the temporary result table: </p>
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<pre><strong>SELECT</strong> WORKDEPT, AVG(SALARY) <strong>AS</strong> AVGSAL
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<strong>FROM</strong> CORPDATA.EMPLOYEE
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<strong>WHERE</strong> WORKDEPT <strong>LIKE</strong> 'D%'
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<strong>GROUP BY</strong> WORKDEPT
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<strong>HAVING AVG</strong>(SALARY)>10000 </pre>
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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="viewopt.htm" title="Views, derived tables (nested table expressions or NTEs), and common table expressions (CTEs) are implemented by the query optimizer using one of two methods.">View implementation</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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