97 lines
6.6 KiB
HTML
97 lines
6.6 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.Type" content="reference" />
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<meta name="DC.Title" content="Predicate Pushing" />
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<meta name="abstract" content="When processing most queries with a non-recursive common table expressions or views, local predicates specified on the main query are pushed down so fewer records need to be materialized. Pushing local predicates from the main query in to the defined recursive part of the query (through the Union ALL), however, may considerably alter the process of recursion itself. So as a general rule, the Union All specified in a recursive query is currently a predicate fence and predicates are not pushed down or up, through this fence." />
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<meta name="description" content="When processing most queries with a non-recursive common table expressions or views, local predicates specified on the main query are pushed down so fewer records need to be materialized. Pushing local predicates from the main query in to the defined recursive part of the query (through the Union ALL), however, may considerably alter the process of recursion itself. So as a general rule, the Union All specified in a recursive query is currently a predicate fence and predicates are not pushed down or up, through this fence." />
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<meta name="DC.Relation" scheme="URI" content="rzajqrecursive.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Predicate Pushing</title>
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</head>
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<body id="rzajqrctepredicate"><a name="rzajqrctepredicate"><!-- --></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">Predicate Pushing</h1>
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<div><p>When processing most queries with a non-recursive common table
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expressions or views, local predicates specified on the main query are pushed
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down so fewer records need to be materialized. Pushing local predicates from
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the main query in to the defined recursive part of the query (through the
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Union ALL), however, may considerably alter the process of recursion
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itself. So as a general rule, the Union All specified in a recursive query
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is currently a predicate fence and predicates are not pushed down or up, through
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this fence.</p>
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<div class="section"><p>The following is an example of how pushing a predicate in to the
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recursion limits the recursive results and alter the intent of the query.</p>
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</div>
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<div class="example">If the intent of the query is to find all destinations accessible
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from 'Chicago' but do not include the final destinations of 'Dallas', pushing
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the "arrival<>'Dallas'" predicate in to the recursive query alters the
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output of the intended results, preventing the output of final destinations
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that are not 'Dallas' but where 'Dallas' was an intermediate stop. <pre><strong>WITH</strong> destinations (departure, arrival, connects, cost ) <strong>AS</strong>
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(
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<strong>SELECT</strong> f.departure,f.arrival, 0, ticket
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<strong>FROM</strong> flights f
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<strong>WHERE</strong> f.departure='Chicago'
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<strong>UNION ALL</strong>
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<strong>SELECT</strong>
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r.departure, b.arrival, r.connects + 1 ,
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r.cost + b.ticket
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<strong>FROM</strong> destinations r, flights b
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<strong>WHERE</strong> r.arrival=b.departure
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)
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<strong>SELECT</strong> departure, arrival, connects, cost
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<strong>FROM</strong> destinations
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<strong>WHERE</strong> arrival != 'Dallas'
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</pre>
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<p>Conversely, the following is an example where a local predicate
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applied to all the recursive results is a good predicate to put in the body
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of the recursive definition because it may greatly decrease the amount of
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rows materialized from the RCTE/View. The better query request here is to
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specify the r.connects <=3 local predicate with in the RCTE definition,
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in the iterative fullselect.</p>
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<pre><strong>WITH</strong> destinations (departure, arrival, connects, cost ) <strong>AS</strong>
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(
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<strong>SELECT</strong> f.departure,f.arrival, 0, ticket
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<strong>FROM</strong> flights f
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<strong>WHERE</strong> f.departure='Chicago' <strong>OR</strong>
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f.departure='New York'
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<strong>UNION ALL</strong>
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<strong>SELECT</strong>
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r.departure, b.arrival, r.connects + 1 ,
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r.cost + b.ticket
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<strong>FROM</strong> destinations r, flights b
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<strong>WHERE</strong> r.arrival=b.departure
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)
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<strong>SELECT</strong> departure, arrival, connects, cost
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<strong>FROM</strong> destinations
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<strong>WHERE</strong> r.connects<=3 </pre>
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</div>
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<div class="section"><p>Placement of local predicates is key in recursive queries as they
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can incorrectly alter the recursive results if pushed in to a recursive definition
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or can cause unnecessary rows to be materialized and then rejected when a
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local predicate may legitimately help limit the recursion.</p>
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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="rzajqrecursive.htm" title="Certain applications and data are recursive by nature. Examples of such applications are a bill-of-material, reservation, trip planner or networking planning system where data in one results row has a natural relationship (call it a parent, child relationship) with data in another row or rows. Although the kinds of recursion implemented in these systems can be performed by using SQL Stored Procedures and temporary results tables, the use of a recursive query to facilitate the access of this hierarchical data can lead to a more elegant and better performing application.">Recursive query optimization</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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