103 lines
6.5 KiB
HTML
103 lines
6.5 KiB
HTML
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<title>Single table optimization</title>
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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">Single table optimization</h1>
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<div><p>At run time, the optimizer chooses an optimal access method for
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the query by calculating an<em> implementation cost</em> based on the current
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state of the database. The optimizer uses 2 costs when making decisions:
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an I/O cost and a CPU cost. The goal of the optimizer is to minimize both
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I/O and CPU cost.</p>
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<div class="section"><h4 class="sectiontitle">Optimizing Access to each table</h4><p>The optimizer uses
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a general set of guidelines to choose the best method for accessing data of
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each table. The optimizer:</p>
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<ul><li>Determines the default filter factor for each predicate in the selection
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clause.</li>
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<li>Determines the true filter factor of the predicates by doing a key range
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estimate when the selection predicates match the left most keys of an index
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or by using columns statistic when available.</li>
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<li>Determines the cost of table scan processing if an index is not required.</li>
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<li>Determines the cost of creating an index over a table if an index is required.
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This index is created by performing either a table scan or creating an index-from-index.</li>
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<li>Determines the cost of using a sort routine or hashing method if appropriate.</li>
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<li>Determines the cost of using existing indexes using Index Probe or Index
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Scan <ul><li>Orders the indexes. For SQE, the indexes are ordered in general such
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that the indexes that access the smallest number of entries are examined first.
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For CQE, the indexes are generally ordered from mostly recently created to
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oldest.</li>
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<li>For each index available, the optimizer does the following: <ul><li>Determines if the index meets the selection criteria.</li>
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<li>Determines the cost of using the index by estimating the number of I/Os
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and the CPU cost that will be needed to perform the Index Probe or the Index
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Scan and the possible Table Probes.</li>
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<li>Compares the cost of using this index with the previous cost (current
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best).</li>
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<li>Picks the cheaper one.</li>
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<li>Continues to search for best index until the optimizer decides to look
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at no more indexes.</li>
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</ul>
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<p>For SQE, since the indexes are ordered so that the best indexes are
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examined first, once an index that is more expensive than the previously chosen
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best index, the search is ended.</p>
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<p>For CQE, the <em>time limit</em> controls
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how much time the optimizer spends choosing an implementation. It is based
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on how much time was spent so far and the current best implementation cost
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found. The idea is to prevent the optimizer from spending more time optimizing
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the query than it takes to actually execute the query. Dynamic
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SQL queries are subject to the optimizer time restrictions. Static SQL queries
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optimization time is not limited. For OPNQRYF, if you specify OPTALLAP(*YES),
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the optimization time is not limited. For small tables, the query optimizer
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spends little time in query optimization. For large tables, the query optimizer
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considers more indexes. Generally, the optimizer considers five or six indexes
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(for each table of a join) before running out of optimization time. Because
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of this, it is normal for the optimizer to spend longer lengths of time analyzing
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queries against larger tables.</p>
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</li>
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</ul>
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</li>
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<li>Determines the cost of using a temporary bitmap <ul><li>Orders the indexes that can be used for bitmapping. In general the indexes
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that select the smallest number of entries are examined first.</li>
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<li>Determine the cost of using this index for bitmapping and the cost of
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merging this bitmap with any previously generated bitmaps.</li>
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<li>If the cost of this bitmap plan is cheaper than the previous bitmap plan,
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continue searching for bitmap plans.</li>
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</ul>
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</li>
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<li>After examining the possible methods of access the data for the table,
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the optimizer chooses the best plan from all the plans examined.</li>
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</ul>
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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="per0001.htm" title="This overview of the query optimizer provides guidelines for designing queries that will perform and will use server resources more efficiently.">Processing queries: Overview</a></div>
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</div>
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</div>
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</body>
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