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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=us-ascii" /> <meta http-equiv="Content-Style-Type" content="text/css" /> <meta http-equiv="Content-Script-Type" content="text/javascript" /> <title>Tuning SQL Statements</title> <meta name="generator" content="Oracle DARB XHTML Converter (Mode = ohj/ohw) - Version 5.1.1 Build 005" /> <meta name="date" content="2009-05-27T15:13:2Z" /> <meta name="robots" content="noarchive" /> <meta name="doctitle" content=" Tuning SQL Statements" /> <meta name="relnum" content="11g Release 2 (11.2)" /> <meta name="partnum" content="E10822-01" /> <link rel="copyright" href="./dcommon/html/cpyr.htm" title="Copyright" type="text/html" /> <link rel="stylesheet" href="./dcommon/css/blafdoc.css" title="Oracle BLAFDoc" type="text/css" /> <link rel="contents" href="toc.htm" title="Contents" type="text/html" /> <link rel="prev" href="tdppt_sqlid015.htm" title="Previous" type="text/html" /> <link rel="next" href="tdppt_sqltune001.htm" title="Next" type="text/html" /> <script src="./callback.js" type="text/javascript"></script> <noscript>Your browser does not support JavaScript. This help page requires JavaScript to render correctly.</noscript> </head> <body> <div class="zz-skip-header"><a href="#BEGIN">Skip Headers</a></div> <table class="simple oac_no_warn" summary="" cellspacing="0" cellpadding="0" width="100%"> <col width="86%" /> <col width="*" /> <tr valign="bottom"> <td align="left"></td> <td align="center"><a href="tdppt_sqlid015.htm"><img src="./dcommon/gifs/leftnav.gif" alt="Previous" /><br /> <span class="icon">Previous</span></a> </td> <td align="center"><a href="tdppt_sqltune001.htm"><img src="./dcommon/gifs/rightnav.gif" alt="Next" /><br /> <span class="icon">Next</span></a></td> </tr> </table> <p><a id="CAEBFCFJ" name="CAEBFCFJ"></a><a id="TDPPT160" name="TDPPT160"></a></p> <h1>Tuning SQL Statements</h1> <p><a name="BEGIN" id="BEGIN"></a></p> <p>A SQL statement expresses the data you want Oracle Database to retrieve. For example, a SQL statement can retrieve the names of employees in a department. When Oracle Database executes the SQL statement, the <span class="bold">query optimizer</span> (also called the <span class="bold">optimizer</span>) first determines the best and most efficient way to retrieve the results.</p> <p>The optimizer determines whether it is more efficient to read all data in the table, called a full table scan, or use an index. It compares the cost of all possible approaches and chooses the approach with the least cost. The access method for physically executing a SQL statement is called an execution plan<a id="sthref261" name="sthref261"></a>, which the optimizer is responsible for generating. The determination of an execution plan is an important step in the processing of any SQL statement, and can greatly affect execution time.</p> <p>The query optimizer can also help you tune SQL statements. By using SQL Tuning Advisor<a id="sthref262" name="sthref262"></a> and SQL Access Advisor<a id="sthref263" name="sthref263"></a>, you can run the query optimizer in advisory mode to examine a SQL statement or set of statements and determine how to improve their efficiency. SQL Tuning Advisor and SQL Access Advisor can make various recommendations, such as creating SQL profiles, restructuring SQL statements, creating additional indexes or materialized views, and refreshing optimizer statistics. Additionally, Oracle Enterprise Manager (Enterprise Manager) enables you to accept and implement many of these recommendations easily.</p> <p>SQL Access Advisor is primarily responsible for making schema modification recommendations, such as adding or dropping indexes and materialized views. SQL Tuning Advisor makes other types of recommendations, such as creating SQL profiles and restructuring SQL statements. If significant performance improvements can be gained by creating a new index, then SQL Tuning Advisor may recommend it. However, such recommendations should be verified by running SQL Access Advisor using a SQL workload that contains a set of representative SQL statements.</p> <div class="helpinfonotealso"> <h2>Related Topics</h2> <p><a href="tdppt_sqlid.htm#BABFHBFJ">Identifying High-Load SQL Statements</a></p> <p><a href="tdppt_sqlopt.htm#BABGHCAE">Optimizing Data Access Paths</a> for information about SQL Access Advisor</p> <p><a href="tdppt_sqltune001.htm#CACHECBD">Tuning SQL Statements Using SQL Tuning Advisor</a></p> <p><a href="tdppt_sqltune004.htm#CBBGEAJI">Managing SQL Tuning Sets</a></p> <p><a href="tdppt_sqltune019.htm#CHDDAGBE">Managing SQL Profiles</a></p> </div> <!-- Start Footer --> <div class="footer"> <table class="simple oac_no_warn" summary="" cellspacing="0" cellpadding="0" width="100%"> <col width="86%" /> <col width="*" /> <tr> <td align="left"><span class="copyrightlogo">Copyright © 2007, 2009, Oracle and/or its affiliates. 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