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<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xml:lang="en" lang="en" xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> <meta http-equiv="Content-Style-Type" content="text/css" /> <!-- MOTW-DISABLED saved from url=(0014)about:internet --> <title>Improving Join Performance</title> <link rel="StyleSheet" href="css/useindx.css" type="text/css" media="all" /> <link rel="StyleSheet" href="css/webworks.css" type="text/css" media="all" /> <script type="text/javascript" language="JavaScript1.2" src="wwhdata/common/context.js"></script> <script type="text/javascript" language="JavaScript1.2" src="wwhdata/common/towwhdir.js"></script> <script type="text/javascript" language="JavaScript1.2" src="wwhdata/common/wwhpagef.js"></script> <script type="text/javascript" language="JavaScript1.2"> <!-- var WebWorksRootPath = ""; // --> </script> <script type="text/javascript" language="JavaScript1.2"> <!-- // Set reference to top level help frame // var WWHFrame = WWHGetWWHFrame("", true); // --> </script> <script type="text/javascript" language="JavaScript1.2" src="scripts/expand.js"></script> </head> <body class="" style="background-color: #FFFFEE;" onload="WWHUpdate();" onunload="WWHUnload();" onkeydown="WWHHandleKeyDown((document.all||document.getElementById||document.layers)?event:null);" onkeypress="WWHHandleKeyPress((document.all||document.getElementById||document.layers)?event:null);" onkeyup="WWHHandleKeyUp((document.all||document.getElementById||document.layers)?event:null);"> <br /> <div class="WebWorks_Breadcrumbs" style="text-align: left;"> <a class="WebWorks_Breadcrumb_Link" href="useindx.08.1.html#106239">6 Using Indexes</a> : Improving Join Performance</div> <hr align="left" /> <blockquote> <div class="head_1"><a name="106239">Improving Join Performance</a></div> <div class="Body"><a name="106240">When joining database tables, index tables can greatly improve performance. Unless the </a>proper indexes are available, queries that use joins can take a long time.</div> <div class="Body"><a name="106241">Assume you have the following Select statement:</a></div> <div class="syntax_wide_first"><a name="106242">SELECT * FROM dept, emp WHERE dept.dept_id = emp.dept_id</a></div> <div class="Body"><a name="106243">In this example, the </a><span class="Syntax">dept</span> and <span class="Syntax">emp</span> database tables are being joined using the <span class="Syntax">dept_id</span> field. When the driver executes a query that contains a join, it processes the tables from left to right and uses an index on the second tableā??s join field (the <span class="Syntax">dept</span> field of the <span class="Syntax">emp</span> table). To improve join performance, you need an index on the join field of the second table in the FROM clause. </div> <div class="Body"><a name="106244">If the FROM clause includes a third table, the driver also uses an index on the field in the </a>third table that joins it to any previous table. For example: </div> <div class="syntax_wide_first"><a name="106245">SELECT * FROM dept, emp, addr WHERE dept.dept_id = emp.dept AND emp.loc = addr.loc</a></div> <div class="Body"><a name="106247">In this case, you should have an index on the emp.dept field and the addr.loc field.</a></div> <div class="Body"><a name="106056"> </a></div> <script type="text/javascript" language="JavaScript1.2"> <!-- // Clear related topics // WWHClearRelatedTopics(); document.writeln(WWHRelatedTopicsInlineHTML()); // --> </script> </blockquote> <script type="text/javascript" language="JavaScript1.2"> <!-- document.write(WWHRelatedTopicsDivTag() + WWHPopupDivTag() + WWHALinksDivTag()); // --> </script> </body> </html>
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