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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>Introduction</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#106170">6 Using Indexes</a> : Introduction</div> <hr align="left" /> <blockquote> <div class="head_1"><a name="106170">Introduction</a></div> <div class="Body"><a name="106171">An index is a database structure that you can use to improve the performance of database </a>activity. A database table can have one or more indexes associated with it.</div> <div class="Body"><a name="107013">An index is defined by a</a><span style="font-style: italic;"> </span>field expression that you specify when you create the index. Typically, the field expression is a single field name, like emp_id. An index created on the emp_id field, for example, contains a sorted list of the employee ID values in the table. Each value in the list is accompanied by references to the rows that contain that value.</div> <div class="picture"><a name="107014"><img class="Default" src="images/useindex.gif" width="272" height="213" style="display: block; float: none; left: 0.0; top: 0.0;" alt="A diagram showing an index created on the EMP_ID field" /></a></div> <div class="Body"><a name="107025">A database driver can use indexes to find rows quickly. An index on the emp_id field, for </a>example, greatly reduces the time that the driver spends searching for a particular employee ID value. Consider the following Where clause:</div> <div class="syntax_first"><a name="106184">WHERE EMP_id = 'E10001'</a></div> <div class="Body"><a name="106185">Without an index, the server must search the entire database table to find those rows having </a>an employee ID of E10001. By using an index on the emp_id field, however, the server can quickly find those rows.</div> <div class="Body"><a name="106186">Indexes may improve the performance of SQL statements. You may not notice this </a>improvement with small tables, but it can be significant for large tables; however, there can be disadvantages to having too many indexes. Indexes can slow down the performance of some inserts, updates, and deletes when the driver has to maintain the indexes as well as the database tables. Also, indexes take additional disk space.</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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