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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>About Distributed SQL</title> <meta name="generator" content="Oracle DARB XHTML Converter (Mode = ohj/ohw) - Version 5.1.1 Build 005" /> <meta name="date" content="2009-06-04T17:1:1Z" /> <meta name="robots" content="noarchive" /> <meta name="doctitle" content="About Distributed SQL" /> <meta name="relnum" content="11g Release 2 (11.2)" /> <meta name="partnum" content="E10703-01" /> <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="tdpii_distdbs001.htm" title="Previous" type="text/html" /> <link rel="next" href="tdpii_distdbs003.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="tdpii_distdbs001.htm"><img src="./dcommon/gifs/leftnav.gif" alt="Previous" /><br /> <span class="icon">Previous</span></a> </td> <td align="center"><a href="tdpii_distdbs003.htm"><img src="./dcommon/gifs/rightnav.gif" alt="Next" /><br /> <span class="icon">Next</span></a></td> </tr> </table> <p><a id="CIHFHIBF" name="CIHFHIBF"></a><a id="TDPII015" name="TDPII015"></a></p> <div class="sect2"><!-- infolevel="all" infotype="General" --> <h1>About Distributed SQL<a id="sthref37" name="sthref37"></a><a id="sthref38" name="sthref38"></a><a id="sthref39" name="sthref39"></a><a id="sthref40" name="sthref40"></a><a id="sthref41" name="sthref41"></a></h1> <a name="BEGIN" id="BEGIN"></a> <p><span class="bold">Distributed SQL</span> enables applications and users to query or modify information in multiple databases with a single SQL statement. Because distributed SQL masks the physical location of your data, you can change the location of your data without changing your application. Distributed SQL includes the following: <span class="bold">distributed queries</span> (which access data) and <span class="bold">distributed transactions</span> (which modify data). In distributed transactions, the <span class="bold">two-phase commit mechanism</span> guarantees the integrity of your data by ensuring that all statements in a transaction either commit or roll back as a unit at each database involved in the distributed transaction.</p> <p>When an application or user tries to commit a distributed transaction, the database to which the application or user is connected is called the <span class="bold">global coordinator</span>. The global coordinator completes the two-phase commit by initiating the following phases:</p> <ul> <li> <p><span class="bold">Prepare Phase:</span> The global coordinator asks the other databases involved in the distributed transaction to confirm that they can either commit or roll back the transaction, even if there is a failure. If any database cannot complete the prepare phase, then the transaction is rolled back.</p> </li> <li> <p><span class="bold">Commit Phase:</span> If all of the other databases inform the global coordinator that they are prepared, then the global coordinator commits the transaction and asks all of the other databases to commit the transaction.</p> </li> </ul> <div class="helpinfonotealso"> <h2>Related Topics</h2> <p><a href="javascript:open('http://www.oracle.com/pls/db112/lookup?id=ADMIN031','newWindow').focus()"><span class="italic">Oracle Database Administrator's Guide</span></a></p> <p><a href="tdpii_distdbs006.htm#BABJDEED">Preparing to Access and Modify Information in Multiple Oracle Databases</a></p> <p><a href="tdpii_distdbs007.htm#BABFJGFG">Tutorial: Querying Multiple Oracle Databases</a></p> <p><a href="tdpii_distdbs008.htm#BABCCCGB">Tutorial: Modifying Data in Multiple Oracle Databases</a></p> <p><a href="tdpii_distdbs009.htm#BABFCFJJ">Tutorial: Running a Stored Procedure in a Remote Oracle Database</a></p> </div> </div> <!-- class="sect2" --> <!-- 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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