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216 lines
8.5 KiB
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Handling Exceptions</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Collections Tutorial" />
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<link rel="up" href="BasicProgram.html" title="Chapter 2. The Basic Program" />
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<link rel="prev" href="retrievingdatabaseitems.html" title="Retrieving Database Items" />
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<link rel="next" href="UsingSecondaries.html" title="Chapter 3. Using Secondary Indices" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.2</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">
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Handling Exceptions
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="retrievingdatabaseitems.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2.
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The Basic Program
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="UsingSecondaries.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="handlingexceptions"></a>
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Handling Exceptions
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</h2>
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</div>
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</div>
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</div>
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<p>
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Exception handling was illustrated previously in
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<a class="xref" href="implementingmain.html" title="Implementing the Main Program">
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Implementing the Main Program
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</a>
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and
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<a class="xref" href="usingtransactions.html" title="Using Transactions">
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Using Transactions
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</a>
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exception handling in a DB Java Collections API application in
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more detail.
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</p>
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<p>
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There are two exceptions that must be treated specially:
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<a class="ulink" href="../../java/com/sleepycat/db/RunRecoveryException.html" target="_top">RunRecoveryException</a>
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and
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<span>
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<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>.
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</span>
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</p>
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<p>
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<a class="ulink" href="../../java/com/sleepycat/db/RunRecoveryException.html" target="_top">RunRecoveryException</a>
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is thrown when the only solution is to shut down the application
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and run recovery. All applications must catch this exception and
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follow the recovery procedure.
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</p>
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<p>
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When
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<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>
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is thrown, the application should normally retry the operation. If
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a deadlock continues to occur for some maximum number of retries,
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the application should give up and try again later or take other
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corrective actions. The DB Java Collections API provides two APIs
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for transaction execution.
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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When using the
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<a class="ulink" href="../../java/com/sleepycat/collections/CurrentTransaction.html" target="_top">CurrentTransaction</a>
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class directly, the application must catch
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<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>
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and follow the procedure described previously.
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</p>
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</li>
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<li>
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<p>
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When using the
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
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class, retries are performed automatically and the application need
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only handle the case where the maximum number of retries has been
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reached. In that case,
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html#run(com.sleepycat.collections.TransactionWorker)" target="_top">TransactionRunner.run</a>
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will throw
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<span>
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<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>.
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</span>
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</p>
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</li>
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</ul>
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</div>
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<p>
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When using the
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
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class there are two other considerations.
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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First, if the application-defined
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionWorker.html#doWork()" target="_top">TransactionWorker.doWork</a>
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method throws an exception the
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transaction will automatically be aborted, and otherwise the
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transaction will automatically be committed. Applications should
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design their transaction processing with this in mind.
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</p>
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</li>
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<li>
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<p>
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Second, please be aware that
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html#run(com.sleepycat.collections.TransactionWorker)" target="_top">TransactionRunner.run</a>
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unwraps exceptions in order to discover whether a nested exception is a
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<span>
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<a class="ulink" href="../../java/com/sleepycat/db/DeadlockException.html" target="_top">DeadlockException</a>.
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</span>
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This is particularly important since all Berkeley DB exceptions
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that occur while calling a stored collection method are wrapped
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with a
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<a class="ulink" href="../../java/com/sleepycat/util/RuntimeExceptionWrapper.html" target="_top">RuntimeExceptionWrapper</a>.
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This wrapping is necessary because Berkeley DB exceptions are
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checked exceptions, and the Java collections API does not allow
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such exceptions to be thrown.
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</p>
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</li>
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</ul>
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</div>
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<p>
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When calling
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html#run(com.sleepycat.collections.TransactionWorker)" target="_top">TransactionRunner.run</a>,
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the unwrapped (nested) exception will be unwrapped and thrown
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automatically. If you are not using
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<a class="ulink" href="../../java/com/sleepycat/collections/TransactionRunner.html" target="_top">TransactionRunner</a>
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or if you are handling exceptions directly for some other reason,
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use the
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<a class="ulink" href="../../java/com/sleepycat/util/ExceptionUnwrapper.html#unwrap(java.lang.Exception)" target="_top">ExceptionUnwrapper.unwrap</a>
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method to get the nested exception. For example, this can be used
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to discover that an exception is a
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<a class="ulink" href="../../java/com/sleepycat/db/RunRecoveryException.html" target="_top">RunRecoveryException</a>
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as shown below.
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</p>
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<a id="cb_java_manageexceptions"></a>
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<pre class="programlisting"><strong class="userinput"><code>import com.sleepycat.db.RunRecoveryException;
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import com.sleepycat.util.ExceptionUnwrapper;
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...
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catch (Exception e)
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{
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e = ExceptionUnwrapper.unwrap(e);
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if (e instanceof RunRecoveryException)
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{
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// follow recovery procedure
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}
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}</code></strong> </pre>
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</div>
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<div class="navfooter">
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<hr />
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|
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<table width="100%" summary="Navigation footer">
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|||
|
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<tr>
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|||
|
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<td width="40%" align="left"><a accesskey="p" href="retrievingdatabaseitems.html">Prev</a> </td>
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|||
|
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<td width="20%" align="center">
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|
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<a accesskey="u" href="BasicProgram.html">Up</a>
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|||
|
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</td>
|
|||
|
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<td width="40%" align="right"> <a accesskey="n" href="UsingSecondaries.html">Next</a></td>
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</tr>
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<tr>
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|||
|
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<td width="40%" align="left" valign="top">
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|||
|
|
Retrieving Database Items
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</td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Chapter 3.
|
|||
|
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Using Secondary Indices
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</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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