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<title>Chapter 3. Using Secondary Indices and Foreign keys</title>
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<link rel="up" href="index.html" title="Berkeley DB Java Edition Collections Tutorial" />
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<link rel="prev" href="handlingexceptions.html" title="Handling Exceptions" />
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<div class="libver">
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<p>Library Version 12.2.7.5</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">Chapter 3.
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Using Secondary Indices<span xmlns="http://www.w3.org/1999/xhtml"> and Foreign keys</span>
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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="handlingexceptions.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="openingforeignkeys.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="chapter" 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"><a id="UsingSecondaries"></a>Chapter 3.
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Using Secondary Indices<span> and Foreign keys</span>
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</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="UsingSecondaries.html#opensecondaryindices">
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Opening Secondary Key Indices
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="openingforeignkeys.html">
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<span>Opening Foreign Key Indices</span>
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="indexedcollections.html">
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Creating Indexed Collections
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</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="retrievingbyindexkey.html">
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Retrieving Items by Index Key
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</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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In the Basic example, each store has a single <span class="emphasis"><em>primary
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key</em></span>. The Index example extends the Basic example to add the use of
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<span class="emphasis"><em>secondary keys</em></span><span> and <span class="emphasis"><em>foreign keys</em></span></span>.
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</p>
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<p>
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The complete source of the final version of the example program
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is included in the Berkeley DB distribution.
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</p>
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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="opensecondaryindices"></a>
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Opening Secondary Key Indices
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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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<span class="emphasis"><em>Secondary indices</em></span> or <span class="emphasis"><em>secondary databases</em></span> are used
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to access a primary database by a key other than the primary key.
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Recall that the Supplier Number field is the primary key of the
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Supplier database. In this section, the Supplier City field will be
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used as a secondary lookup key. Given a city value, we would like
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to be able to find the Suppliers in that city. Note that more than
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one Supplier may be in the same city.
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</p>
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<p>
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Both primary and secondary databases contain key-value records.
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The key of an index record is the secondary key, and its value is
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the key of the associated record in the primary database. When lookups by
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secondary key are performed, the associated record in the primary
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database is transparently retrieved by its primary key and returned
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to the caller.
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</p>
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<p>
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Secondary indices are maintained automatically when index key
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fields (the City field in this case) are added, modified or removed
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in the records of the primary database. However, the application
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must implement a
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</a>
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that extracts the index key from the database record.
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</p>
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<p>
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It is useful to contrast opening an secondary index with opening
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a primary database (as described earlier in
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<a class="xref" href="opendatabases.html" title="Opening and Closing Databases">
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Opening and Closing Databases
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</a>.
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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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A primary database may be associated with one or more secondary
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indices. A secondary index is always associated with exactly one
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primary database.
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</p>
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</li>
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<li>
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<p>
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For a secondary index, a
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</a>
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must be implemented by the application to extract the index key
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from the record of its associated primary database.
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</p>
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</li>
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<li>
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<p>
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A primary database is represented by a
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<a class="ulink" href="../../java/com/sleepycat/je/Database.html" target="_top">Database</a>
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object and a secondary index is represented by a
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>
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object. The
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>
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class extends the
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<a class="ulink" href="../../java/com/sleepycat/je/Database.html" target="_top">Database</a>
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class.
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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 a
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>
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is created it is associated with a primary
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<a class="ulink" href="../../java/com/sleepycat/je/Database.html" target="_top">Database</a>
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object and a
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<span>
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</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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The <code class="classname">SampleDatabase</code> class is extended to open the
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Supplier-by-City secondary key index.
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</p>
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<a id="index_je_sampledatabase"></a>
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<pre class="programlisting"><strong class="userinput"><code>import com.sleepycat.bind.serial.SerialSerialKeyCreator;
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import com.sleepycat.je.SecondaryConfig;
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import com.sleepycat.je.SecondaryDatabase;</code></strong>
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...
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public class SampleDatabase
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{
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...
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<strong class="userinput"><code> private static final String SUPPLIER_CITY_INDEX =
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"supplier_city_index";
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...
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private SecondaryDatabase supplierByCityDb;
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...</code></strong>
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public SampleDatabase(String homeDirectory)
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throws DatabaseException, FileNotFoundException
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{
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...
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<strong class="userinput"><code> SecondaryConfig secConfig = new SecondaryConfig();
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secConfig.setTransactional(true);
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secConfig.setAllowCreate(true);
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secConfig.setSortedDuplicates(true);
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secConfig.setKeyCreator(
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new SupplierByCityKeyCreator(javaCatalog,
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SupplierKey.class,
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SupplierData.class,
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String.class));
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supplierByCityDb = env.openSecondaryDatabase(null,
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SUPPLIER_CITY_INDEX,
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supplierDb,
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secConfig);</code></strong>
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...
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}
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} </pre>
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<p>
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A
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryConfig.html" target="_top">SecondaryConfig</a>
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object is used to configure the secondary database. The
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryConfig.html" target="_top">SecondaryConfig</a>
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class extends the
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<a class="ulink" href="../../java/com/sleepycat/je/DatabaseConfig.html" target="_top">DatabaseConfig</a>
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class, and most steps for configuring a secondary database are the
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same as for configuring a primary database. The main difference in
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the example above is that the
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<code class="methodname">SecondaryConfig.setSortedDuplicates()</code> method is called to
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allow duplicate index keys. This is how more than one Supplier may
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be in the same City. If this property is not specified, the default is
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that the index keys of all records must be unique.
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</p>
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<p>
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For a primary database, duplicate keys are not normally used
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since a primary database with duplicate keys may not have any
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associated secondary indices. If primary database keys are not
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unique, there is no way for a secondary key to reference a specific
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record in the primary database.
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</p>
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<p>
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Opening a secondary key index requires creating a
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<span>
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</a>.
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</span>
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The <code class="classname">SupplierByCityKeyCreator</code> class implements the
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</a>
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interface and will be defined below.
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</p>
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<p>
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The
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>
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object is opened last. If you compare the
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<code class="methodname">openSecondaryDatabase()</code> and <code class="methodname">openDatabase()</code> methods you'll
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notice only two differences:
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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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<code class="methodname">openSecondaryDatabase()</code> has an extra parameter for
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specifying the associated primary database. The primary database is
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<code class="literal">supplierDb</code> in this case.
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</p>
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</li>
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<li>
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<p>
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The last parameter of <code class="methodname">openSecondaryDatabase()</code> is a
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<code class="literal">SecondaryConfig</code> instead of a <code class="literal">DatabaseConfig</code>.
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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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How to use the secondary index to access records will be shown
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in a later section.
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</p>
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<p>
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The application-defined <code class="classname">SupplierByCityKeyCreator</code> class is
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shown below. It was used above to configure the secondary
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database.
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</p>
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<a id="index_supplierbycitykeycreator"></a>
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<pre class="programlisting">public class SampleDatabase
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{
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...
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<strong class="userinput"><code> private static class SupplierByCityKeyCreator
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extends SerialSerialKeyCreator
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{
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private SupplierByCityKeyCreator(ClassCatalog catalog,
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Class primaryKeyClass,
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Class valueClass,
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Class indexKeyClass)
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{
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super(catalog, primaryKeyClass, valueClass, indexKeyClass);
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}
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public Object createSecondaryKey(Object primaryKeyInput,
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Object valueInput)
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{
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SupplierData supplierData = (SupplierData) valueInput;
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return supplierData.getCity();
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}
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}</code></strong>
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...
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} </pre>
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<p>
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In general, a key creator class must implement the
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</a>
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interface. This interface has methods that operate on the record
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data as raw bytes. In practice, it is easiest to use an abstract
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base class that performs the conversion of record data to and from
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the format defined for the database's key and value. The base class
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implements the
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<a class="ulink" href="../../java/com/sleepycat/je/SecondaryKeyCreator.html" target="_top">SecondaryKeyCreator</a>
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interface and has abstract methods that must be implemented in turn
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by the application.
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</p>
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<p>
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In this example the
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<a class="ulink" href="../../java/com/sleepycat/bind/serial/SerialSerialKeyCreator.html" target="_top">SerialSerialKeyCreator</a>
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base class is used because the database record uses the serial
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format for both its key and its value. The abstract methods of this
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class have key and value parameters of type
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<a class="ulink" href="http://download.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html" target="_top">Object</a>
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which are automatically converted to and from the raw record data
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by the base class.
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</p>
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<p>
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To perform the conversions properly, the key creator must be
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||
aware of all three formats involved: the key format of the primary
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database record, the value format of the primary database record,
|
||
and the key format of the index record. The
|
||
<a class="ulink" href="../../java/com/sleepycat/bind/serial/SerialSerialKeyCreator.html" target="_top">SerialSerialKeyCreator</a>
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|
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constructor is given the base classes for these three formats as
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||
parameters.
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</p>
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<p>
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The <code class="methodname">SerialSerialKeyCreator.createSecondaryKey</code> method is
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||
given the key and value of the primary database record as
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parameters, and it returns the key of the index record. In this
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||
example, the index key is a field in the primary database record
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value. Since the record value is known to be a <code class="classname">SupplierData</code>
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object, it is cast to that class and the city field is
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||
returned.
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</p>
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<p>
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Note that the <code class="literal">primaryKeyInput</code> parameter is not used in
|
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the example. This parameter is needed only when an index key is
|
||
derived from the key of the primary database record. Normally an
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index key is derived only from the primary database record value,
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||
but it may be derived from the key, value or both.
|
||
</p>
|
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<p>
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The following getter methods return the secondary database
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||
object for use by other classes in the example program. The
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secondary database object is used to create Java collections for
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accessing records via their secondary keys.
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</p>
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<a id="index_getsupplierbycitydatabase"></a>
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||
<pre class="programlisting">public class SampleDatabase
|
||
{
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||
...
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<strong class="userinput"><code> public final SecondaryDatabase getSupplierByCityDatabase()
|
||
{
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||
return supplierByCityDb;
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||
}</code></strong>
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||
...
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||
} </pre>
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<p>
|
||
The following statement closes the secondary database.
|
||
</p>
|
||
<a id="index_close"></a>
|
||
<pre class="programlisting">public class SampleDatabase
|
||
{
|
||
...
|
||
public void close()
|
||
throws DatabaseException {
|
||
|
||
<strong class="userinput"><code> supplierByCityDb.close();</code></strong>
|
||
partDb.close();
|
||
supplierDb.close();
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shipmentDb.close();
|
||
javaCatalog.close();
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env.close();
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}
|
||
...
|
||
} </pre>
|
||
<p>
|
||
Secondary databases must be closed before closing their
|
||
associated primary database.
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="handlingexceptions.html">Prev</a> </td>
|
||
<td width="20%" align="center"> </td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="openingforeignkeys.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">
|
||
Handling Exceptions
|
||
</td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top">
|
||
<span>Opening Foreign Key Indices</span>
|
||
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|