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<h1><a name="kanchor104"></a>Comparison Of Different IDs</h1>
<p>There are a lot of possibilities for additional ids to identify objects across the object container boundary. </p>
<h2>Internal db4o ids</h2>
<p>db4o has internal ids to identify each object in the database. You can access these ids and use them yourself. <a target="" title="" alt="" class="MCXref" xrefformat="See "{paratext}"">See "Internal IDs"</a>. Take a look at the example. <a href="example_internal_id.htm" target="" title="" alt="" class="MCXref">See "Example Internal Id"</a></p>
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<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColSep">Advantages</th>
<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">Disadvantages</th>
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<li value="1">Internal ids are fast.</li>
<li value="2">No additional field on the class required.</li>
<li value="3">No additional configuration required.</li>
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<li value="1">The id may change when defragmentating the database.</li>
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<h2>db4o UUIDs</h2>
<p>db4o supports special UUIDs. You can enable them by configuration. <a target="" title="" alt="" class="MCXref" xrefformat="See "{paratext}"">See "Unique Universal IDs"</a>. Take a look at the example. <a href="example_db4o_uuid.htm" target="" title="" alt="" class="MCXref">See "Example db4o UUID"</a></p>
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<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColSep">Advantages</th>
<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">Disadvantages</th>
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<li value="1">A UUID is a worldwide unique id.</li>
<li value="2">No additional field on the class required.</li>
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<li value="1">db4o UUIDs are large. </li>
<li value="2">db4o UUID is db4o-specific type.</li>
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<h2><span class="PrimaryUUID">Guid</span>-Fields on Classes</h2>
<p>You can add <span class="PrimaryUUID">Guid</span>-fields to your classes. In the constructor of the object you assign a new <span class="PrimaryUUID">Guid</span> to the object. Then you can find the object by a regular query. . Don't forget <a href="../../basics/indexing.htm">to index the id-field</a>. Take a look at the example. <a href="example_plattform_uuid.htm" target="" title="" alt="" class="MCXref">See "Example Guid"</a></p>
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<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColSep">Advantages</th>
<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">Disadvantages</th>
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<li value="1">A <span class="PrimaryUUID">Guid</span> is a worldwide unique id .</li>
<li value="2"><span class="PrimaryUUID">Guid</span> are easy to generate and portable.</li>
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<li value="1">You need an id-field on your objects.</li>
<li value="2"><span class="PrimaryUUID">Guid</span>s are quite large.</li>
<li value="3">Additional index required.</li>
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<h2>ID-Field On Classes With a ID-Generator</h2>
<p>You can add a id-field to your classes and then use an ID-Generator to assign new ids to stored objects. Don't forget <a href="../../basics/indexing.htm">to index the id-field</a>. Take a look at the example. <a href="example_id_generator.htm" target="" title="" alt="" class="MCXref">See "Example ID-Generator"</a></p>
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<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColSep">Advantages</th>
<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">Disadvantages</th>
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<li value="1">A simple id on objects.</li>
<li value="2">Familiar model from the <span class="MCTextPopup"><a href="javascript:void(0);" class="MCTextPopupSpot" onclick="FMCTextPopup( event, this ); return false;">RDBMS<img style="border: none;margin-left: 5px;" src="../../SkinSupport/ExpandingClosed.gif" MadCap:altsrc="../../SkinSupport/ExpandingOpen.gif" class="MCExpandingIcon" onload="if ( typeof( FMCPreloadImage ) == 'function' ) { FMCPreloadImage( '../../SkinSupport/ExpandingOpen.gif' ); }" /></a><span class="MCTextPopupBody" style="display: none; ">Relational Database Management System</span></span>-world.</li>
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<li value="1">You need an id-field on your objects.</li>
<li value="2">You need to implement an ID-Generator. Which isn't trivial.</li>
<li value="3">Additional index required.</li>
</ul>
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