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<a name="outline228"></a><br><h1>15. Indexes</h1><br>
db4o allows to index fields to provide maximum querying performance. To request an index to be created, you would issue the following API method call in your global <a href="Configuration.html#Configuration">db4o configuration method</a> before you open an IObjectContainer/IObjectServer:<br>
<br>
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<code>// assuming<br>
class Foo<br>
{<br>
String bar;<br>
}<br>
<br>
ICommonConfiguration#ObjectClass(typeof(Foo)).ObjectField("bar").Indexed(true);</code></td></tr></table>
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<br>
If the configuration is set in this way, an index on the Foo#bar field will be created (if not present already) the next time you open an<br>
IObjectContainer/IObjectServer and you use the Foo class the first time<br>
in your application.<br>
<br>
Contrary to all other <a href="Configuration.html#Configuration">configuration calls</a> indexes - once created - will remain in a database even if the index configuration call is not issued before opening an IObjectContainer/IObjectServer. <br>
<br>
To drop an index you would also issue a configuration call in your db4o configuration method:<br>
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<code>ICommonConfiguration#ObjectClass(typeof(Foo)).ObjectField("bar").Indexed(false);</code></td></tr></table>
<br>
Actually dropping the index will take place the next time the respective class is used.<br>
<br>
db4o will tell you when it creates and drops indexes, if you choose a message level of 1 or higher:<br>
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<code>ICommonConfiguration#MessageLevel(1);</code></td></tr></table>
<br>
<br>
For creating and dropping indexes on large amounts of objects there are two possible strategies:<br>
(1) Import all objects with indexing off, configure the index and reopen the ObjectContainer/ObjectServer.<br>
(2) Import all objects with indexing turned on and commit regularly for a fixed amount of objects (~10,000).<br>
<br>
(1) will be faster.<br>
(2) will keep memory consumption lower.<br>
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