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<title>GNOME Data Access 5 manual: Dictionary - metadata</title>
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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="gda-dict"></a>Dictionary - metadata</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="sect1"><a href="gda-dict.html#GdaMetaStoreSetup">Setup</a></span></dt>
<dt><span class="sect1"><a href="GdaMetaStoreCustomData.html">Adding custom data</a></span></dt>
<dt><span class="sect1"><a href="information_schema.html">Database structure</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="information_schema.html#information_schema:data_types">Data types</a></span></dt>
<dt><span class="sect2"><a href="information_schema.html#information_schema:sql_identifiers">SQL identifiers</a></span></dt>
<dt><span class="sect2"><a href="information_schema.html#id-1.3.11.8.5">Short and full names</a></span></dt>
<dt><span class="sect2"><a href="information_schema.html#id-1.3.11.8.6">Declared foreign keys</a></span></dt>
<dt><span class="sect2"><a href="information_schema.html#id-1.3.11.8.7">Individual table description</a></span></dt>
</dl></dd>
<dt>
<span class="refentrytitle"><a href="GdaMetaStore.html">GdaMetaStore</a></span><span class="refpurpose"> — Dictionary object</span>
</dt>
<dt>
<span class="refentrytitle"><a href="GdaMetaStruct.html">GdaMetaStruct</a></span><span class="refpurpose"> — In memory representation of some database objects</span>
</dt>
</dl></div>
<p>
Each connection has a dictionary object (a <a class="link" href="GdaMetaStore.html" title="GdaMetaStore">GdaMetaStore</a>) attached to it. That
dictionary is either created by the connection when it needs it, or is created and set by the user
(to be able to re-use a dictionary), using the <a class="link" href="GdaConnection.html#GdaConnection--meta-store" title="The “meta-store” property">"meta-store"</a>
property of the <a class="link" href="GdaConnection.html" title="GdaConnection">GdaConnection</a> object.
</p>
<p>
Previous versions of <span class="application">Libgda</span> used an XML file based dictionary which had several drawbacks (see the
<a class="link" href="migration-2-dict.html" title="Dictionary changes">migrations notes</a> for more details), now a database is used: the default
is to use an SQLite base database which is easily transportable and can be used in the same way the previous
XML based dictionary was used.
</p>
<p>
Each <a class="link" href="GdaMetaStore.html" title="GdaMetaStore">GdaMetaStore</a> requires (and creates) a dictionary
structure which is a set of tables implementing an
"information schema" schema (as defined in the information schema SQL standard (ISO/IEC 9075), and adapted).
The user is then free to add more database objects (tables and views) to contain his own data in the dictionary,
using the <a class="link" href="GdaMetaStore.html#gda-meta-store-schema-add-custom-object" title="gda_meta_store_schema_add_custom_object ()">gda_meta_store_schema_add_custom_object()</a>
method.
</p>
<p>
Extracting information can be done using SQL statements on the dictionary database (a special
gda_meta_store_extract() method), or, for information about
the database structure, using the <a class="link" href="GdaMetaStruct.html" title="GdaMetaStruct">GdaMetaStruct</a> object which creates
an easy to use in-memory representation of some database objects.
</p>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="GdaMetaStoreSetup"></a>Setup</h2></div></div></div>
<p>
Each <a class="link" href="GdaMetaStore.html" title="GdaMetaStore">GdaMetaStore</a> object internally uses a (private)
<a class="link" href="GdaConnection.html" title="GdaConnection">GdaConnection</a> connection object. The following figure illustrates
the situation when the programmer uses a <a class="link" href="GdaConnection.html" title="GdaConnection">GdaConnection</a> connection object
when the meta data is stored in a different database (usually an SQLite file):
</p>
<div class="mediaobject">
<img src="MetaStore1.png"><div class="caption"><p>
GdaConnection object and its associated GdaMetaStore using its own database to store the
meta data.
</p></div>
</div>
<p>
</p>
<p>
From a programmer's point of view, the following code example shows how to get a connection's associated
meta store object, without being able to specify anything about the meta store's private connection (in which
case the private connection will be an in-memory database destroyed when the meta store object is destroyed):
</p>
<pre class="programlisting">
GdaConnection *cnc;
GdaMetaStore *store;
cnc = gda_connection_open_from_dsn (...);
g_object_get (G_OBJECT (cnc), "meta-store", &store, NULL);
[... use the meta store object ...]
g_object_unref (store);
</pre>
<p>
</p>
<p>
One can also specify the meta store object to be used by a connection, as in:
</p>
<pre class="programlisting">
GdaConnection *cnc;
GdaMetaStore *store;
cnc = gda_connection_open_from_dsn (...);
store = gda_meta_store_new_with_file ("/path/to/file");
/* or */
store = gda_meta_store_new ("PostgreSQL://DB_NAME=meta_db");
g_object_set (G_OBJECT (cnc), "meta-store", store, NULL);
g_object_unref (store);
</pre>
<p>
</p>
<p>
The meta data can also be stored in the same database as the connection the meta data is for. In this case
(and if the associated database provider supports it), the dictionary structure can be placed into a spearate
schema. The next figure illustrates this situation:
</p>
<div class="mediaobject">
<img src="MetaStore2.png"><div class="caption"><p>
GdaConnection object and its associated GdaMetaStore using the same database.
</p></div>
</div>
<p>
</p>
<p>
From a programmer's point of view, the following code example shows how to do the setup:
</p>
<pre class="programlisting">
GdaConnection *cnc;
GdaMetaStore *store;
cnc = gda_connection_open_from_dsn (...);
store = GDA_META_STORE (g_object_new (GDA_TYPE_META_STORE, "cnc", cnc, NULL));
g_object_set (G_OBJECT (cnc), "meta-store", store, NULL);
g_object_unref (store);
</pre>
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