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<title>Binary large objects (BLOBs): GNOME Data Access 5 manual</title>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="gen_blobs"></a>Binary large objects (BLOBs)</h2></div></div></div>
<p>
Binary large objects (BLOBs) are potentially very big (several GigaBytes) pieces of binary
data which databases can store. Because of their potential size, they are not manipulated
like any other data, and are usually not transfered to the database engine at the same time
as the SQL text is transfered: first
a reference to a blob is created for an existing or a new blob, and then
that reference is used, <span class="emphasis"><em>before or after the statement is being executed,
depending on the database engine accessed</em></span>, to get or set some parts of the blob.
</p>
<p>
<span class="application">Libgda</span> makes its best to hide the complexity of this and most of the time blobs can be used
like any other type of values with the following limitations:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>When getting a blob through a SELECT statement, a transaction will automatically
be started if it's not already started, and usually the transaction (implicitly or explicitly)
will be locked untill all the ressources associated to the fetched blobs are liberated (by
calling <a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref()</a> on the model).</p></li>
<li class="listitem"><p>When fetching a blob value (using
gda_data_model_get_value_at() for example), the blob
itself is not fetched, only a reference to the blob is fetched, and one has to use the
<a class="link" href="GdaBlobOp.html" title="Blobs">GdaBlobOp</a> object embedded in a blob value to get the blob's
contents.</p></li>
</ul></div>
<p>
</p>
<p>
Please consult each database provider's <a class="link" href="provider-notes.html" title="Provider's notes">notes</a> and
<a class="link" href="limitations.html" title="Limitations">limitations</a> for database specific rules
regarding BLOBs. Also have a look at the
<a class="link" href="blobs_example.html" title="Binary large objects (BLOBs) example">Blobs manipulation source code example</a>.
</p>
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