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<!-- doc/src/sgml/lo.sgml -->
<sect1 id="lo" xreflabel="lo">
<title>lo</title>
<indexterm zone="lo">
<primary>lo</primary>
</indexterm>
<para>
The <filename>lo</filename> module provides support for managing Large Objects
(also called LOs or BLOBs). This includes a data type <type>lo</type>
and a trigger <function>lo_manage</function>.
</para>
<para>
This module is considered <quote>trusted</quote>, that is, it can be
installed by non-superusers who have <literal>CREATE</literal> privilege
on the current database.
</para>
<sect2>
<title>Rationale</title>
<para>
One of the problems with the JDBC driver (and this affects the ODBC driver
also), is that the specification assumes that references to BLOBs (Binary
Large OBjects) are stored within a table, and if that entry is changed, the
associated BLOB is deleted from the database.
</para>
<para>
As <productname>PostgreSQL</productname> stands, this doesn't occur. Large objects
are treated as objects in their own right; a table entry can reference a
large object by OID, but there can be multiple table entries referencing
the same large object OID, so the system doesn't delete the large object
just because you change or remove one such entry.
</para>
<para>
Now this is fine for <productname>PostgreSQL</productname>-specific applications, but
standard code using JDBC or ODBC won't delete the objects, resulting in
orphan objects — objects that are not referenced by anything, and
simply occupy disk space.
</para>
<para>
The <filename>lo</filename> module allows fixing this by attaching a trigger
to tables that contain LO reference columns. The trigger essentially just
does a <function>lo_unlink</function> whenever you delete or modify a value
referencing a large object. When you use this trigger, you are assuming
that there is only one database reference to any large object that is
referenced in a trigger-controlled column!
</para>
<para>
The module also provides a data type <type>lo</type>, which is really just
a <glossterm linkend="glossary-domain">domain</glossterm> over
the <type>oid</type> type. This is useful for differentiating
database columns that hold large object references from those that are
OIDs of other things. You don't have to use the <type>lo</type> type to
use the trigger, but it may be convenient to use it to keep track of which
columns in your database represent large objects that you are managing with
the trigger. It is also rumored that the ODBC driver gets confused if you
don't use <type>lo</type> for BLOB columns.
</para>
</sect2>
<sect2>
<title>How to Use It</title>
<para>
Here's a simple example of usage:
</para>
<programlisting>
CREATE TABLE image (title text, raster lo);
CREATE TRIGGER t_raster BEFORE UPDATE OR DELETE ON image
FOR EACH ROW EXECUTE FUNCTION lo_manage(raster);
</programlisting>
<para>
For each column that will contain unique references to large objects,
create a <literal>BEFORE UPDATE OR DELETE</literal> trigger, and give the column
name as the sole trigger argument. You can also restrict the trigger
to only execute on updates to the column by using <literal>BEFORE UPDATE
OF</literal> <replaceable class="parameter">column_name</replaceable>.
If you need multiple <type>lo</type>
columns in the same table, create a separate trigger for each one,
remembering to give a different name to each trigger on the same table.
</para>
</sect2>
<sect2>
<title>Limitations</title>
<itemizedlist>
<listitem>
<para>
Dropping a table will still orphan any objects it contains, as the trigger
is not executed. You can avoid this by preceding the <command>DROP
TABLE</command> with <command>DELETE FROM <replaceable>table</replaceable></command>.
</para>
<para>
<command>TRUNCATE</command> has the same hazard.
</para>
<para>
If you already have, or suspect you have, orphaned large objects, see the
<xref linkend="vacuumlo"/> module to help
you clean them up. It's a good idea to run <application>vacuumlo</application>
occasionally as a back-stop to the <function>lo_manage</function> trigger.
</para>
</listitem>
<listitem>
<para>
Some frontends may create their own tables, and will not create the
associated trigger(s). Also, users may not remember (or know) to create
the triggers.
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2>
<title>Author</title>
<para>
Peter Mount <email>peter@retep.org.uk</email>
</para>
</sect2>
</sect1>
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