postgres/doc/src/sgml/lo.sgml

<!-- 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 &mdash; 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 domain of 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>