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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>F.22. lo</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="isn.html" title="F.21. isn" /><link rel="next" href="ltree.html" title="F.23. ltree" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.22. lo</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="isn.html" title="F.21. isn">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ltree.html" title="F.23. ltree">Next</a></td></tr></table><hr /></div><div class="sect1" id="LO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.22. lo</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="lo.html#id-1.11.7.31.5">F.22.1. Rationale</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.31.6">F.22.2. How to Use It</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.31.7">F.22.3. Limitations</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.31.8">F.22.4. Author</a></span></dt></dl></div><a id="id-1.11.7.31.2" class="indexterm"></a><p>
  The <code class="filename">lo</code> module provides support for managing Large Objects
  (also called LOs or BLOBs).  This includes a data type <code class="type">lo</code>
  and a trigger <code class="function">lo_manage</code>.
 </p><p>
  This module is considered <span class="quote"><span class="quote">trusted</span></span>, that is, it can be
  installed by non-superusers who have <code class="literal">CREATE</code> privilege
  on the current database.
 </p><div class="sect2" id="id-1.11.7.31.5"><div class="titlepage"><div><div><h3 class="title">F.22.1. Rationale</h3></div></div></div><p>
   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.
  </p><p>
   As <span class="productname">PostgreSQL</span> 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.
  </p><p>
   Now this is fine for <span class="productname">PostgreSQL</span>-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.
  </p><p>
   The <code class="filename">lo</code> module allows fixing this by attaching a trigger
   to tables that contain LO reference columns.  The trigger essentially just
   does a <code class="function">lo_unlink</code> 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!
  </p><p>
   The module also provides a data type <code class="type">lo</code>, which is really just
   a <a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN" title="Domain">domain</a></em></a> over
   the <code class="type">oid</code> 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 <code class="type">lo</code> 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 <code class="type">lo</code> for BLOB columns.
  </p></div><div class="sect2" id="id-1.11.7.31.6"><div class="titlepage"><div><div><h3 class="title">F.22.2. How to Use It</h3></div></div></div><p>
   Here's a simple example of usage:
  </p><pre class="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);
</pre><p>
   For each column that will contain unique references to large objects,
   create a <code class="literal">BEFORE UPDATE OR DELETE</code> 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 <code class="literal">BEFORE UPDATE
   OF</code> <em class="replaceable"><code>column_name</code></em>.
   If you need multiple <code class="type">lo</code>
   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.
  </p></div><div class="sect2" id="id-1.11.7.31.7"><div class="titlepage"><div><div><h3 class="title">F.22.3. Limitations</h3></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
     Dropping a table will still orphan any objects it contains, as the trigger
     is not executed.  You can avoid this by preceding the <code class="command">DROP
     TABLE</code> with <code class="command">DELETE FROM <em class="replaceable"><code>table</code></em></code>.
    </p><p>
     <code class="command">TRUNCATE</code> has the same hazard.
    </p><p>
     If you already have, or suspect you have, orphaned large objects, see the
     <a class="xref" href="vacuumlo.html" title="vacuumlo"><span class="refentrytitle"><span class="application">vacuumlo</span></span></a> module to help
     you clean them up.  It's a good idea to run <span class="application">vacuumlo</span>
     occasionally as a back-stop to the <code class="function">lo_manage</code> trigger.
    </p></li><li class="listitem"><p>
     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.
    </p></li></ul></div></div><div class="sect2" id="id-1.11.7.31.8"><div class="titlepage"><div><div><h3 class="title">F.22.4. Author</h3></div></div></div><p>
   Peter Mount <code class="email">&lt;<a class="email" href="mailto:peter@retep.org.uk">peter@retep.org.uk</a>&gt;</code>
  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="isn.html" title="F.21. isn">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ltree.html" title="F.23. ltree">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.21. isn </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.23. ltree</td></tr></table></div></body></html>