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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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.20. 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 V1.79.1" /><link rel="prev" href="isn.html" title="F.19. isn" /><link rel="next" href="ltree.html" title="F.21. ltree" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.20. lo</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="isn.html" title="F.19. 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 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ltree.html" title="F.21. ltree">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.20. lo</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.4">F.20.1. Rationale</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.5">F.20.2. How to Use It</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.6">F.20.3. Limitations</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.7">F.20.4. Author</a></span></dt></dl></div><a id="id-1.11.7.29.2" class="indexterm"></a><p>

  3.   The <code class="filename">lo</code> module provides support for managing Large Objects

  4.   (also called LOs or BLOBs).  This includes a data type <code class="type">lo</code>

  5.   and a trigger <code class="function">lo_manage</code>.

  6.  </p><div class="sect2" id="id-1.11.7.29.4"><div class="titlepage"><div><div><h3 class="title">F.20.1. Rationale</h3></div></div></div><p>

  7.    One of the problems with the JDBC driver (and this affects the ODBC driver

  8.    also), is that the specification assumes that references to BLOBs (Binary

  9.    Large OBjects) are stored within a table, and if that entry is changed, the

  10.    associated BLOB is deleted from the database.

  11.   </p><p>

  12.    As <span class="productname">PostgreSQL</span> stands, this doesn't occur.  Large objects

  13.    are treated as objects in their own right; a table entry can reference a

  14.    large object by OID, but there can be multiple table entries referencing

  15.    the same large object OID, so the system doesn't delete the large object

  16.    just because you change or remove one such entry.

  17.   </p><p>

  18.    Now this is fine for <span class="productname">PostgreSQL</span>-specific applications, but

  19.    standard code using JDBC or ODBC won't delete the objects, resulting in

  20.    orphan objects — objects that are not referenced by anything, and

  21.    simply occupy disk space.

  22.   </p><p>

  23.    The <code class="filename">lo</code> module allows fixing this by attaching a trigger

  24.    to tables that contain LO reference columns.  The trigger essentially just

  25.    does a <code class="function">lo_unlink</code> whenever you delete or modify a value

  26.    referencing a large object.  When you use this trigger, you are assuming

  27.    that there is only one database reference to any large object that is

  28.    referenced in a trigger-controlled column!

  29.   </p><p>

  30.    The module also provides a data type <code class="type">lo</code>, which is really just

  31.    a domain of the <code class="type">oid</code> type.  This is useful for differentiating

  32.    database columns that hold large object references from those that are

  33.    OIDs of other things.  You don't have to use the <code class="type">lo</code> type to

  34.    use the trigger, but it may be convenient to use it to keep track of which

  35.    columns in your database represent large objects that you are managing with

  36.    the trigger.  It is also rumored that the ODBC driver gets confused if you

  37.    don't use <code class="type">lo</code> for BLOB columns.

  38.   </p></div><div class="sect2" id="id-1.11.7.29.5"><div class="titlepage"><div><div><h3 class="title">F.20.2. How to Use It</h3></div></div></div><p>

  39.    Here's a simple example of usage:

  40.   </p><pre class="programlisting">

  41. CREATE TABLE image (title text, raster lo);

  42. 

  43. CREATE TRIGGER t_raster BEFORE UPDATE OR DELETE ON image

  44.     FOR EACH ROW EXECUTE FUNCTION lo_manage(raster);

  45. </pre><p>

  46.    For each column that will contain unique references to large objects,

  47.    create a <code class="literal">BEFORE UPDATE OR DELETE</code> trigger, and give the column

  48.    name as the sole trigger argument.  You can also restrict the trigger

  49.    to only execute on updates to the column by using <code class="literal">BEFORE UPDATE

  50.    OF</code> <em class="replaceable"><code>column_name</code></em>.

  51.    If you need multiple <code class="type">lo</code>

  52.    columns in the same table, create a separate trigger for each one,

  53.    remembering to give a different name to each trigger on the same table.

  54.   </p></div><div class="sect2" id="id-1.11.7.29.6"><div class="titlepage"><div><div><h3 class="title">F.20.3. Limitations</h3></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>

  55.      Dropping a table will still orphan any objects it contains, as the trigger

  56.      is not executed.  You can avoid this by preceding the <code class="command">DROP

  57.      TABLE</code> with <code class="command">DELETE FROM <em class="replaceable"><code>table</code></em></code>.

  58.     </p><p>

  59.      <code class="command">TRUNCATE</code> has the same hazard.

  60.     </p><p>

  61.      If you already have, or suspect you have, orphaned large objects, see the

  62.      <a class="xref" href="vacuumlo.html" title="vacuumlo"><span class="refentrytitle"><span class="application">vacuumlo</span></span></a> module to help

  63.      you clean them up.  It's a good idea to run <span class="application">vacuumlo</span>

  64.      occasionally as a back-stop to the <code class="function">lo_manage</code> trigger.

  65.     </p></li><li class="listitem"><p>

  66.      Some frontends may create their own tables, and will not create the

  67.      associated trigger(s).  Also, users may not remember (or know) to create

  68.      the triggers.

  69.     </p></li></ul></div></div><div class="sect2" id="id-1.11.7.29.7"><div class="titlepage"><div><div><h3 class="title">F.20.4. Author</h3></div></div></div><p>

  70.    Peter Mount <code class="email">&lt;<a class="email" href="mailto:peter@retep.org.uk">peter@retep.org.uk</a>&gt;</code>

  71.   </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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ltree.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.19. isn </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> F.21. ltree</td></tr></table></div></body></html>
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