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diff --git a/doc/src/sgml/html/lo.html b/doc/src/sgml/html/lo.html new file mode 100644 index 0000000..ffe74df --- /dev/null +++ b/doc/src/sgml/html/lo.html @@ -0,0 +1,75 @@ +<?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.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 Vsnapshot" /><link rel="prev" href="isn.html" title="F.19. isn" /><link rel="next" href="ltree.html" title="F.21. ltree" /></head><body id="docContent" class="container-fluid col-10"><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 14.5 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-1.11.7.29.5">F.20.1. Rationale</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.6">F.20.2. How to Use It</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.7">F.20.3. Limitations</a></span></dt><dt><span class="sect2"><a href="lo.html#id-1.11.7.29.8">F.20.4. Author</a></span></dt></dl></div><a id="id-1.11.7.29.2" 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-1.11.7.29.5"><div class="titlepage"><div><div><h3 class="title">F.20.1. 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 domain of 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-1.11.7.29.6"><div class="titlepage"><div><div><h3 class="title">F.20.2. 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-1.11.7.29.7"><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> + 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-1.11.7.29.8"><div class="titlepage"><div><div><h3 class="title">F.20.4. Author</h3></div></div></div><p> + Peter Mount <code class="email"><<a class="email" href="mailto:peter@retep.org.uk">peter@retep.org.uk</a>></code> + </p></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="isn.html" title="F.19. isn">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. 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