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<!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>9.28. Trigger Functions</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="functions-admin.html" title="9.27. System Administration Functions" /><link rel="next" href="functions-event-triggers.html" title="9.29. Event Trigger Functions" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">9.28. Trigger Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="functions-admin.html" title="9.27. System Administration Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><th width="60%" align="center">Chapter 9. Functions and Operators</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="functions-event-triggers.html" title="9.29. Event Trigger Functions">Next</a></td></tr></table><hr /></div><div class="sect1" id="FUNCTIONS-TRIGGER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.28. Trigger Functions</h2></div></div></div><p>
While many uses of triggers involve user-written trigger functions,
<span class="productname">PostgreSQL</span> provides a few built-in trigger
functions that can be used directly in user-defined triggers. These
are summarized in <a class="xref" href="functions-trigger.html#BUILTIN-TRIGGERS-TABLE" title="Table 9.101. Built-In Trigger Functions">Table 9.101</a>.
(Additional built-in trigger functions exist, which implement foreign
key constraints and deferred index constraints. Those are not documented
here since users need not use them directly.)
</p><p>
For more information about creating triggers, see
<a class="xref" href="sql-createtrigger.html" title="CREATE TRIGGER"><span class="refentrytitle">CREATE TRIGGER</span></a>.
</p><div class="table" id="BUILTIN-TRIGGERS-TABLE"><p class="title"><strong>Table 9.101. Built-In Trigger Functions</strong></p><div class="table-contents"><table class="table" summary="Built-In Trigger Functions" border="1"><colgroup><col /></colgroup><thead><tr><th class="func_table_entry"><p class="func_signature">
Function
</p>
<p>
Description
</p>
<p>
Example Usage
</p></th></tr></thead><tbody><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.34.4.2.2.1.1.1.1" class="indexterm"></a>
<code class="function">suppress_redundant_updates_trigger</code> ( )
→ <code class="returnvalue">trigger</code>
</p>
<p>
Suppresses do-nothing update operations. See below for details.
</p>
<p>
<code class="literal">CREATE TRIGGER ... suppress_redundant_updates_trigger()</code>
</p></td></tr><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.34.4.2.2.2.1.1.1" class="indexterm"></a>
<code class="function">tsvector_update_trigger</code> ( )
→ <code class="returnvalue">trigger</code>
</p>
<p>
Automatically updates a <code class="type">tsvector</code> column from associated
plain-text document column(s). The text search configuration to use
is specified by name as a trigger argument. See
<a class="xref" href="textsearch-features.html#TEXTSEARCH-UPDATE-TRIGGERS" title="12.4.3. Triggers for Automatic Updates">Section 12.4.3</a> for details.
</p>
<p>
<code class="literal">CREATE TRIGGER ... tsvector_update_trigger(tsvcol, 'pg_catalog.swedish', title, body)</code>
</p></td></tr><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.34.4.2.2.3.1.1.1" class="indexterm"></a>
<code class="function">tsvector_update_trigger_column</code> ( )
→ <code class="returnvalue">trigger</code>
</p>
<p>
Automatically updates a <code class="type">tsvector</code> column from associated
plain-text document column(s). The text search configuration to use
is taken from a <code class="type">regconfig</code> column of the table. See
<a class="xref" href="textsearch-features.html#TEXTSEARCH-UPDATE-TRIGGERS" title="12.4.3. Triggers for Automatic Updates">Section 12.4.3</a> for details.
</p>
<p>
<code class="literal">CREATE TRIGGER ... tsvector_update_trigger_column(tsvcol, tsconfigcol, title, body)</code>
</p></td></tr></tbody></table></div></div><br class="table-break" /><p>
The <code class="function">suppress_redundant_updates_trigger</code> function,
when applied as a row-level <code class="literal">BEFORE UPDATE</code> trigger,
will prevent any update that does not actually change the data in the
row from taking place. This overrides the normal behavior which always
performs a physical row update
regardless of whether or not the data has changed. (This normal behavior
makes updates run faster, since no checking is required, and is also
useful in certain cases.)
</p><p>
Ideally, you should avoid running updates that don't actually
change the data in the record. Redundant updates can cost considerable
unnecessary time, especially if there are lots of indexes to alter,
and space in dead rows that will eventually have to be vacuumed.
However, detecting such situations in client code is not
always easy, or even possible, and writing expressions to detect
them can be error-prone. An alternative is to use
<code class="function">suppress_redundant_updates_trigger</code>, which will skip
updates that don't change the data. You should use this with care,
however. The trigger takes a small but non-trivial time for each record,
so if most of the records affected by updates do actually change,
use of this trigger will make updates run slower on average.
</p><p>
The <code class="function">suppress_redundant_updates_trigger</code> function can be
added to a table like this:
</p><pre class="programlisting">
CREATE TRIGGER z_min_update
BEFORE UPDATE ON tablename
FOR EACH ROW EXECUTE FUNCTION suppress_redundant_updates_trigger();
</pre><p>
In most cases, you need to fire this trigger last for each row, so that
it does not override other triggers that might wish to alter the row.
Bearing in mind that triggers fire in name order, you would therefore
choose a trigger name that comes after the name of any other trigger
you might have on the table. (Hence the <span class="quote">“<span class="quote">z</span>”</span> prefix in the
example.)
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="functions-admin.html" title="9.27. System Administration Functions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="functions-event-triggers.html" title="9.29. Event Trigger Functions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">9.27. System Administration Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 9.29. Event Trigger Functions</td></tr></table></div></body></html>
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