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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>44.6. Trigger Functions in PL/Tcl</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="pltcl-dbaccess.html" title="44.5. Database Access from PL/Tcl" /><link rel="next" href="pltcl-event-trigger.html" title="44.7. Event Trigger Functions in PL/Tcl" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">44.6. Trigger Functions in PL/Tcl</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pltcl-dbaccess.html" title="44.5. Database Access from PL/Tcl">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="pltcl.html" title="Chapter 44. PL/Tcl — Tcl Procedural Language">Up</a></td><th width="60%" align="center">Chapter 44. PL/Tcl — Tcl Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pltcl-event-trigger.html" title="44.7. Event Trigger Functions in PL/Tcl">Next</a></td></tr></table><hr /></div><div class="sect1" id="PLTCL-TRIGGER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">44.6. Trigger Functions in PL/Tcl <a href="#PLTCL-TRIGGER" class="id_link">#</a></h2></div></div></div><a id="id-1.8.9.10.2" class="indexterm"></a><p>
Trigger functions can be written in PL/Tcl.
<span class="productname">PostgreSQL</span> requires that a function that is to be called
as a trigger must be declared as a function with no arguments
and a return type of <code class="literal">trigger</code>.
</p><p>
The information from the trigger manager is passed to the function body
in the following variables:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="varname">$TG_name</code></span></dt><dd><p>
The name of the trigger from the <code class="command">CREATE TRIGGER</code> statement.
</p></dd><dt><span class="term"><code class="varname">$TG_relid</code></span></dt><dd><p>
The object ID of the table that caused the trigger function
to be invoked.
</p></dd><dt><span class="term"><code class="varname">$TG_table_name</code></span></dt><dd><p>
The name of the table that caused the trigger function
to be invoked.
</p></dd><dt><span class="term"><code class="varname">$TG_table_schema</code></span></dt><dd><p>
The schema of the table that caused the trigger function
to be invoked.
</p></dd><dt><span class="term"><code class="varname">$TG_relatts</code></span></dt><dd><p>
A Tcl list of the table column names, prefixed with an empty list
element. So looking up a column name in the list with <span class="application">Tcl</span>'s
<code class="function">lsearch</code> command returns the element's number starting
with 1 for the first column, the same way the columns are customarily
numbered in <span class="productname">PostgreSQL</span>. (Empty list
elements also appear in the positions of columns that have been
dropped, so that the attribute numbering is correct for columns
to their right.)
</p></dd><dt><span class="term"><code class="varname">$TG_when</code></span></dt><dd><p>
The string <code class="literal">BEFORE</code>, <code class="literal">AFTER</code>, or
<code class="literal">INSTEAD OF</code>, depending on the type of trigger event.
</p></dd><dt><span class="term"><code class="varname">$TG_level</code></span></dt><dd><p>
The string <code class="literal">ROW</code> or <code class="literal">STATEMENT</code> depending on the
type of trigger event.
</p></dd><dt><span class="term"><code class="varname">$TG_op</code></span></dt><dd><p>
The string <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>,
<code class="literal">DELETE</code>, or <code class="literal">TRUNCATE</code> depending on the type of
trigger event.
</p></dd><dt><span class="term"><code class="varname">$NEW</code></span></dt><dd><p>
An associative array containing the values of the new table
row for <code class="command">INSERT</code> or <code class="command">UPDATE</code> actions, or
empty for <code class="command">DELETE</code>. The array is indexed by column
name. Columns that are null will not appear in the array.
This is not set for statement-level triggers.
</p></dd><dt><span class="term"><code class="varname">$OLD</code></span></dt><dd><p>
An associative array containing the values of the old table
row for <code class="command">UPDATE</code> or <code class="command">DELETE</code> actions, or
empty for <code class="command">INSERT</code>. The array is indexed by column
name. Columns that are null will not appear in the array.
This is not set for statement-level triggers.
</p></dd><dt><span class="term"><code class="varname">$args</code></span></dt><dd><p>
A Tcl list of the arguments to the function as given in the
<code class="command">CREATE TRIGGER</code> statement. These arguments are also accessible as
<code class="literal">$1</code> ... <code class="literal">$<em class="replaceable"><code>n</code></em></code> in the function body.
</p></dd></dl></div><p>
</p><p>
The return value from a trigger function can be one of the strings
<code class="literal">OK</code> or <code class="literal">SKIP</code>, or a list of column name/value pairs.
If the return value is <code class="literal">OK</code>,
the operation (<code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code>)
that fired the trigger will proceed
normally. <code class="literal">SKIP</code> tells the trigger manager to silently suppress
the operation for this row. If a list is returned, it tells PL/Tcl to
return a modified row to the trigger manager; the contents of the
modified row are specified by the column names and values in the list.
Any columns not mentioned in the list are set to null.
Returning a modified row is only meaningful
for row-level <code class="literal">BEFORE</code> <code class="command">INSERT</code> or <code class="command">UPDATE</code>
triggers, for which the modified row will be inserted instead of the one
given in <code class="varname">$NEW</code>; or for row-level <code class="literal">INSTEAD OF</code>
<code class="command">INSERT</code> or <code class="command">UPDATE</code> triggers where the returned row
is used as the source data for <code class="command">INSERT RETURNING</code> or
<code class="command">UPDATE RETURNING</code> clauses.
In row-level <code class="literal">BEFORE</code> <code class="command">DELETE</code> or <code class="literal">INSTEAD
OF</code> <code class="command">DELETE</code> triggers, returning a modified row has the same
effect as returning <code class="literal">OK</code>, that is the operation proceeds.
The trigger return value is ignored for all other types of triggers.
</p><div class="tip"><h3 class="title">Tip</h3><p>
The result list can be made from an array representation of the
modified tuple with the <code class="literal">array get</code> Tcl command.
</p></div><p>
Here's a little example trigger function that forces an integer value
in a table to keep track of the number of updates that are performed on the
row. For new rows inserted, the value is initialized to 0 and then
incremented on every update operation.
</p><pre class="programlisting">
CREATE FUNCTION trigfunc_modcount() RETURNS trigger AS $$
switch $TG_op {
INSERT {
set NEW($1) 0
}
UPDATE {
set NEW($1) $OLD($1)
incr NEW($1)
}
default {
return OK
}
}
return [array get NEW]
$$ LANGUAGE pltcl;
CREATE TABLE mytab (num integer, description text, modcnt integer);
CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab
FOR EACH ROW EXECUTE FUNCTION trigfunc_modcount('modcnt');
</pre><p>
Notice that the trigger function itself does not know the column
name; that's supplied from the trigger arguments. This lets the
trigger function be reused with different tables.
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