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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>43.6. 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 V1.79.1" /><link rel="prev" href="pltcl-dbaccess.html" title="43.5. Database Access from PL/Tcl" /><link rel="next" href="pltcl-event-trigger.html" title="43.7. Event Trigger Functions in PL/Tcl" /></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">43.6. Trigger Functions in PL/Tcl</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pltcl-dbaccess.html" title="43.5. Database Access from PL/Tcl">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="pltcl.html" title="Chapter 43. PL/Tcl - Tcl Procedural Language">Up</a></td><th width="60%" align="center">Chapter 43. PL/Tcl - Tcl Procedural Language</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="pltcl-event-trigger.html" title="43.7. Event Trigger Functions in PL/Tcl">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PLTCL-TRIGGER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">43.6. Trigger Functions in PL/Tcl</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.
    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pltcl-dbaccess.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pltcl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pltcl-event-trigger.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">43.5. Database Access from PL/Tcl </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 43.7. Event Trigger Functions in PL/Tcl</td></tr></table></div></body></html>