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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>39.3. Writing Trigger Functions in C</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="trigger-datachanges.html" title="39.2. Visibility of Data Changes" /><link rel="next" href="trigger-example.html" title="39.4. A Complete Trigger Example" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">39.3. Writing Trigger Functions in C</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="trigger-datachanges.html" title="39.2. Visibility of Data Changes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="triggers.html" title="Chapter 39. Triggers">Up</a></td><th width="60%" align="center">Chapter 39. Triggers</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="trigger-example.html" title="39.4. A Complete Trigger Example">Next</a></td></tr></table><hr /></div><div class="sect1" id="TRIGGER-INTERFACE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">39.3. Writing Trigger Functions in C <a href="#TRIGGER-INTERFACE" class="id_link">#</a></h2></div></div></div><a id="id-1.8.4.7.2" class="indexterm"></a><a id="id-1.8.4.7.3" class="indexterm"></a><p>
This section describes the low-level details of the interface to a
trigger function. This information is only needed when writing
trigger functions in C. If you are using a higher-level language then
these details are handled for you. In most cases you should consider
using a procedural language before writing your triggers in C. The
documentation of each procedural language explains how to write a
trigger in that language.
</p><p>
Trigger functions must use the <span class="quote">“<span class="quote">version 1</span>”</span> function manager
interface.
</p><p>
When a function is called by the trigger manager, it is not passed
any normal arguments, but it is passed a <span class="quote">“<span class="quote">context</span>”</span>
pointer pointing to a <code class="structname">TriggerData</code> structure. C
functions can check whether they were called from the trigger
manager or not by executing the macro:
</p><pre class="programlisting">
CALLED_AS_TRIGGER(fcinfo)
</pre><p>
which expands to:
</p><pre class="programlisting">
((fcinfo)->context != NULL && IsA((fcinfo)->context, TriggerData))
</pre><p>
If this returns true, then it is safe to cast
<code class="literal">fcinfo->context</code> to type <code class="literal">TriggerData
*</code> and make use of the pointed-to
<code class="structname">TriggerData</code> structure. The function must
<span class="emphasis"><em>not</em></span> alter the <code class="structname">TriggerData</code>
structure or any of the data it points to.
</p><p>
<code class="structname">struct TriggerData</code> is defined in
<code class="filename">commands/trigger.h</code>:
</p><pre class="programlisting">
typedef struct TriggerData
{
NodeTag type;
TriggerEvent tg_event;
Relation tg_relation;
HeapTuple tg_trigtuple;
HeapTuple tg_newtuple;
Trigger *tg_trigger;
TupleTableSlot *tg_trigslot;
TupleTableSlot *tg_newslot;
Tuplestorestate *tg_oldtable;
Tuplestorestate *tg_newtable;
const Bitmapset *tg_updatedcols;
} TriggerData;
</pre><p>
where the members are defined as follows:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="structfield">type</code></span></dt><dd><p>
Always <code class="literal">T_TriggerData</code>.
</p></dd><dt><span class="term"><code class="structfield">tg_event</code></span></dt><dd><p>
Describes the event for which the function is called. You can use the
following macros to examine <code class="literal">tg_event</code>:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">TRIGGER_FIRED_BEFORE(tg_event)</code></span></dt><dd><p>
Returns true if the trigger fired before the operation.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_AFTER(tg_event)</code></span></dt><dd><p>
Returns true if the trigger fired after the operation.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_INSTEAD(tg_event)</code></span></dt><dd><p>
Returns true if the trigger fired instead of the operation.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_FOR_ROW(tg_event)</code></span></dt><dd><p>
Returns true if the trigger fired for a row-level event.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_FOR_STATEMENT(tg_event)</code></span></dt><dd><p>
Returns true if the trigger fired for a statement-level event.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_INSERT(tg_event)</code></span></dt><dd><p>
Returns true if the trigger was fired by an <code class="command">INSERT</code> command.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_UPDATE(tg_event)</code></span></dt><dd><p>
Returns true if the trigger was fired by an <code class="command">UPDATE</code> command.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_DELETE(tg_event)</code></span></dt><dd><p>
Returns true if the trigger was fired by a <code class="command">DELETE</code> command.
</p></dd><dt><span class="term"><code class="literal">TRIGGER_FIRED_BY_TRUNCATE(tg_event)</code></span></dt><dd><p>
Returns true if the trigger was fired by a <code class="command">TRUNCATE</code> command.
</p></dd></dl></div><p>
</p></dd><dt><span class="term"><code class="structfield">tg_relation</code></span></dt><dd><p>
A pointer to a structure describing the relation that the trigger fired for.
Look at <code class="filename">utils/rel.h</code> for details about
this structure. The most interesting things are
<code class="literal">tg_relation->rd_att</code> (descriptor of the relation
tuples) and <code class="literal">tg_relation->rd_rel->relname</code>
(relation name; the type is not <code class="type">char*</code> but
<code class="type">NameData</code>; use
<code class="literal">SPI_getrelname(tg_relation)</code> to get a <code class="type">char*</code> if you
need a copy of the name).
</p></dd><dt><span class="term"><code class="structfield">tg_trigtuple</code></span></dt><dd><p>
A pointer to the row for which the trigger was fired. This is
the row being inserted, updated, or deleted. If this trigger
was fired for an <code class="command">INSERT</code> or
<code class="command">DELETE</code> then this is what you should return
from the function if you don't want to replace the row with
a different one (in the case of <code class="command">INSERT</code>) or
skip the operation. For triggers on foreign tables, values of system
columns herein are unspecified.
</p></dd><dt><span class="term"><code class="structfield">tg_newtuple</code></span></dt><dd><p>
A pointer to the new version of the row, if the trigger was
fired for an <code class="command">UPDATE</code>, and <code class="symbol">NULL</code> if
it is for an <code class="command">INSERT</code> or a
<code class="command">DELETE</code>. This is what you have to return
from the function if the event is an <code class="command">UPDATE</code>
and you don't want to replace this row by a different one or
skip the operation. For triggers on foreign tables, values of system
columns herein are unspecified.
</p></dd><dt><span class="term"><code class="structfield">tg_trigger</code></span></dt><dd><p>
A pointer to a structure of type <code class="structname">Trigger</code>,
defined in <code class="filename">utils/reltrigger.h</code>:
</p><pre class="programlisting">
typedef struct Trigger
{
Oid tgoid;
char *tgname;
Oid tgfoid;
int16 tgtype;
char tgenabled;
bool tgisinternal;
bool tgisclone;
Oid tgconstrrelid;
Oid tgconstrindid;
Oid tgconstraint;
bool tgdeferrable;
bool tginitdeferred;
int16 tgnargs;
int16 tgnattr;
int16 *tgattr;
char **tgargs;
char *tgqual;
char *tgoldtable;
char *tgnewtable;
} Trigger;
</pre><p>
where <code class="structfield">tgname</code> is the trigger's name,
<code class="structfield">tgnargs</code> is the number of arguments in
<code class="structfield">tgargs</code>, and <code class="structfield">tgargs</code> is an array of
pointers to the arguments specified in the <code class="command">CREATE
TRIGGER</code> statement. The other members are for internal use
only.
</p></dd><dt><span class="term"><code class="structfield">tg_trigslot</code></span></dt><dd><p>
The slot containing <code class="structfield">tg_trigtuple</code>,
or a <code class="symbol">NULL</code> pointer if there is no such tuple.
</p></dd><dt><span class="term"><code class="structfield">tg_newslot</code></span></dt><dd><p>
The slot containing <code class="structfield">tg_newtuple</code>,
or a <code class="symbol">NULL</code> pointer if there is no such tuple.
</p></dd><dt><span class="term"><code class="structfield">tg_oldtable</code></span></dt><dd><p>
A pointer to a structure of type <code class="structname">Tuplestorestate</code>
containing zero or more rows in the format specified by
<code class="structfield">tg_relation</code>, or a <code class="symbol">NULL</code> pointer
if there is no <code class="literal">OLD TABLE</code> transition relation.
</p></dd><dt><span class="term"><code class="structfield">tg_newtable</code></span></dt><dd><p>
A pointer to a structure of type <code class="structname">Tuplestorestate</code>
containing zero or more rows in the format specified by
<code class="structfield">tg_relation</code>, or a <code class="symbol">NULL</code> pointer
if there is no <code class="literal">NEW TABLE</code> transition relation.
</p></dd><dt><span class="term"><code class="structfield">tg_updatedcols</code></span></dt><dd><p>
For <code class="literal">UPDATE</code> triggers, a bitmap set indicating the
columns that were updated by the triggering command. Generic trigger
functions can use this to optimize actions by not having to deal with
columns that were not changed.
</p><p>
As an example, to determine whether a column with attribute number
<code class="varname">attnum</code> (1-based) is a member of this bitmap set,
call <code class="literal">bms_is_member(attnum -
FirstLowInvalidHeapAttributeNumber,
trigdata->tg_updatedcols))</code>.
</p><p>
For triggers other than <code class="literal">UPDATE</code> triggers, this will
be <code class="symbol">NULL</code>.
</p></dd></dl></div><p>
</p><p>
To allow queries issued through SPI to reference transition tables, see
<a class="xref" href="spi-spi-register-trigger-data.html" title="SPI_register_trigger_data"><span class="refentrytitle">SPI_register_trigger_data</span></a>.
</p><p>
A trigger function must return either a
<code class="structname">HeapTuple</code> pointer or a <code class="symbol">NULL</code> pointer
(<span class="emphasis"><em>not</em></span> an SQL null value, that is, do not set <em class="parameter"><code>isNull</code></em> true).
Be careful to return either
<code class="structfield">tg_trigtuple</code> or <code class="structfield">tg_newtuple</code>,
as appropriate, if you don't want to modify the row being operated on.
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