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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>38.4. A Complete Trigger Example</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="trigger-interface.html" title="38.3. Writing Trigger Functions in C" /><link rel="next" href="event-triggers.html" title="Chapter 39. Event Triggers" /></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">38.4. A Complete Trigger Example</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="trigger-interface.html" title="38.3. Writing Trigger Functions in C">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="triggers.html" title="Chapter 38. Triggers">Up</a></td><th width="60%" align="center">Chapter 38. Triggers</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="event-triggers.html" title="Chapter 39. Event Triggers">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="TRIGGER-EXAMPLE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.4. A Complete Trigger Example</h2></div></div></div><p>
    Here is a very simple example of a trigger function written in C.
    (Examples of triggers written in procedural languages can be found
    in the documentation of the procedural languages.)
   </p><p>
    The function <code class="function">trigf</code> reports the number of rows in the
    table <code class="structname">ttest</code> and skips the actual operation if the
    command attempts to insert a null value into the column
    <code class="structfield">x</code>. (So the trigger acts as a not-null constraint but
    doesn't abort the transaction.)
   </p><p>
    First, the table definition:
</p><pre class="programlisting">
CREATE TABLE ttest (
    x integer
);
</pre><p>
   </p><p>
    This is the source code of the trigger function:
</p><pre class="programlisting">
#include "postgres.h"
#include "fmgr.h"
#include "executor/spi.h"       /* this is what you need to work with SPI */
#include "commands/trigger.h"   /* ... triggers ... */
#include "utils/rel.h"          /* ... and relations */

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(trigf);

Datum
trigf(PG_FUNCTION_ARGS)
{
    TriggerData *trigdata = (TriggerData *) fcinfo-&gt;context;
    TupleDesc   tupdesc;
    HeapTuple   rettuple;
    char       *when;
    bool        checknull = false;
    bool        isnull;
    int         ret, i;

    /* make sure it's called as a trigger at all */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not called by trigger manager");

    /* tuple to return to executor */
    if (TRIGGER_FIRED_BY_UPDATE(trigdata-&gt;tg_event))
        rettuple = trigdata-&gt;tg_newtuple;
    else
        rettuple = trigdata-&gt;tg_trigtuple;

    /* check for null values */
    if (!TRIGGER_FIRED_BY_DELETE(trigdata-&gt;tg_event)
        &amp;&amp; TRIGGER_FIRED_BEFORE(trigdata-&gt;tg_event))
        checknull = true;

    if (TRIGGER_FIRED_BEFORE(trigdata-&gt;tg_event))
        when = "before";
    else
        when = "after ";

    tupdesc = trigdata-&gt;tg_relation-&gt;rd_att;

    /* connect to SPI manager */
    if ((ret = SPI_connect()) &lt; 0)
        elog(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret);

    /* get number of rows in table */
    ret = SPI_exec("SELECT count(*) FROM ttest", 0);

    if (ret &lt; 0)
        elog(ERROR, "trigf (fired %s): SPI_exec returned %d", when, ret);

    /* count(*) returns int8, so be careful to convert */
    i = DatumGetInt64(SPI_getbinval(SPI_tuptable-&gt;vals[0],
                                    SPI_tuptable-&gt;tupdesc,
                                    1,
                                    &amp;isnull));

    elog (INFO, "trigf (fired %s): there are %d rows in ttest", when, i);

    SPI_finish();

    if (checknull)
    {
        SPI_getbinval(rettuple, tupdesc, 1, &amp;isnull);
        if (isnull)
            rettuple = NULL;
    }

    return PointerGetDatum(rettuple);
}

</pre><p>
   </p><p>
    After you have compiled the source code (see <a class="xref" href="xfunc-c.html#DFUNC" title="37.10.5. Compiling and Linking Dynamically-Loaded Functions">Section 37.10.5</a>), declare the function and the triggers:
</p><pre class="programlisting">
CREATE FUNCTION trigf() RETURNS trigger
    AS '<em class="replaceable"><code>filename</code></em>'
    LANGUAGE C;

CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest
    FOR EACH ROW EXECUTE FUNCTION trigf();

CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
    FOR EACH ROW EXECUTE FUNCTION trigf();
</pre><p>
   </p><p>
    Now you can test the operation of the trigger:
</p><pre class="screen">
=&gt; INSERT INTO ttest VALUES (NULL);
INFO:  trigf (fired before): there are 0 rows in ttest
INSERT 0 0

-- Insertion skipped and AFTER trigger is not fired

=&gt; SELECT * FROM ttest;
 x
---
(0 rows)

=&gt; INSERT INTO ttest VALUES (1);
INFO:  trigf (fired before): there are 0 rows in ttest
INFO:  trigf (fired after ): there are 1 rows in ttest
                                       ^^^^^^^^
                             remember what we said about visibility.
INSERT 167793 1
vac=&gt; SELECT * FROM ttest;
 x
---
 1
(1 row)

=&gt; INSERT INTO ttest SELECT x * 2 FROM ttest;
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 2 rows in ttest
                                       ^^^^^^
                             remember what we said about visibility.
INSERT 167794 1
=&gt; SELECT * FROM ttest;
 x
---
 1
 2
(2 rows)

=&gt; UPDATE ttest SET x = NULL WHERE x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
UPDATE 0
=&gt; UPDATE ttest SET x = 4 WHERE x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired after ): there are 2 rows in ttest
UPDATE 1
vac=&gt; SELECT * FROM ttest;
 x
---
 1
 4
(2 rows)

=&gt; DELETE FROM ttest;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 0 rows in ttest
INFO:  trigf (fired after ): there are 0 rows in ttest
                                       ^^^^^^
                             remember what we said about visibility.
DELETE 2
=&gt; SELECT * FROM ttest;
 x
---
(0 rows)
</pre><p>

   </p><p>
    There are more complex examples in
    <code class="filename">src/test/regress/regress.c</code> and
    in <a class="xref" href="contrib-spi.html" title="F.36. spi">spi</a>.
   </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="trigger-interface.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="triggers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="event-triggers.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">38.3. Writing Trigger Functions in C </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 39. Event Triggers</td></tr></table></div></body></html>