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.) C言語で作成したトリガ関数に関するとても簡単な例をここに示します （手続き言語で作成したトリガの例は、その手続き言語の文書に記載されています。）

The function <function>trigf</function> reports the number of rows in the table <structname>ttest</structname> and skips the actual operation if the command attempts to insert a null value into the column <structfield>x</structfield>. (So the trigger acts as a not-null constraint but doesn't abort the transaction.) trigf関数は、ttestテーブル内にある行数を報告し、問い合わせがxにNULL値を挿入しようとしていた場合は、その操作をスキップします （つまり、このトリガは、トランザクションを中断させないNOT NULL制約のような動作をします。）

First, the table definition: まず、以下のようにテーブルを定義します。

CREATE TABLE ttest (
    x integer
);

This is the source code of the trigger function: 以下がトリガ関数のソースコードです。

#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 */

#include "executor/spi.h"       /* これはSPIを使用する場合に必要なもの */
#include "commands/trigger.h"   /* これはトリガで必要なもの */
#include "utils/rel.h"          /* これはリレーションで必要なもの */

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(trigf);

Datum
trigf(PG_FUNCTION_ARGS)
{
    TriggerData *trigdata = (TriggerData *) fcinfo->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 */

    /* トリガとして呼び出されたかどうかを確認 */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not called by trigger manager");


    /* tuple to return to executor */

    /* エグゼキュータに返すタプル */
    if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
        rettuple = trigdata->tg_newtuple;
    else
        rettuple = trigdata->tg_trigtuple;


    /* check for null values */

    /* NULL値をチェック */
    if (!TRIGGER_FIRED_BY_DELETE(trigdata->tg_event)
        && TRIGGER_FIRED_BEFORE(trigdata->tg_event))
        checknull = true;

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

    tupdesc = trigdata->tg_relation->rd_att;


    /* connect to SPI manager */

    /* SPIマネージャに接続 */
    if ((ret = SPI_connect()) < 0)
        elog(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret);


    /* get number of rows in table */

    /* テーブル中の行数を取得 */
    ret = SPI_exec("SELECT count(*) FROM ttest", 0);

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


    /* count(*) returns int8, so be careful to convert */

    /* count(*)はint8を返す。変換に注意してください*/
    i = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
                                    SPI_tuptable->tupdesc,
                                    1,
                                    &isnull));

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

    SPI_finish();

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

    return PointerGetDatum(rettuple);
}

After you have compiled the source code (see <xref linkend="dfunc"/>), declare the function and the triggers: ソースコードをコンパイル（36.10.5を参照してください）した後に、以下の様に関数とトリガを宣言します。

CREATE FUNCTION trigf() RETURNS trigger
    AS 'filename'
    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();

Now you can test the operation of the trigger: これで、トリガの操作を確認することができます。

=> 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

-- 挿入操作は飛ばされ、また、AFTERトリガも発行されません。

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

=> 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.

                             可視性の説明を思い出してください。
INSERT 167793 1
vac=> SELECT * FROM ttest;
 x
---
 1
(1 row)

=> 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.

                             可視性の説明を思い出してください。
INSERT 167794 1
=> SELECT * FROM ttest;
 x
---
 1
 2
(2 rows)

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

=> 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.

                             可視性の説明を思い出してください。
DELETE 2
=> SELECT * FROM ttest;
 x
---
(0 rows)

There are more complex examples in <filename>src/test/regress/regress.c</filename> and in <xref linkend="contrib-spi"/>. src/test/regress/regress.cとspiにはもっと複雑な例があります。