35.4. 一個完整的例子

這裡是一個用 C 寫的非常簡單的觸發器使用的例子。 函數trigf報告表ttest中行數量， 並且如果命令試圖把空值插入到字段 x 裡（也就是說 －它做為一個非空約束但不退出事務的約束）時略過操作。

首先，表定義：

CREATE TABLE ttest (
    x integer
);

這裡是觸發器函數的源代碼：

#include "postgres.h"
#include "executor/spi.h"	 /* 你用SPI的時候要用的頭文件 */
#include "commands/trigger.h"	 /* 用觸發器時要用的頭文件 */

extern Datum trigf(PG_FUNCTION_ARGS);

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;

    /* 確信自己是作為觸發器觸發的 */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not fired by trigger manager");

    /* 返回給執行者的行 */
    if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
        rettuple = trigdata->tg_newtuple;
    else
        rettuple = trigdata->tg_trigtuple;

    /* 檢查空值 */
    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;

    /* 與 SPI 管理器連接 */
    if ((ret = SPI_connect()) < 0)
        elog(INFO, "trigf (fired %s): SPI_connect returned %d", when, ret);

    /* 獲取關系中的行數量 */
    ret = SPI_exec("SELECT count(*) FROM ttest", 0);

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

    /* count(*) 返回 int8，所以要小心轉換 */
    i = (int) DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
                                          SPI_tuptable->tupdesc,
                                          1,
                                          &isnull));

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

    SPI_finish();

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

    return PointerGetDatum(rettuple);
}

編譯完源代碼後，聲明函數並創建觸發器：

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

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

CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest 
    FOR EACH ROW EXECUTE PROCEDURE trigf();

現在你可以測試觸發器的操作：

vac=> INSERT INTO ttest VALUES (NULL);
INFO:trigf (fired before): there are 0 tuples in ttest
INSERT 0 0

-- 插入被忽略，AFTER 觸發器沒有觸發

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

vac=> 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
				       ^^^^^^^^
			     回憶一下我們講的有關可視性的東西。
INSERT 167793 1
vac=> SELECT * FROM ttest;
 x
---
 1
(1 row)

vac=> 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
				       ^^^^^^^^
			     還記得我們講過的關于可視性的原則嗎
INSERT 167794 1
vac=> SELECT * FROM ttest;
 x
---
 1
 2
(2 rows)

vac=> UPDATE ttest SET x = NULL where x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
UPDATE 0
vac=> 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)

vac=> DELETE FROM ttest;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired after ): there are 1 rows in ttest
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 0 rows in ttest
				       ^^^^^^^^
			     還記得我們講過的關于可視性的原則嗎
DELETE 2
vac=> SELECT * FROM ttest;
 x
---
(0 rows)

在 src/test/regress/regress.c 和 contrib/spi 裡還有更復雜的例子。