35.4. 一个完整的例子

这里是一个用 C 写的非常简单的触发器使用的例子。 函数trigf报告表ttest中行数量， 并且如果命令试图把空值插入到字段 x 里（也就是说 －它做为一个非空约束但不退出事务的约束）时略过操作。

首先，表定义：

CREATE TABLE ttest (
    x integer
);

这里是触发器函数的源代码：

#include "postgres.h"
#include "executor/spi.h"	 /* 你用SPI的时候要用的头文件 */
#include "commands/trigger.h"	 /* 用触发器时要用的头文件 */

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;

    /* 确信自己是作为触发器触发的 */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not fired by trigger manager");

    /* 返回给执行者的行 */
    if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
        rettuple = trigdata->tg_newtuple;
    else
        rettuple = trigdata->tg_trigtuple;

    /* 检查空值 */
    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;

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

    /* 获取关系中的行数量 */
    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，所以要小心转换 */
    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);
}

编译完源代码后，声明函数并创建触发器：

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();

现在你可以测试触发器的操作：

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 触发器没有触发

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
				       ^^^^^^^^
			     回忆一下我们讲的有关可视性的东西。
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
				       ^^^^^^^^
			     还记得我们讲过的关于可视性的原则吗
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
				       ^^^^^^^^
			     还记得我们讲过的关于可视性的原则吗
DELETE 2
vac=> SELECT * FROM ttest;
 x
---
(0 rows)

在 src/test/regress/regress.c 和 contrib/spi 里还有更复杂的例子。