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这里是一个用 C 写的非常简单的触发器例子。用过程语言写的触发器例子可以在文档的过程语言那部分找到。
如果命令尝试向trigf中插入空值，那么trigf会报告ttest表中的函数，
并跳过实际的操作。因此触发实际上执行了一个非空约束，但不会中止事务。
First, the table definition: 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" /* 用 SPI 的时候要调用的头文件*/
#include "commands/trigger.h" /* ...和触发器 */
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif
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调用的 */
if (!CALLED_AS_TRIGGER(fcinfo))
elog(ERROR, "trigf: not called 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 NULL 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(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret);
/* 获取表中的行数量 */
ret = SPI_exec("SELECT count(*) FROM ttest", 0);
if (ret < 0)
elog(ERROR, "trigf (fired %s): SPI_exec returned %d", when, ret);
/* count(*) 返回 int8 ，所以要小心转换*/
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);
}
编译完源代码后(参阅Section 35.9.6)，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：
=> INSERT INTO ttest VALUES (NULL);
INFO: trigf (fired before): there are 0 rows in ttest
INSERT 0 0
-- 插入被忽略，AFTER 触发器没有触发
=> 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
^^^^^^^^
还记得讲过的关于可视性的原则吗？
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
^^^^^^
还记得讲过的关于可视性的原则吗？
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
^^^^^^
还记得讲过的关于可视性的原则吗？
DELETE 2
=> SELECT * FROM ttest;
x
---
(0 rows)
在src/test/regress/regress.c和contrib/spi里还有更复杂的例子