37.4. 一个完整的触发器实例

这里有一个用 C 编写的触发器函数的非常简单的例子(用过程语言编写的触发器的例子可以在过程语言的文档中找到)。

如果该命令试图向列x中插入一个空值,函数trigf报告表ttest中的行数并且跳过实际的操作(这样该触发器会作为一个非空约束但不会中止事务)。

首先,表定义:

CREATE TABLE ttest (
 x integer
);

这是该触发器函数的源代码:

#include "postgres.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->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->tg_event))
 rettuple = trigdata->tg_newtuple;
 else
 rettuple = trigdata->tg_trigtuple;
 /* check for null values */
 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 */
 if ((ret = SPI_connect()) < 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 < 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->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);
}

在你编译了该源代码(见第 36.9.6 节)之后,声明该函数和触发器:

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();

现在你可以测试该触发器的操作:

=> 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
=> 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.
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.
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.
DELETE 2
=> SELECT * FROM ttest;
 x
---
(0 rows)

在src/test/regress/regress.c和spi中有更多复杂的例子。