# 39.4.
一个完整的触发器示例这是一个用 C 编写的触发器函数的非常简单的示例。(用过程语言编写的触发器示例可以在过程语言的文档中找到。)
功能触发
报告表中的行数测试
如果命令尝试在列中插入空值,则跳过实际操作x
.(因此触发器充当非空约束,但不会中止事务。)
一、表定义:
CREATE TABLE ttest (
x integer
);
这是触发函数的源代码:
#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 */
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);
}
编译源代码后(请参阅第 38.10.5 节),声明函数和触发器:
CREATE FUNCTION trigf() RETURNS trigger
AS 'filename'
LANGUAGE C;
CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest
FOR EACH ROW EXECUTE FUNCTION trigf();
CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
FOR EACH ROW EXECUTE FUNCTION trigf();
现在可以测试触发器的操作了:
=> 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