20.4. 例子

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

这里是一个非常简单的触发器使用的例子． 函数 trigf 报告在被触发的关系 ttest 中元组数量， 并且如果查询试图把NULL 插入到 x 里 （也就是说 －它做为一个 NOT NULL 约束但不退出事务的约束）时略过操作．

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

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;

    /* Make sure trigdata is pointing at what I expect */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not fired 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 NULLs ? */
    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(NOTICE, "trigf (fired %s): SPI_connect returned %d", when, ret);
    
    /* Get number of tuples in relation */
    ret = SPI_exec("SELECT count(*) FROM ttest", 0);
    
    if (ret < 0)
        elog(NOTICE, "trigf (fired %s): SPI_exec returned %d", when, ret);
    

       /* count(*) returns int8 as of PG 7.2, so be careful to convert */
       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);
}
    

然后，编译和创建触发器函数：

CREATE FUNCTION trigf () RETURNS OPAQUE AS 
'...path_to_so' LANGUAGE 'C';

create table ttest (x int4);
    

vac=> CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest 
FOR EACH ROW EXECUTE PROCEDURE trigf();
CREATE
vac=> CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest 
FOR EACH ROW EXECUTE PROCEDURE trigf();
CREATE
vac=> INSERT INTO ttest VALUES (null);
NOTICE:trigf (fired before): there are 0 tuples in ttest
INSERT 0 0

-- Insertion skipped and AFTER trigger is not fired

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

vac=> INSERT INTO ttest VALUES (1);
NOTICE:trigf (fired before): there are 0 tuples in ttest
NOTICE:trigf (fired after ): there are 1 tuples in ttest
                                       ^^^^^^^^
                             remember what we said about visibility.
INSERT 167793 1
vac=> SELECT * FROM ttest;
x
-
1
(1 row)

vac=> INSERT INTO ttest SELECT x * 2 FROM ttest;
NOTICE:trigf (fired before): there are 1 tuples in ttest
NOTICE:trigf (fired after ): there are 2 tuples in ttest
                                       ^^^^^^^^
                             remember what we said about visibility.
INSERT 167794 1
vac=> SELECT * FROM ttest;
x
-
1
2
(2 rows)

vac=> UPDATE ttest SET x = null where x = 2;
NOTICE:trigf (fired before): there are 2 tuples in ttest
UPDATE 0
vac=> UPDATE ttest SET x = 4 where x = 2;
NOTICE:trigf (fired before): there are 2 tuples in ttest
NOTICE:trigf (fired after ): there are 2 tuples in ttest
UPDATE 1
vac=> SELECT * FROM ttest;
x
-
1
4
(2 rows)

vac=> DELETE FROM ttest;
NOTICE:trigf (fired before): there are 2 tuples in ttest
NOTICE:trigf (fired after ): there are 1 tuples in ttest
NOTICE:trigf (fired before): there are 1 tuples in ttest
NOTICE:trigf (fired after ): there are 0 tuples in ttest
                                       ^^^^^^^^
                             remember what we said about visibility.
DELETE 2
vac=> SELECT * FROM ttest;
x
-
(0 rows)