Examples

There are more complex examples in src/test/regress/regress.c and in contrib/spi.

Here is a very simple example of trigger usage. Function trigf reports the number of tuples in the triggered relation ttest and skips the operation if the query attempts to insert NULL into x (i.e - it acts as a NOT NULL constraint but doesn't abort the transaction).

#include "executor/spi.h"	/* this is what you need to work with SPI */
#include "commands/trigger.h"	/* -"- and triggers */

HeapTuple		trigf(void);

HeapTuple
trigf()
{
	TupleDesc	tupdesc;
	HeapTuple	rettuple;
	char		*when;
	bool		checknull = false;
	bool		isnull;
	int		ret, i;

	if (!CurrentTriggerData)
		elog(WARN, "trigf: triggers are not initialized");
	
	/* tuple to return to Executor */
	if (TRIGGER_FIRED_BY_UPDATE(CurrentTriggerData->tg_event))
		rettuple = CurrentTriggerData->tg_newtuple;
	else
		rettuple = CurrentTriggerData->tg_trigtuple;
	
	/* check for NULLs ? */
	if (!TRIGGER_FIRED_BY_DELETE(CurrentTriggerData->tg_event) &&
		TRIGGER_FIRED_BEFORE(CurrentTriggerData->tg_event))
		checknull = true;
	
	if (TRIGGER_FIRED_BEFORE(CurrentTriggerData->tg_event))
		when = "before";
	else
		when = "after ";
	
	tupdesc = CurrentTriggerData->tg_relation->rd_att;
	CurrentTriggerData = NULL;
	
	/* Connect to SPI manager */
	if ((ret = SPI_connect()) < 0)
		elog(WARN, "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(WARN, "trigf (fired %s): SPI_exec returned %d", when, ret);
	
	i = 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)
	{
		i = SPI_getbinval(rettuple, tupdesc, 1, &isnull);
		if (isnull)
			rettuple = NULL;
	}

	return (rettuple);
}
    

Now, compile and create table ttest (x int4):

create function trigf () returns opaque as 
'...path_to_so' language 'c';
    

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)