Examples

There are more complex examples in 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)