21.4. 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 */
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);
 
 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 PointerGetDatum(rettuple);
}
 

Now, compile and create the trigger function:

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)