16.4. 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 a null value 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 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(INFO, "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);
 }

Now, compile and create the trigger function:

CREATE FUNCTION trigf () RETURNS TRIGGER 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);
 WARNING:  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);
 INFO:  trigf (fired before): there are 0 tuples in ttest
 INFO:  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;
 INFO:  trigf (fired before): there are 1 tuples in ttest
 INFO:  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;
 INFO:  trigf (fired before): there are 2 tuples in ttest
 UPDATE 0
 vac=> UPDATE ttest SET x = 4 WHERE x = 2;
 INFO:  trigf (fired before): there are 2 tuples in ttest
 INFO:  trigf (fired after ): there are 2 tuples in ttest
 UPDATE 1
 vac=> SELECT * FROM ttest;
  x
 ---
  1
  4
 (2 rows)
 
 vac=> DELETE FROM ttest;
 INFO:  trigf (fired before): there are 2 tuples in ttest
 INFO:  trigf (fired after ): there are 1 tuples in ttest
 INFO:  trigf (fired before): there are 1 tuples in ttest
 INFO:  trigf (fired after ): there are 0 tuples in ttest
                                        ^^^^^^^^
                              remember what we said about visibility.
 DELETE 2
 vac=> SELECT * FROM ttest;
  x
 ---
 (0 rows)